Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults

Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults Pritam Banerjee, Pasquale Orgiani, Arno Meingast, Sorin Lazar, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5883878/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The study of rare-earth nickelates, such as LaNiO 3 (LNO), is significant due to their complex electronic properties. Ordered oxygen vacancies (OOV) in LaNiO 3 − x decrease conductivity, converting it from metallic to insulating state as 'x' approaches 0.5, and semiconducting behavior near x = 0.75. These OOV also influence magnetic properties, causing LNO to exhibit anti-ferromagnetic and ferromagnetic behavior instead of its usual paramagnetic state. Interfacial strain in thin-film heterostructures is utilized to regulate the creation of oxygen vacancies and Ruddlesden-Popper (RP) faults, leading to notable impacts on materials' structural and electronic phases. The effect of strain on the formation of RP faults and the critical thickness of a fault-free layer in LNO has been studied, but atomic-scale insights into the relationship between strain, OOV, and RP faults are still limited. In this paper, we systematically investigated the effect of strain and RP faults on the formation of OOV in LNO thin films grown on SrTiO 3 (STO) substrates. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM imaging, we conducted atomic-scale structural and compositional analyses of OOV. Geometric phase analysis (GPA) was employed to measure the strain in fault-free and RP fault regions, while density functional theory (DFT) calculations explored different OOV arrangements in the LNO phase. Simulated iDPC-STEM imaging of energy-stabilized structures was performed to correlate with experimental results. Our findings reveal superstructure modulation in the chemical composition and atomic-scale lattice structure in LNO, primarily due to the formation of the OOV in Ni-O layer of LaNiO 2.5 phase. The out-of-plane compressive strain of about 2% stabilizes this phase, reducing the strain, diminishing OOV, and transforming them into LNO. Physical sciences/Nanoscience and technology/Techniques and instrumentation/Microscopy/Transmission electron microscopy Physical sciences/Nanoscience and technology/Nanoscale materials/Structural properties Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction LaNiO 3 (LNO), a member of the rare-earth nickelate family ( RE NiO 3 , where RE is a rare-earth element), stands out due to its unique electronic properties 1 2 3 4 5 . Unlike other rare-earth nickelates, LNO remains metallic across all temperatures, a characteristic that has garnered significant scientific interest. This behavior is intricately linked to the structural dynamics of the NiO 6 octahedra and the size of the rare-earth ion, which influence the material's transition between metallic and insulating states, as well as its shift from paramagnetic to antiferromagnetic phases 6 . The need to understand the behavior of LNO becomes even more compelling in the context of epitaxial thin films, where novel phases can emerge, distinct from those observed in bulk materials 7 . Notably, when LNO is incorporated into superlattices with band-insulator perovskite oxides, sufficiently thin layers exhibit insulating behavior, highlighting the profound impact of dimensional constraints on its electronic properties 8 . Oxygen vacancies represent a prevalent type of defect encountered in transition metal oxides, capable of altering the oxidation state of metal atoms, consequently generating charge carriers that can instigate a transition from a metallic to an insulating state (MIT). Experimental investigations into the conductivity of LaNiO 3 − x have revealed that an elevation in the vacancy concentration, denoted as x reduces conductivity. Notably, the transition from a metallic to an insulating state occurs as x approaches a value of 0.5 9 10 . Furthermore, as the oxygen vacancy level x continues to increase, LaNiO 2.75 is observed to exhibit semiconducting behavior 11 12 13 . Beyond their influence on transport properties, oxygen vacancies also exert substantial effects on the magnetic behavior of materials. While LNO is known to maintain a paramagnetic ( PM ) state across all temperature ranges, the emergence of anti-ferromagnetic ( AFM ) behavior in LNO can be attributed to the presence of minor oxygen vacancies 14 15 . As the oxygen vacancy level increases, LaNiO 2.5 is observed to exhibit AFM properties below 152 K, and LaNiO 2.75 displays a ferromagnetic ( FM ) structure below 225 K 16 . In recent years, researchers have harnessed interfacial strain within thin-film heterostructures to exert control over oxygen vacancies. The biaxial strain has been employed to manipulate both ordered and disordered arrangements of vacancies, as well as octahedral tilts within the LNO system 1718 . Ordered oxygen vacancies (OOV) have been observed within the oxygen-deficient phases of LaNiO 2.5 , primarily along the [110] crystallographic direction. This structural transformation leads to the conversion of NiO6 octahedra into NiO4 square planar units that align parallel to the crystallographic c-axis 19 . The resultant alterations in coordination number, accompanied by changes in electron filling, provide an explanation for the insulating AFM characteristics observed in LaNiO 2.5 5 20 . Liu et al. investigated 1 unit cell thick LNO superlattices grown on STO and LaAlO 3 (LAO) to explore the influence of polar mismatch on their electronic and structural properties 21 . They found that LNO growth on the nonpolar STO surface results in a rough morphology and an unusual 2 + Ni valence state, which is not observed on the polar LAO surface. Tung et al. studied how thickness affects the properties of epitaxial LNO ultrathin films on STO (001) 22 . They found that ultrathin films initially form as LaNiO 2.5 and transition to LNO as thickness increases, driven by polar energetics influencing oxygen vacancy formation and phase stability. L. López-Conesa et al. investigated local superstructure modulation in 35 nm LNO films grown on LAO and LSAT substrates 23 . They discovered that this modulation corresponds to LaNiO 2.5 , a monoclinic oxygen-deficient phase. The study reveals the differing signs of strain conditions, such as out-of-plane tensile strain from the LAO substrate and compressive strain from the LSAT substrate, resulting in two distinct orientations of the monoclinic axes of LaNiO 2.5 . However, detailed atomic-resolution studies of the crystal structure of OOV phases under compressive strain conditions remain limited in the literature. Detemple et al. investigated the origin and atomic structure of Ruddlesden-Popper (RP) faults in LNO film grown on LAO substrate 24 . Bak et al. studied the impact of lattice strain on the formation of RP faults and the critical thickness of a fault-free layer in LNO thin films using LAO, STO, and DyScO 3 (DSO) substrates 25 . They found that strain is effectively managed for STO by forming RP faults without the need for misfit dislocations. In contrast, many misfit dislocations are formed for LAO and DSO substrates to relieve strain. The effect of RP fault in forming OOV in LNO thin film has not been studied. The formation of ordered oxygen vacancy phases has also been observed in ferrite and cobaltite perovskite thin films, with strain manipulation influencing the orientation of these phases 26 27 28 29 30 31 32 . Typically, the atomic-scale structure and compositional characterization of oxygen vacancies and OOV phases are conducted using HAADF-STEM imaging 26 27 28 29 30 33 34 . However, HAADF-STEM imaging has limitations, particularly in visualizing changes in lighter elements like oxygen, which appear invisible unless highly ordered or have reduced contrast due to the collection of high-angle scattered electrons 35 . This makes direct observation of oxygen atomic columns in perovskite thin films and quantification of oxygen vacancies challenging. Integrated differential phase contrast (iDPC) STEM imaging, a phase imaging technique, utilizes atomic electrostatic potential to image the phase directly 36 . The iDPC image contrast is linearly related to the atomic number ( Z ), allowing for better imaging of light elements adjacent to heavy elements with an improved signal-to-noise ratio 37 38 . While iDPC-STEM imaging has been applied to characterize oxygen vacancies in a few perovskite thin films 39 40 41 42 43 , it presents opportunities for more precise quantification and ordering of oxygen vacancies 44 . We systematically investigated the effect of strain and RP fault in forming ordered oxygen vacancy in LNO thin film grown on STO substrate. The atomic-scale structural and compositional analysis of OOV was done using HAADF-STEM imaging and iDPC-STEM imaging. Geometric phase analysis (GPA) was implemented to measure the strain state of the fault-free and near the RP-fault region. Density functional theory (DFT) calculations of the OOV phase LaNiO 2.5 with different arrangements of oxygen vacancies were also conducted. The iDPC-STEM image simulation was done on the energy-stabilized structures to correlate with the experimental results. Our investigation reveals the presence of superstructure modulation in the chemical composition and in the atomic-scale lattice structure of the LNO thin film in the fault-free and RP fault regions, primarily attributable to the formation of ordered oxygen vacant phase LaNiO 2.5 . The out-of-plane compressive strain of about 2% was shown to stabilize the ordered oxygen-vacant LaNiO 2.5 phase; reduction in the strain leads to diminishing OOV and transformation to LNO. 2. Methods 2.1 Thin film growth: LNO films were grown by layer-by-layer laser molecular beam epitaxy (ALL-Laser MBE) on (100)-oriented STO substrates. The ablation was conducted using a KrF excimer laser with 1 Hz repetition rate and 1 J cm − 2 energy density. Samples were grown at a temperature of 650°C in an oxygen pressure of 0.07 mbar and post-growth annealed in about 600 mbar at the growth temperature of 650°C for 30 min. 2.2 TEM sample preparation: A conventional sandwich technique was employed to prepare cross-sectional STEM samples. Initially, the samples underwent mechanical grinding, followed by a dimpling process, and were subsequently thinned to the point of electron transparency using argon ion beam milling of about 0.2 time mean-free path. High-resolution STEM imaging was conducted using a probe-corrected ThermoFisher Spectra 60–300 instrument operating at 300 kV equipped with a cold field emission gun, with a convergence angle of 21 mrad and a scan step size of 8.9 pm. The probe aberration correction is done with a C 3 value of -632 nm and a defocus value of -107 nm. Both HAADF and iDPC-STEM images were acquired simultaneously during the imaging process. The HAADF detector had inner and outer detection angles of 40 and 200 mrad, while the segmented detector utilized 10 to 38 mrad detection angles. The orientation of the sample was aligned with the [010] axis of the LNO thin films, and the scanning direction was set to be perpendicular to the interface. 2.3 DFT calculations: The epitaxially strained LNO was represented using the periodic model approach; the supercell was constructed as the 2×2×4 extension on the cubic perovskite crystallographic cell. The calculations used the VASP package 45 46 and the PBEsol density functional 47 . All calculations were performed in the spin-polarized mode. The projector-augmented wave potentials were used to approximate the effect of the core electrons 48 . The plane-wave basis-set cutoff was set at 500 eV. The Γ-centered 4×4×2 k-mesh was used throughout. The k-mesh density was doubled along the short dimensions of each supercell for the DOS calculations. The total energy convergence criterion was set to 10 − 5 eV. LNO samples with a high oxygen vacancy content were modeled assuming 8 oxygen vacancies per supercell, corresponding to LaNiO2.5. 2.4 Strain analysis: To measure the strain within the film region, the GPA (Geometric Phase Analysis) method developed by Hÿtch and colleagues was employed 49 . Strain mapping was accomplished using HAADF-STEM images. The calculation of strain maps was performed using strain + + software 50 . This analysis utilized two Bragg spots corresponding to the crystallographic directions [001] and [100]. These Bragg spots were employed to calculate two-dimensional symmetric strain components. 2.5 Data Evaluation: The experimental HAADF-STEM and iDPC image analysis was done using Digital Micrograph software (version 3.51.66). The structure file of the LNO and LaNiO 2.5 phase is generated using VESTA software 51 . The simulated diffraction patterns of the LaNiO 2.5 phase are generated using ReciPro software 52 . 2.6 iDPC-STEM simulation: The iDPC-STEM images were simulated using a multislice algorithm with Frozen Phonon approximation to allow quantitative comparison with experimental micrographs. The simulations were executed using ToTEM software 53 , with parameters as of the experiment: 300 kV acceleration voltage, 21 mrad convergence angle, -632 nm Cs, 0.3 eV energy spread (d E ), and − 1.4 nm defocus. LaNiO 2.5 structures derived from DFT calculations were expanded into a 2×2×2 supercell. This supercell was aligned along the [010] viewing direction, with a thickness of approximately 11.76 Å along the electron beam direction. To perform a multislice algorithm, the slice thickness of the LaNiO 2.5 structure was chosen 0.2 Å. The scan areas were 80 × 80 pixels, and the probe array and resolution were kept as 128 × 128 pixels and 0.089 Å, respectively. Collection angles between 10 to 38 mrad were used for iDPC-STEM image simulation. 3. Results Figure 1 (a) shows the HAADF-STEM image of the LNO film epitaxially grown on the STO substrate. The LNO film thickness is around 18 nm (shown in Fig. S1 ). The interface between the LNO and STO substrate is atomically sharp with the cube-on-cube epitaxial relationship of [001] LNO (100) // [001] STO (100). The growth direction of LNO film on STO is [001], and the viewing direction is [010]. The HAADF-STEM image shows that the LNO film has no faults. Figure 1 (b) shows the iDPC-STEM image of the STO-LNO film acquired from the corresponding region of the HAADF-STEM image in Fig. 1 (a). Figure 1 (c) and 1(e) show the magnified cropped image of the blue region in Fig. 1 (a) and the red region in Fig. 1 (b) respectively. The bright atomic columns at the corner and center of the unit cell in the HAADF-STEM image in Fig. 1 (c) represent La and Ni atoms, respectively. The inset represents a schematic of a LNO unit cell viewed along [010] direction with the La atom in green at the corner, the Ni atom in grey at the center, and the oxygen atom in red color at the center on top of the Ni atom and edge of the unit cell respectively. The oxygen atomic columns at the center of the unit cell overlap with the Ni atomic column, and the edges are not visible as O has low scattering potential, which is not effectively collected by HAADF detectors. Conversely, iDPC-STEM imaging is a phase contrast technique sensitive to elements with low and high atomic numbers. The intensity of the atomic columns in the iDPC-STEM image is directly proportional to the atomic number of the elements present, similar to the HAADF-STEM image. The advantage of this method is that the O ( Z = 8) atomic columns at the edge center of the unit cell are clearly visible in Fig. 1 (e), along with the atomic columns of La ( Z = 57) and Ni ( Z = 28). The intensity of the atomic column at the center of the LNO unit cell in the iDPC-STEM image results from the combination of Ni and O atomic columns. The iDPC-STEM image in Fig. 1 (e) shows the compositional modulation in the Ni-O layers in the LNO film. In contrast, no significant compositional modulation is observed in the HAADF-STEM image of the LNO film in Fig. 1 (c). Figure 1 (d) and (f) represent the average intensity profile of the (Ni + O) atomic columns in the Ni-O layer of Fig. 1 (c) and 1(e), respectively. Figure 1 (f) shows significantly better compositional modulation in the average intensity of the (Ni + O) atomic columns in the iDPC-STEM image compared to the HAADF-STEM image in Fig. 1 (d). To investigate the effect of compositional modulation in Ni-O layer on the structure of the LNO film, the in-plane and out-of-plane lattice parameter is measured, indicated by La-La distance. Figure 2 (a) shows the iDPC-STEM image of the red square region in Fig. 1 (b) contains the alternating unit cell with bright and dark Ni-O atomic layers along the out-of-plane [001] direction. Figure 2 (b) and 2(c) show the alternating unit cells' average out-of-plane and in-plane lattice parameters, respectively. It is clear from the plot in Fig. 2 (b) that unit cells with dark Ni-O layers have larger out-of-plane lattice parameters than the unit cells with brighter Ni-O layers. The average lattice parameters of the unit cells with bright and dark Ni-O layers are 4.12 ± 0.03 Å and 3.85 ± 0.03 Å, respectively. The average out-of-plane lattice parameter (d 001 ) of the LNO film is around 3.93 Å, calculated from the (001) Bragg spot from the power spectrum of the iDPC-STEM image in Fig. 2 (a), shown as Fig. 2 (d). The bright and dark superlattice structure repeats itself with a magnitude twice d 001 , reflected in the power spectrum with a superlattice Bragg spot of (00 \(\:\frac{1}{2}\) ). The average in-plane lattice parameter (d 100 ) is approximately 3.96 Å, with minimal deviation, as depicted in Fig. 2 (c). Additionally, no superlattice reflection is observed in the power spectrum along the [100] direction, as shown in Fig. 2 (d). The combined findings from Figs. 1 and 2 suggest that out-of-plane unit cell expansion occurs in the dark Ni-O layers, and the opposite happens in other layers. This phenomenon could be attributed to OOV formation. To quantify the effect of the oxygen vacancy distribution on the structure and composition of the oxygen-deficient phase, two different methods are adopted: 1. Atomic-scale simulation of iDPC-STEM images of the LaNiO 2.5 structures with different ordering of oxygen vacancies along the same zone axis and comparison with the experimental iDPC image. 2. Simulation of the diffraction pattern of oxygen-deficient structure with the experimental power spectrum along the same zone axis. Figure 3 (a) shows the experimental iDPC-STEM image of the 2×2 unit cell of LNO film marked with the green square region in Fig. 2 (a). The green rectangle in Fig. 3 (a) represents the intensity line profile along the out-of-plane direction, and the orange and blue rectangles represent the intensity line profile along the diagonals, respectively. Figure 3 (b) represents the intensity line profile along the out-of-plane direction. The out-of-plane lattice parameters of the unit cell containing a bright Ni-O layer and dark Ni-O layer are 3.7 Å and 4.2 Å, respectively, as shown in Fig. 3 (b). The La-La distance or out-of-plane lattice parameter is observed to be larger in the darker Ni-O layer. The intensity of the La atom is nearly the same, which signifies no compositional change in the La atomic columns. The diagonal intensity line profiles of the 2×2 unit cell of LNO marked with blue and orange rectangles are shown in Fig. 3 (c) and (d), respectively. The Ni and O atomic columns in the lower and upper-unit cells are represented by Ni + O(L) and Ni + O(U), respectively. Ni + O(U) 's intensity is lower than the Ni + O(L) atomic columns in both diagonals and has an average intensity ratio Ni + O(L) / Ni + O(U) of about 1.3. The out-of-plane lattice parameter modulation, lattice expansion in the Ni-O dark layer, and reduction in Ni + O atomic column intensity indicate that ordered oxygen vacancies are present in the LNO film. The ball-and-stick representation of the LaNiO 2.5 configurations with different ordering of oxygen vacancies, C1-C7, respectively, are shown in Fig. S2 (a-g). In each case, the supercell total energy was minimized with respect to the internal coordinates and out-of-plane supercell parameter. Structures C1-C3 contain ordered oxygen vacancies in the La-O layer, and C4-C7 have ordered oxygen vacancies in the Ni-O layer. Then we performed the iDPC-STEM image simulation of all the structures C1-C7 and measured the out-of-plane lattice parameter (La-La distance) and intensity ratio of Ni + O atoms in different Ni-O layers. Figure 3 (e) presents a simulated iDPC-STEM image of LaNiO 2.5 , generated from the unit cell structure C4. The green rectangle represents the intensity line profile along the out-of-plane direction, and the orange and blue rectangles represent the intensity line profile along the diagonals, respectively. Figure 3 (f) shows the intensity profile of La atoms along the out-of-plane direction. In contrast, Figs. 3 (g) and 3(h) display the intensity profiles of La and Ni + O atoms along the diagonals marked by blue and orange rectangles in Fig. 3 (e), respectively. The out-of-plane lattice parameters of the unit cell containing a dark Ni-O layer and bright Ni-O layer of the simulated iDPC image are 3.6 Å and 3.8 Å, respectively, as shown in Fig. 3 (f). The unit cell containing a dark Ni-O layer has a larger out-of-plane lattice parameter than the bright Ni-O layer, as shown in the experimental results in Fig. 3 (b). Also, it is observed that the intensity of Ni + O(U) is lower than the Ni + O(L) atomic columns in both diagonals, as shown in Fig. 3 (g) and (h), respectively. It has an average intensity ratio of about 1.28, which matches well with the average intensity ratio of about 1.3 in the experimental iDPC image in Fig. 3 (c) and (d), respectively. Figure S3(a) and S3(b) illustrate the LaNiO 2.5 structures (C4) with an oxygen vacancy in the Ni-O plane, viewed along the [010] direction and the simulated diffraction pattern of the C4 structure along the [010] direction and contains superlattice reflection at (00 \(\:\frac{1}{2}\) ) like the experimental power spectrum shown in Fig. 2 (d). The out-of-plane lattice parameter measurement, the intensity of the Ni + O atomic columns, and the superlattice reflection in the diffraction pattern of the simulated C4 structure of the LaNiO 2.5 phase match well with the experimental results. The results of the iDPC-STEM image simulation of other LaNiO 2.5 structures C1-C3 and C5-C7 with ordered oxygen vacancy are shown in Fig. S4 and S5, respectively. Figure S4 shows the cropped region of LaNiO 2.5 structures C1-C3 viewed along the [010] direction in panels (a), (d), and (g), respectively. Panels (b), (e), and (h) display simulated iDPC-STEM images of the C1-C3 structures. Panels (c), (f), and (i) present the intensity line profiles of La and Ni + O atoms along the diagonals marked with orange rectangles in Fig. S3(b), (e), and (h), respectively. The simulated structures C1-C3 do not show any modulation in the out-of-plane lattice parameter, and the intensity ratio of Ni + O(L) and Ni + O(U) is about 1.0, which does not match with the experimental results in Fig. 3 (b-d). Figure S5(a), (f), and (k) depict the LaNiO 2.5 structures (C5-C7) viewed along the [010] direction, respectively. Figures S5(b), (g), and (i) show the simulated iDPC-STEM images of LaNiO 2.5 generated from the structures (C5-C7) shown in Figures S5(a), (f), and (k), with intensity line profiles along the out-of-plane and diagonals marked with green, orange, and blue rectangles. Figures S5(c), (h), and (m) present the intensity profiles of La atoms along the out-of-plane direction, while Figures S5(d), (e), (i), (j), (n), and (o) display the intensity profiles of La and Ni + O atoms along the diagonals marked with blue and orange rectangles for structures C5-C7, respectively. The structure C5-C7 shows out-of-plane lattice parameter modulation, but the intensity ratio of Ni + O(L) and Ni + O(U) does not match the experimental results in Fig. 3 (c-d). Therefore, it can be stated that the LNO film contains OOV in the Ni-O layer and has a composition of LaNiO 2.5 . To understand strain's effect on OOV's stability, the strain map across the interface is generated using GPA on the iDPC-STEM image of the fault-free region of LNO, considering STO as a reference. Figure 4 (a) illustrates the cropped region of the iDPC-STEM image of the fault-free region of the LNO film, as shown in Fig. 1 (b). The out-of-plane strain component inset the iDPC-STEM image is represented by e yy . The STO substrate is taken as a reference to measure the strain variation in the LNO film. The color bar represents the strain variation between − 0.15 to 0.15 values. The result shows that e yy component within the fault-free LNO film is compressive, with a maximum strain of about 3.5% and an average strain of about 2.5%. Figure 4 (b) shows the out-of-plane La-La atomic distance within the fault-free region of the STO-LNO film. As the compressive strain increases away from the interface, the modulation in the out-of-plane lattice parameter increases, which signifies that the out-of-plane compressive strain stabilizes the OOV. The iDPC-STEM image in Fig. 4 (c) shows the defective LNO film with a RP fault, 4 unit cells away from the STO-LNO interface, which occurs due to the relative displacement of two perfect perovskite blocks by half a unit cell. Strain component e yy across the interface is shown as an inset. The e yy strain component map shows that the defective LNO film near the RP fault has compressive strain with a maximum value of 2.5%, which reduces to zero along the out-of-plane direction. The defective LNO film with RP fault has a less compressive strain than the fault-free LNO region of 3.5%, as shown in Fig. 4 (b). Figure 4 (d) shows the variation in out-of-plane La-La atomic distance beyond the RP fault. The out-of-plane lattice modulation is observed near the RP fault region, like Fig. 2 (b), which indicates the presence of OOV. Fig. S6 shows the power spectrum of the iDPC-STEM image of LNO film taken from the purple rectangular region in Fig. 4 (c). The power spectrum shows superlattice reflection at (00 \(\:\frac{1}{2}\) ) like Fig. 2 (d). It shows that the film near the RP fault has the compressive strain and OOV LaNiO 2.5 phase. As we move away from the film, the compressive strain diminishes and eventually reaches zero at a thickness of 20 unit cells away from the RP fault. Correspondingly, the modulation in the out-of-plane lattice parameter also decreases and gradually ceases. In comparison, at the same film thickness, the compressive strain and the modulation in the out-of-plane lattice parameter are still present in the fault-free region of LNO, as shown in Fig. 4 (a). 4. Discussion: 4.1. Strain-dependent oxygen vacancy ordering: Studies have shown a significant relationship between the strain and the energy required for oxygen vacancy formation 54 . Due to lattice parameter mismatch between the film and the substrate, the tensile or compressive strain can be introduced in the film 55 , which can influence the ordering of oxygen vacancy in the film, as it is considered a strain relaxation mechanism 54 . 4.1.1. Unstrained conditions: The LNO structure lacks ordered oxygen vacancies, as indicated by the absence of superlattice reflections in the diffraction pattern. Figure S7(a-c) illustrates the unit cell and the simulated diffraction pattern of the LNO phase along the [010] and [001] directions, respectively. 4.1.2. Tensile strain: The substrates, such as LAO, having smaller lattice parameters, introduce out-of-plane tensile strain to the LNO film. Lo´pez-Conesa et al. investigated the presence of monolithic LaNiO 2.5 phase in the LNO thin film grown on LAO substrate 23 . The substrate introduced + 2% tensile strain on the film and ordered oxygen vacancy was observed perpendicular to the interface. The LaNiO 2.5 phase shows vacancy ordering along [110] direction, forming chains of NiO 6 octahedra and NiO 4 square planes aligned when viewed along the [001] zone axis. They also observed the contrast modulation along the [110] direction, which resembles a checkerboard pattern in the HRTEM image with [001] zone axis. Rawat et al. performed a first principle calculation examining how biaxial strain influences the ordering and disordering of oxygen vacancies in pseudocubic LaNiO 2.5 , employing STO as the substrate 56 . They introduced 2% tensile strain on the LaNiO 2.5 supercell to determine the stable structure with different arrangement of oxygen vacancy ordering. They observed that under tensile strain conditions, LaNiO 2.5 featuring NiO 4 square planar units oriented parallel to the c-axis exhibited greater stability, consistent with findings from Lo´pez-Conesa et al. and previous first-principle studies 22 23 . The schematic of the LaNiO 2.5 phase under tensile strain and simulated diffraction patterns along the [010] and [001] direction is shown in fig. S7(d-f). No superlattice reflection is observed in the simulated diffraction pattern when viewed along [010] direction, as shown in Fig. S7(e). The presence of superlattice reflection at ( \(\:\frac{1}{2}\frac{1}{2}\) 0) corresponds to the doubling of the (110) interplanar distances, as shown in Fig. S7(f). The lattice parameters of the LaNiO 2.5 are double those of the pseudocubic LNO unit cell, which are also observed by Moriga et al. 57 from X-ray diffraction analysis and Alonso et al. 58 from high-resolution neutron diffraction experiments. We observed superlattice reflection at (00 \(\:\frac{1}{2}\) ) in the power spectrum along [010] zone axis as shown in Fig. 2 (d). It indicates that the ordered oxygen vacancy in LaNiO 2.5 under tensile strain is different as compared to compressive strain. 4.1.3. Compressive strain: The substrates, such as STO and LSAT, have larger lattice parameters and introduce out-of-plane compressive strain to the LNO film. We observed a compressive strain of about 3.5% of the fault-free region of the LNO film, as shown in Fig. 1 (b). The application of compressive strains resulted in the organization of ordered oxygen vacancies, aligning perpendicularly with respect to the substrate-film interface. The LNO film shows superlattice reflection at (00 \(\:\frac{1}{2}\) ) in the power spectrum along [010] zone axis, as shown in Fig. 2 (d), which does not match with the LaNiO 2.5 structure under tensile strain condition, as shown in Fig. S7(e-f). It indicates that the ordered oxygen vacancy in LaNiO 2.5 under tensile strain is different as compared to compressive strain. The simulated diffraction pattern from the structure C4 with ordered oxygen vacancy in Ni-O layer shows superlattice reflection at (00 \(\:\frac{1}{2}\) ) as shown in Fig. S4(b), which matches well with the experimental power spectrum in Fig. 2 (d). The C4 structure exhibits oxygen vacancy ordering, with NiO₄ square planar units oriented perpendicular to the c-axis (or [001] direction). Rawat et al. observed similar outcomes in their first-principles calculations, applying a 2% compressive strain to the LaNiO 2.5 supercell. 56 . 4.2. Effect of RP fault on the strain and oxygen vacancy ordering: We observed fault-free and RP fault regions in the iDPC-STEM images of the STO-LNO thin film, as shown in Fig. 4 (b) and 4(c), respectively. We observed that the out-of-plane compressive strain in the LNO film with the RP fault was approximately 2.5%, lower than the 3.5% strain measured in the fault-free region, as illustrated in Fig. 4 (b). In LNO perovskite oxides, the formation of RP faults occurs through inserting [LaO]–[LaO] layers, which increases the effective lattice parameter of the fault-containing material compared to fault-free LNO. This insertion of additional crystallographic monolayers leads to an overall expansion of the crystal lattice 25 . In contrast, the lattice parameter of the STO substrate is larger than that of LNO. Consequently, the presence of RP faults reduces the lattice mismatch between the LNO film and the STO substrate, thereby alleviating the misfit strain in the film. Hence, the presence of RP fault reduces the strain compared to the fault-free region and relaxes the LNO film. The LNO film with RP fault still maintains a significant compressive strain of about 2.5%, which promotes the formation of the ordered oxygen-vacant phase LaNiO 2.5 . However, as the distance from the RP fault increases, the compressive strain gradually decreases, leading to a corresponding reduction in the OOV and gradually transforming to the LNO phase. This observation underscores the critical role of compressive strain in driving the ordering of oxygen vacancies in LNO thin films. 5. Conclusion: In this study, we investigated the effect of strain and RP faults on ordering oxygen vacancies in LNO thin films using atomic-scale HAADF-STEM and iDPC-STEM imaging, complemented by iDPC-STEM simulations and first-principles calculations. The iDPC-STEM imaging provided a much clearer depiction of compositional modulation in the Ni-O layer than HAADF-STEM. This modulation, caused by ordering oxygen vacancies within the dark Ni-O layers, resulted in variations in the out-of-plane lattice parameter and the emergence of superlattice reflections at the (00 \(\:\frac{1}{2}\) ) Bragg plane along the [010] zone axis. The simulated LaNiO 2.5 structure, characterized by NiO 4 square planar units aligned perpendicular to the [001] direction obtained from DFT calculations, aligned well with the experimental iDPC-STEM observations. An out-of-plane compressive strain of about 2% was shown to stabilize the ordered oxygen-vacant LaNiO 2.5 phase. The presence of RP faults mitigates the lattice mismatch between the film and the substrate, reducing the misfit strain in the film. As the compressive strain diminishes, leading to a corresponding reduction in the OOV. These findings emphasize the critical role of compressive strain and RP faults in stabilizing the ordered oxygen-vacant phase in LNO thin films. Declarations Competing interests The authors declare no competing financial interest. Author contribution statement Conceptualization: P.B, P.O, and R.C; methodology: thin-film growth by P.O, TEM experiment conducted by A.M, S.L and R.C; formal analysis: P.B, P.O, and R.C; S.R.S. contributed to STEM data interpretation; P.V.S contributed to DFT calculations; All authors contributed to the data interpretation and preparation of the article. Acknowledgment PB acknowledges the receipt of a fellowship from the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. This work was partly authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. The views expressed in the presentation do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains, and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work or allow others to do so for the U.S. Government purposes. Ab initio modeling was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, Synthesis and Processing Science program (FWP 10122). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0028636. Data availability statement The data shown will be available upon request. References 1. Catalan, G. Progress in perovskite nickelate research. Phase Transitions 81 , 729–749 (2008). 2. Zubko, P., Gariglio, S., Gabay, M., Ghosez, P. & Triscone, J.-M. Interface Physics in Complex Oxide Heterostructures. Annu. Rev. Condens. Matter Phys. 2 , 141–165 (2011). 3. Alonso, J. A., Martínez-Lope, M. J., Casais, M. T., Aranda, M. A. G. & Fernández-Díaz, M. T. Metal − Insulator Transitions, Structural and Microstructural Evolution of RNiO 3 (R = Sm, Eu, Gd, Dy, Ho, Y) Perovskites: Evidence for Room-Temperature Charge Disproportionation in Monoclinic HoNiO 3 and YNiO 3. J. Am. Chem. Soc. 121 , 4754–4762 (1999). 4. Park, H., Millis, A. J. & Marianetti, C. A. Site-Selective Mott Transition in Rare-Earth-Element Nickelates. Phys. Rev. Lett. 109 , 156402 (2012). 5. Liao, X., Singh, V. & Park, H. Oxygen vacancy induced site-selective Mott transition in LaNiO3. Phys. Rev. B 103 , 1–10 (2021). 6. Catalano, S. et al. Rare-earth nickelates R NiO 3 : thin films and heterostructures. Reports Prog. Phys. 81 , 046501 (2018). 7. Di Pietro, P. et al. Spectroscopic Evidence of a Dimensionality-Induced Metal-to-Insulator Transition in the Ruddlesden–Popper La n + 1 Ni n O 3 n + 1 Series. ACS Appl. Mater. Interfaces 13 , 6813–6819 (2021). 8. Boris, A. V. et al. Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices. Science (80-. ). 332 , 937–940 (2011). 9. Golalikhani, M. et al. Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films. Nat. Commun. 9 , 2206 (2018). 10. Gunkel, F., Christensen, D. V., Chen, Y. Z. & Pryds, N. Oxygen vacancies: The (in)visible friend of oxide electronics. Appl. Phys. Lett. 116 , (2020). 11. Qiao, L. & Bi, X. Direct observation of Ni 3 + and Ni 2 + in correlated LaNiO 3 − δ films. EPL (Europhysics Lett. 93 , 57002 (2011). 12. Kaneko, D., Yamagishi, K., Tsukada, A., Manabe, T. & Naito, M. Synthesis of infinite-layer LaNiO2 films by metal organic decomposition. Phys. C Supercond. its Appl. (2009) doi:10.1016/j.physc.2009.05.104. 13. Ikeda, A., Manabe, T. & Naito, M. Improved conductivity of infinite-layer LaNiO2 thin films by metal organic decomposition. Phys. C Supercond. 495 , 134–140 (2013). 14. Guo, H. et al. Antiferromagnetic correlations in the metallic strongly correlated transition metal oxide LaNiO3. Nat. Commun. (2018) doi:10.1038/s41467-017-02524-x. 15. Zhang, J., Zheng, H., Ren, Y. & Mitchell, J. F. High-Pressure Floating-Zone Growth of Perovskite Nickelate LaNiO 3 Single Crystals. Cryst. Growth Des. 17 , 2730–2735 (2017). 16. Wang, B.-X. et al. Antiferromagnetic defect structure in LaNiO3 − δ single crystals. Phys. Rev. Mater. 2 , 064404 (2018). 17. McBride, P. M., Janotti, A., Dreyer, C. E., Himmetoglu, B. & Van de Walle, C. G. Effects of biaxial stress and layer thickness on octahedral tilts in LaNiO3. Appl. Phys. Lett. 107 , (2015). 18. Ziese, M., Semmelhack, H. C. & Han, K. H. Strain-induced orbital ordering in thin La0.7Ca0.3MnO3 films on SrTiO3. Phys. Rev. B 68 , 134444 (2003). 19. Sánchez, R. D. et al. Metal-insulator transition in oxygen-deficient LaNiO 3-x perovskites. Phys. Rev. B 54 , 16574–16578 (1996). 20. Misra, D. & Kundu, T. K. Oxygen vacancy induced metal-insulator transition in LaNiO3. Eur. Phys. J. B 89 , 1–8 (2016). 21. Liu, J. et al. Effect of polar discontinuity on the growth of LaNiO3/LaAlO3 superlattices. Appl. Phys. Lett. 96 , (2010). 22. Tung, I.-C. et al. Polarity-driven oxygen vacancy formation in ultrathin LaNiO3 films on SrTiO3. Phys. Rev. Mater. 1 , 053404 (2017). 23. López-Conesa, L. et al. Evidence of a minority monoclinic LaNiO 2.5 phase in lanthanum nickelate thin films. Phys. Chem. Chem. Phys. 19 , 9137–9142 (2017). 24. Detemple, E. et al. Ruddlesden-Popper faults in LaNiO 3/LaAlO 3 superlattices. J. Appl. Phys. (2012) doi:10.1063/1.4731249. 25. Bak, J. & Chung, S. Y. Observation of fault-free coherent layer during Ruddlesden–Popper faults generation in LaNiO3 thin films. J. Korean Ceram. Soc. 58 , 169–177 (2021). 26. Hirai, K. et al. Melting of Oxygen Vacancy Order at Oxide–Heterostructure Interface. ACS Appl. Mater. Interfaces 9 , 30143–30148 (2017). 27. Gazquez, J. et al. Lattice mismatch accommodation via oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3-δ thin films. APL Mater. 1 , 0–7 (2013). 28. Jeen, H. et al. Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge. Nat. Mater. 12 , 1057–1063 (2013). 29. Ivanov, Y. P. et al. Strain-induced structure and oxygen transport interactions in epitaxial La0.6Sr0.4CoO3 − δ thin films. Commun. Mater. (2020) doi:10.1038/s43246-020-0027-0. 30. Khare, A. et al. Directing Oxygen Vacancy Channels in SrFeO 2.5 Epitaxial Thin Films. ACS Appl. Mater. Interfaces 10 , 4831–4837 (2018). 31. Shin, Y., Poeppelmeier, K. R. & Rondinelli, J. M. Informatics-Based Learning of Oxygen Vacancy Ordering Principles in Oxygen-Deficient Perovskites. Inorg. Chem. 63 , 12785–12802 (2024). 32. Jin, L. et al. Understanding Structural Incorporation of Oxygen Vacancies in Perovskite Cobaltite Films and Potential Consequences for Electrocatalysis. Chem. Mater. 34 , 10373–10381 (2022). 33. Yang, Z. et al. Guided anisotropic oxygen transport in vacancy ordered oxides. Nat. Commun. 14 , 1–9 (2023). 34. Yao, L., Inkinen, S., Komsa, H. P. & van Dijken, S. Structural Phase Transitions to 2D and 3D Oxygen Vacancy Patterns in a Perovskite Film Induced by Electrical and Mechanical Nanoprobing. Small 17 , 1–9 (2021). 35. Krivanek, O. L. et al. Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy. Nature (2010) doi:10.1038/nature08879. 36. Bosch, E. G. T. & Lazić, I. Analysis of HR-STEM theory for thin specimen. Ultramicroscopy 156 , 59–72 (2015). 37. Majert, S. & Kohl, H. High-resolution STEM imaging with a quadrant detector-Conditions for differential phase contrast microscopy in the weak phase object approximation. Ultramicroscopy (2015) doi:10.1016/j.ultramic.2014.09.009. 38. Yücelen, E., Lazić, I. & Bosch, E. G. T. Phase contrast scanning transmission electron microscopy imaging of light and heavy atoms at the limit of contrast and resolution. Sci. Rep. 8 , 2676 (2018). 39. Goossens, A. S. et al. Memristive Memory Enhancement by Device Miniaturization for Neuromorphic Computing. Adv. Electron. Mater. (2023) doi:10.1002/aelm.202201111. 40. Nukala, P. et al. In situ heating studies on temperature-induced phase transitions in epitaxial Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 heterostructures. Appl. Phys. Lett. 118 , (2021). 41. Jiang, H. et al. Atomic-resolution characterization on the structure of strontium doped barium titanate nanoparticles. Nano Res. 14 , 4802–4807 (2021). 42. Penn, A. N. et al. Explaining the Magnetic Properties of Oxygen Deficient LSMO Thin Films by iDPC. Microsc. Microanal. 25 , 1748–1749 (2019). 43. Penn, A., Koohfar, S., Kumah, D. & LeBeau, J. Combined iDPC and EELS analyses for quantifying oxygen vacancy concentration in LSMO. Microsc. Microanal. 27 , 1196–1197 (2021). 44. Nukala, P. et al. Reversible oxygen migration and phase transitions in hafnia-based ferroelectric devices. Science (80-. ). (2021) doi:10.1126/science.abf3789. 45. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B - Condens. Matter Mater. Phys. 54 , 11169–11186 (1996). 46. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59 , 1758–1775 (1999). 47. Perdew, J. P. et al. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 100 , 136406 (2008). 48. P.E., B. Projector augmented-wave method. Phys. Rev. B 50 , 17953–17979 (1994). 49. Hÿtch, M. J., Snoeck, E. & Kilaas, R. Quantitative measurement of displacement and strain fields from HREM micrographs. Ultramicroscopy 74 , 131–146 (1998). 50. Peters, J. JJPPeters/Strainpp: v1.8. at https://doi.org/10.5281/zenodo.7422773 (2022). 51. Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 44 , 1272–1276 (2011). 52. Seto, Y. & Ohtsuka, M. ReciPro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools. J. Appl. Crystallogr. (2022) doi:10.1107/S1600576722000139. 53. Yuan, P. J. et al. ToTEM: A software for fast TEM image simulation. J. Microsc. 287 , 93–104 (2022). 54. Aidhy, D. S. & Rawat, K. Coupling between interfacial strain and oxygen vacancies at complex-oxides interfaces. J. Appl. Phys. 129 , (2021). 55. Weber, M. C. et al. Multiple strain-induced phase transitions in LaNiO 3 thin films. Phys. Rev. B 94 , 014118 (2016). 56. Rawat, K., Fong, D. D. & Aidhy, D. S. Breaking atomic-level ordering via biaxial strain in functional oxides: A DFT study. J. Appl. Phys. 129 , 095301 (2021). 57. Moriga, T. et al. Synthesis, Crystal Structure, and Properties of Oxygen-Deficient Lanthanum Nickelate LaNiO 3 − x (0 ≤ x ≤ 0.5). Bull. Chem. Soc. Jpn. 67 , 687–693 (1994). 58. Alonso, J. A., Martínez-Lope, M. J., García-Muñoz, J. L. & Fernández-Díaz, M. T. A structural and magnetic study of the defect perovskite LaNiO2.5 from high-resolution neutron diffraction data. J. Phys. Condens. Matter (1997) doi:10.1088/0953-8984/9/30/010. Additional Declarations There is NO Competing Interest. Supplementary Files supplementaryinformationV5.docx supplementary information Onlinefloatimage5.png graphical abstract Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5883878","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":407502022,"identity":"bb7c79ed-bfb6-48e6-a199-7a9b02dfe72f","order_by":0,"name":"Pritam Banerjee","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+UlEQVRIiWNgGAWjYFACxgYwxcbAkMDAUAFkMTM3ENLSCFbBxgbScgakhZGQFiRrGBjbkARwAXPpw+0PfuYw5PHJNzz+XDmvNpq/HajlR8U2nFos+xIbG3u3MRQDHZYmeXbb8dwZhxkbGHvO3MapxeAM0C+82xgS24BaGBu3HcttAGphZmzDr6XxL0RL8sfGOcdy5xOjpRlqS4JkY0NN7gZCWix7GBtny26TAGpJSJNsOHYgdyNQy0F8fjHnYX/w8e02m8T5zWeSPzbU1OXOO3/44IMfFXgcBqEkgJgnAUgcBnMP4FSP0AIC7CCFdfgUj4JRMApGwQgFAOecW0TG7VndAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-5201-8972","institution":"Technical University of Denmark (DTU)","correspondingAuthor":true,"prefix":"","firstName":"Pritam","middleName":"","lastName":"Banerjee","suffix":""},{"id":407502023,"identity":"2a9db505-08c5-4799-8267-e1ace7ca0c48","order_by":1,"name":"Pasquale Orgiani","email":"","orcid":"https://orcid.org/0000-0002-1082-9651","institution":"CNR-IOM","correspondingAuthor":false,"prefix":"","firstName":"Pasquale","middleName":"","lastName":"Orgiani","suffix":""},{"id":407502024,"identity":"7649a5f9-71d6-4ca3-845f-e368f7517cc4","order_by":2,"name":"Arno Meingast","email":"","orcid":"","institution":"Thermo Fisher Scientific","correspondingAuthor":false,"prefix":"","firstName":"Arno","middleName":"","lastName":"Meingast","suffix":""},{"id":407502025,"identity":"8fcac868-03bc-4cf4-a240-9d5ca2ecb36a","order_by":3,"name":"Sorin Lazar","email":"","orcid":"","institution":"Thermo Fisher Scientific (Netherlands)","correspondingAuthor":false,"prefix":"","firstName":"Sorin","middleName":"","lastName":"Lazar","suffix":""},{"id":407502026,"identity":"e92d86de-b61c-44c2-b137-241a0945e81f","order_by":4,"name":"Peter Sushko","email":"","orcid":"https://orcid.org/0000-0001-7338-4146","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Peter","middleName":"","lastName":"Sushko","suffix":""},{"id":407502027,"identity":"6092c19a-67cd-4e9f-ac1b-fdff4aeacaa6","order_by":5,"name":"Steven Spurgeon","email":"","orcid":"","institution":"National Renewable Energy Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Steven","middleName":"","lastName":"Spurgeon","suffix":""},{"id":407502028,"identity":"c565a099-7a04-4d3d-9d36-5fa41bc33c4f","order_by":6,"name":"Regina Ciancio","email":"","orcid":"","institution":"Area Science Park","correspondingAuthor":false,"prefix":"","firstName":"Regina","middleName":"","lastName":"Ciancio","suffix":""}],"badges":[],"createdAt":"2025-01-22 22:55:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5883878/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5883878/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":75658145,"identity":"b0cb830b-1454-45ee-a6ee-20c0d25be1b8","added_by":"auto","created_at":"2025-02-06 21:29:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":228969,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSTEM imaging of STO-LNO thin film. (a) HAADF-STEM image of the LNO film epitaxially grown on STO substrate. (b) the iDPC-STEM image of the corresponding region of the HAADF-STEM image. (c) The magnified cropped HAADF-STEM image of the LNO film is marked with the blue rectangle in Fig. 1(a); Inset: schematic of LNO unit cell with La atom in green, Ni atom in grey, and oxygen atom in red color. (d) average intensity profile of the Ni atomic columns marked with the yellow arrow in Fig. 1(c). (e) magnified cropped iDPC-STEM image of LNO film marked with the red rectangle in Fig. 1(b) with inset of the schematic of LNO unit cell. (f) average intensity profile of the Ni+O atomic columns marked with the yellow arrow in Fig. 1(e).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/294f13959e7ccae02bc9f3a2.png"},{"id":75657825,"identity":"a8ebf87e-067e-4024-870d-1739381d0bad","added_by":"auto","created_at":"2025-02-06 21:21:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":102186,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) The iDPC-STEM image of the LNO film marked with the red square region in fig. 1(b). (b-c) Plots showing variation in the out-of-plane and in-plane La-La atomic distance, respectively. (d) The power spectrum of the iDPC-STEM image of the LNO film taken from the region in Fig. 2(a).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/ca40b9ff7a24ef06c3a223d5.png"},{"id":75657835,"identity":"2990b646-6bf8-4e89-9b1a-9586825ee89e","added_by":"auto","created_at":"2025-02-06 21:21:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":88040,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) iDPC-STEM image of LNO region taken from fig. 2(a); intensity line profiles regions along out-of-plane and diagonal directions are marked with green, orange, and blue rectangles. (b) Intensity profile of La atoms along the out-of-plane direction, (c, d) intensity profiles of La and Ni+O atoms along diagonals marked with a blue and orange rectangle in Fig. 3(a); Ni and O atomic columns in the lower and upper columns are represented by Ni+O(L) and Ni+O(U) respectively. (e)A simulated iDPC-STEM image of LaNiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2.5\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e was generated from the unit cell structure (C4); intensity line profiles along the out-of-plane and diagonals are marked with green, orange, and blue rectangles. (f) Intensity profile of La atoms along the out-of-plane direction, (g, h) intensity profiles of La and Ni+O atoms along diagonals marked with a blue and orange rectangle in Fig. 3(e).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/d0d57cc41e7688f4a154af76.png"},{"id":75658148,"identity":"24fb8742-75d9-4a74-b620-48cc7f2eae6b","added_by":"auto","created_at":"2025-02-06 21:29:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":109076,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) The iDPC-STEM image of the fault-free region of the LNO film. Inset: the strain component e\u003c/em\u003e\u003csub\u003e\u003cem\u003eyy\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e is shown as an inset. (b) Plots showing variation in out-of-plane La-La atomic distance of fault-free region of the LNO film. (c) The iDPC-STEM image of the defective LNO film with RP fault; the strain component e\u003c/em\u003e\u003csub\u003e\u003cem\u003eyy\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e is shown as an inset. (d) Plots showing variation in out-of-plane La-La atomic distance beyond the RP fault. Inset: The color bar represents the strain variation between -0.15 to 0.15 values.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/1bdfaa9909d587a2b1eb02d0.png"},{"id":76584422,"identity":"927c1348-010e-4da0-a67d-8e16fe2f5a73","added_by":"auto","created_at":"2025-02-18 15:33:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1473659,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/7c4d07e5-6c33-46a3-bcef-cfa47d40721e.pdf"},{"id":75658147,"identity":"6a6a3b2b-8f1b-4c9c-8acd-848b6410e2d9","added_by":"auto","created_at":"2025-02-06 21:29:52","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7977866,"visible":true,"origin":"","legend":"supplementary information","description":"","filename":"supplementaryinformationV5.docx","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/1c786a9f83a308d14df7ab75.docx"},{"id":75657826,"identity":"c956b7b9-e078-477a-ba53-384363890d76","added_by":"auto","created_at":"2025-02-06 21:21:52","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":72940,"visible":true,"origin":"","legend":"\u003cp\u003egraphical abstract\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-5883878/v1/a09c502f4420f7ec4fbcf796.png"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eLaNiO\u003csub\u003e3\u003c/sub\u003e (LNO), a member of the rare-earth nickelate family (\u003cem\u003eRE\u003c/em\u003eNiO\u003csub\u003e3\u003c/sub\u003e, where \u003cem\u003eRE\u003c/em\u003e is a rare-earth element), stands out due to its unique electronic properties \u003csup\u003e1 2 3 4 5\u003c/sup\u003e. Unlike other rare-earth nickelates, LNO remains metallic across all temperatures, a characteristic that has garnered significant scientific interest. This behavior is intricately linked to the structural dynamics of the NiO\u003csub\u003e6\u003c/sub\u003e octahedra and the size of the rare-earth ion, which influence the material's transition between metallic and insulating states, as well as its shift from paramagnetic to antiferromagnetic phases \u003csup\u003e6\u003c/sup\u003e. The need to understand the behavior of LNO becomes even more compelling in the context of epitaxial thin films, where novel phases can emerge, distinct from those observed in bulk materials \u003csup\u003e7\u003c/sup\u003e. Notably, when LNO is incorporated into superlattices with band-insulator perovskite oxides, sufficiently thin layers exhibit insulating behavior, highlighting the profound impact of dimensional constraints on its electronic properties \u003csup\u003e8\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOxygen vacancies represent a prevalent type of defect encountered in transition metal oxides, capable of altering the oxidation state of metal atoms, consequently generating charge carriers that can instigate a transition from a metallic to an insulating state (MIT). Experimental investigations into the conductivity of LaNiO\u003csub\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003e have revealed that an elevation in the vacancy concentration, denoted as x reduces conductivity. Notably, the transition from a metallic to an insulating state occurs as x approaches a value of 0.5 \u003csup\u003e9 10\u003c/sup\u003e. Furthermore, as the oxygen vacancy level x continues to increase, LaNiO\u003csub\u003e2.75\u003c/sub\u003e is observed to exhibit semiconducting behavior \u003csup\u003e11 12 13\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBeyond their influence on transport properties, oxygen vacancies also exert substantial effects on the magnetic behavior of materials. While LNO is known to maintain a paramagnetic (\u003cem\u003ePM\u003c/em\u003e) state across all temperature ranges, the emergence of anti-ferromagnetic (\u003cem\u003eAFM\u003c/em\u003e) behavior in LNO can be attributed to the presence of minor oxygen vacancies \u003csup\u003e14 15\u003c/sup\u003e. As the oxygen vacancy level increases, LaNiO\u003csub\u003e2.5\u003c/sub\u003e is observed to exhibit \u003cem\u003eAFM\u003c/em\u003e properties below 152 K, and LaNiO\u003csub\u003e2.75\u003c/sub\u003e displays a ferromagnetic (\u003cem\u003eFM\u003c/em\u003e) structure below 225 K \u003csup\u003e16\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn recent years, researchers have harnessed interfacial strain within thin-film heterostructures to exert control over oxygen vacancies. The biaxial strain has been employed to manipulate both ordered and disordered arrangements of vacancies, as well as octahedral tilts within the LNO system \u003csup\u003e1718\u003c/sup\u003e. Ordered oxygen vacancies (OOV) have been observed within the oxygen-deficient phases of LaNiO\u003csub\u003e2.5\u003c/sub\u003e, primarily along the [110] crystallographic direction. This structural transformation leads to the conversion of NiO6 octahedra into NiO4 square planar units that align parallel to the crystallographic c-axis \u003csup\u003e19\u003c/sup\u003e. The resultant alterations in coordination number, accompanied by changes in electron filling, provide an explanation for the insulating \u003cem\u003eAFM\u003c/em\u003e characteristics observed in LaNiO\u003csub\u003e2.5\u003c/sub\u003e \u003csup\u003e5 20\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eLiu et al. investigated 1 unit cell thick LNO superlattices grown on STO and LaAlO\u003csub\u003e3\u003c/sub\u003e (LAO) to explore the influence of polar mismatch on their electronic and structural properties\u003csup\u003e21\u003c/sup\u003e. They found that LNO growth on the nonpolar STO surface results in a rough morphology and an unusual 2\u0026thinsp;+\u0026thinsp;Ni valence state, which is not observed on the polar LAO surface. Tung et al. studied how thickness affects the properties of epitaxial LNO ultrathin films on STO (001) \u003csup\u003e22\u003c/sup\u003e. They found that ultrathin films initially form as LaNiO\u003csub\u003e2.5\u003c/sub\u003e and transition to LNO as thickness increases, driven by polar energetics influencing oxygen vacancy formation and phase stability.\u003c/p\u003e \u003cp\u003eL. L\u0026oacute;pez-Conesa et al. investigated local superstructure modulation in 35 nm LNO films grown on LAO and LSAT substrates \u003csup\u003e23\u003c/sup\u003e. They discovered that this modulation corresponds to LaNiO\u003csub\u003e2.5\u003c/sub\u003e, a monoclinic oxygen-deficient phase. The study reveals the differing signs of strain conditions, such as out-of-plane tensile strain from the LAO substrate and compressive strain from the LSAT substrate, resulting in two distinct orientations of the monoclinic axes of LaNiO\u003csub\u003e2.5\u003c/sub\u003e. However, detailed atomic-resolution studies of the crystal structure of OOV phases under compressive strain conditions remain limited in the literature.\u003c/p\u003e \u003cp\u003eDetemple et al. investigated the origin and atomic structure of Ruddlesden-Popper (RP) faults in LNO film grown on LAO substrate \u003csup\u003e24\u003c/sup\u003e. Bak et al. studied the impact of lattice strain on the formation of RP faults and the critical thickness of a fault-free layer in LNO thin films using LAO, STO, and DyScO\u003csub\u003e3\u003c/sub\u003e (DSO) substrates \u003csup\u003e25\u003c/sup\u003e. They found that strain is effectively managed for STO by forming RP faults without the need for misfit dislocations. In contrast, many misfit dislocations are formed for LAO and DSO substrates to relieve strain. The effect of RP fault in forming OOV in LNO thin film has not been studied.\u003c/p\u003e \u003cp\u003eThe formation of ordered oxygen vacancy phases has also been observed in ferrite and cobaltite perovskite thin films, with strain manipulation influencing the orientation of these phases \u003csup\u003e26 27 28 29 30 31 32\u003c/sup\u003e. Typically, the atomic-scale structure and compositional characterization of oxygen vacancies and OOV phases are conducted using HAADF-STEM imaging \u003csup\u003e26 27 28 29 30 33 34\u003c/sup\u003e. However, HAADF-STEM imaging has limitations, particularly in visualizing changes in lighter elements like oxygen, which appear invisible unless highly ordered or have reduced contrast due to the collection of high-angle scattered electrons \u003csup\u003e35\u003c/sup\u003e. This makes direct observation of oxygen atomic columns in perovskite thin films and quantification of oxygen vacancies challenging. Integrated differential phase contrast (iDPC) STEM imaging, a phase imaging technique, utilizes atomic electrostatic potential to image the phase directly \u003csup\u003e36\u003c/sup\u003e. The iDPC image contrast is linearly related to the atomic number (\u003cem\u003eZ\u003c/em\u003e), allowing for better imaging of light elements adjacent to heavy elements with an improved signal-to-noise ratio\u003csup\u003e37 38\u003c/sup\u003e. While iDPC-STEM imaging has been applied to characterize oxygen vacancies in a few perovskite thin films\u003csup\u003e39 40 41 42 43\u003c/sup\u003e, it presents opportunities for more precise quantification and ordering of oxygen vacancies\u003csup\u003e44\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWe systematically investigated the effect of strain and RP fault in forming ordered oxygen vacancy in LNO thin film grown on STO substrate. The atomic-scale structural and compositional analysis of OOV was done using HAADF-STEM imaging and iDPC-STEM imaging. Geometric phase analysis (GPA) was implemented to measure the strain state of the fault-free and near the RP-fault region. Density functional theory (DFT) calculations of the OOV phase LaNiO\u003csub\u003e2.5\u003c/sub\u003e with different arrangements of oxygen vacancies were also conducted. The iDPC-STEM image simulation was done on the energy-stabilized structures to correlate with the experimental results.\u003c/p\u003e \u003cp\u003eOur investigation reveals the presence of superstructure modulation in the chemical composition and in the atomic-scale lattice structure of the LNO thin film in the fault-free and RP fault regions, primarily attributable to the formation of ordered oxygen vacant phase LaNiO\u003csub\u003e2.5\u003c/sub\u003e. The out-of-plane compressive strain of about 2% was shown to stabilize the ordered oxygen-vacant LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase; reduction in the strain leads to diminishing OOV and transformation to LNO.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Thin film growth:\u003c/h2\u003e \u003cp\u003eLNO films were grown by layer-by-layer laser molecular beam epitaxy (ALL-Laser MBE) on (100)-oriented STO substrates. The ablation was conducted using a KrF excimer laser with 1 Hz repetition rate and 1 J cm\u0026thinsp;\u0026minus;\u0026thinsp;2 energy density. Samples were grown at a temperature of 650\u0026deg;C in an oxygen pressure of 0.07 mbar and post-growth annealed in about 600 mbar at the growth temperature of 650\u0026deg;C for 30 min.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 TEM sample preparation:\u003c/h2\u003e \u003cp\u003eA conventional sandwich technique was employed to prepare cross-sectional STEM samples. Initially, the samples underwent mechanical grinding, followed by a dimpling process, and were subsequently thinned to the point of electron transparency using argon ion beam milling of about 0.2 time mean-free path. High-resolution STEM imaging was conducted using a probe-corrected ThermoFisher Spectra 60\u0026ndash;300 instrument operating at 300 kV equipped with a cold field emission gun, with a convergence angle of 21 mrad and a scan step size of 8.9 pm. The probe aberration correction is done with a C\u003csub\u003e3\u003c/sub\u003e value of -632 nm and a defocus value of -107 nm.\u003c/p\u003e \u003cp\u003eBoth HAADF and iDPC-STEM images were acquired simultaneously during the imaging process. The HAADF detector had inner and outer detection angles of 40 and 200 mrad, while the segmented detector utilized 10 to 38 mrad detection angles. The orientation of the sample was aligned with the [010] axis of the LNO thin films, and the scanning direction was set to be perpendicular to the interface.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 DFT calculations:\u003c/h2\u003e \u003cp\u003eThe epitaxially strained LNO was represented using the periodic model approach; the supercell was constructed as the 2\u0026times;2\u0026times;4 extension on the cubic perovskite crystallographic cell. The calculations used the VASP package \u003csup\u003e45 46\u003c/sup\u003e and the PBEsol density functional \u003csup\u003e47\u003c/sup\u003e. All calculations were performed in the spin-polarized mode. The projector-augmented wave potentials were used to approximate the effect of the core electrons \u003csup\u003e48\u003c/sup\u003e. The plane-wave basis-set cutoff was set at 500 eV. The Γ-centered 4\u0026times;4\u0026times;2 k-mesh was used throughout. The k-mesh density was doubled along the short dimensions of each supercell for the DOS calculations. The total energy convergence criterion was set to 10\u0026thinsp;\u0026minus;\u0026thinsp;5 eV. LNO samples with a high oxygen vacancy content were modeled assuming 8 oxygen vacancies per supercell, corresponding to LaNiO2.5.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Strain analysis:\u003c/h2\u003e \u003cp\u003eTo measure the strain within the film region, the GPA (Geometric Phase Analysis) method developed by H\u0026yuml;tch and colleagues was employed \u003csup\u003e49\u003c/sup\u003e. Strain mapping was accomplished using HAADF-STEM images. The calculation of strain maps was performed using strain\u0026thinsp;+\u0026thinsp;+\u0026thinsp;software \u003csup\u003e50\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThis analysis utilized two Bragg spots corresponding to the crystallographic directions [001] and [100]. These Bragg spots were employed to calculate two-dimensional symmetric strain components.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Data Evaluation:\u003c/h2\u003e \u003cp\u003eThe experimental HAADF-STEM and iDPC image analysis was done using Digital Micrograph software (version 3.51.66). The structure file of the LNO and LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase is generated using VESTA software \u003csup\u003e51\u003c/sup\u003e. The simulated diffraction patterns of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase are generated using ReciPro software \u003csup\u003e52\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 iDPC-STEM simulation:\u003c/h2\u003e \u003cp\u003eThe iDPC-STEM images were simulated using a multislice algorithm with Frozen Phonon approximation to allow quantitative comparison with experimental micrographs. The simulations were executed using ToTEM software \u003csup\u003e53\u003c/sup\u003e, with parameters as of the experiment: 300 kV acceleration voltage, 21 mrad convergence angle, -632 nm Cs, 0.3 eV energy spread (d\u003cem\u003eE\u003c/em\u003e), and \u0026minus;\u0026thinsp;1.4 nm defocus. LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures derived from DFT calculations were expanded into a 2\u0026times;2\u0026times;2 supercell. This supercell was aligned along the [010] viewing direction, with a thickness of approximately 11.76 \u0026Aring; along the electron beam direction. To perform a multislice algorithm, the slice thickness of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e structure was chosen 0.2 \u0026Aring;. The scan areas were 80 \u0026times; 80 pixels, and the probe array and resolution were kept as 128 \u0026times; 128 pixels and 0.089 \u0026Aring;, respectively. Collection angles between 10 to 38 mrad were used for iDPC-STEM image simulation.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a) shows the HAADF-STEM image of the LNO film epitaxially grown on the STO substrate. The LNO film thickness is around 18 nm (shown in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The interface between the LNO and STO substrate is atomically sharp with the cube-on-cube epitaxial relationship of [001] LNO (100) // [001] STO (100). The growth direction of LNO film on STO is [001], and the viewing direction is [010]. The HAADF-STEM image shows that the LNO film has no faults. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) shows the iDPC-STEM image of the STO-LNO film acquired from the corresponding region of the HAADF-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c) and 1(e) show the magnified cropped image of the blue region in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) and the red region in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) respectively. The bright atomic columns at the corner and center of the unit cell in the HAADF-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c) represent La and Ni atoms, respectively. The inset represents a schematic of a LNO unit cell viewed along [010] direction with the La atom in green at the corner, the Ni atom in grey at the center, and the oxygen atom in red color at the center on top of the Ni atom and edge of the unit cell respectively.\u003c/p\u003e \u003cp\u003eThe oxygen atomic columns at the center of the unit cell overlap with the Ni atomic column, and the edges are not visible as O has low scattering potential, which is not effectively collected by HAADF detectors. Conversely, iDPC-STEM imaging is a phase contrast technique sensitive to elements with low and high atomic numbers. The intensity of the atomic columns in the iDPC-STEM image is directly proportional to the atomic number of the elements present, similar to the HAADF-STEM image. The advantage of this method is that the O (\u003cem\u003eZ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;8) atomic columns at the edge center of the unit cell are clearly visible in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(e), along with the atomic columns of La (\u003cem\u003eZ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;57) and Ni (\u003cem\u003eZ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;28). The intensity of the atomic column at the center of the LNO unit cell in the iDPC-STEM image results from the combination of Ni and O atomic columns. The iDPC-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(e) shows the compositional modulation in the Ni-O layers in the LNO film. In contrast, no significant compositional modulation is observed in the HAADF-STEM image of the LNO film in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (d) and (f) represent the average intensity profile of the (Ni\u0026thinsp;+\u0026thinsp;O) atomic columns in the Ni-O layer of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c) and 1(e), respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(f) shows significantly better compositional modulation in the average intensity of the (Ni\u0026thinsp;+\u0026thinsp;O) atomic columns in the iDPC-STEM image compared to the HAADF-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(d).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the effect of compositional modulation in Ni-O layer on the structure of the LNO film, the in-plane and out-of-plane lattice parameter is measured, indicated by La-La distance. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows the iDPC-STEM image of the red square region in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) contains the alternating unit cell with bright and dark Ni-O atomic layers along the out-of-plane [001] direction. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) and 2(c) show the alternating unit cells' average out-of-plane and in-plane lattice parameters, respectively. It is clear from the plot in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) that unit cells with dark Ni-O layers have larger out-of-plane lattice parameters than the unit cells with brighter Ni-O layers. The average lattice parameters of the unit cells with bright and dark Ni-O layers are 4.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 \u0026Aring; and 3.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 \u0026Aring;, respectively. The average out-of-plane lattice parameter (d\u003csub\u003e001\u003c/sub\u003e) of the LNO film is around 3.93 \u0026Aring;, calculated from the (001) Bragg spot from the power spectrum of the iDPC-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a), shown as Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). The bright and dark superlattice structure repeats itself with a magnitude twice d\u003csub\u003e001\u003c/sub\u003e, reflected in the power spectrum with a superlattice Bragg spot of (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e). The average in-plane lattice parameter (d\u003csub\u003e100\u003c/sub\u003e) is approximately 3.96 \u0026Aring;, with minimal deviation, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c). Additionally, no superlattice reflection is observed in the power spectrum along the [100] direction, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). The combined findings from Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e suggest that out-of-plane unit cell expansion occurs in the dark Ni-O layers, and the opposite happens in other layers. This phenomenon could be attributed to OOV formation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo quantify the effect of the oxygen vacancy distribution on the structure and composition of the oxygen-deficient phase, two different methods are adopted: 1. Atomic-scale simulation of iDPC-STEM images of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures with different ordering of oxygen vacancies along the same zone axis and comparison with the experimental iDPC image. 2. Simulation of the diffraction pattern of oxygen-deficient structure with the experimental power spectrum along the same zone axis.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows the experimental iDPC-STEM image of the 2\u0026times;2 unit cell of LNO film marked with the green square region in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). The green rectangle in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) represents the intensity line profile along the out-of-plane direction, and the orange and blue rectangles represent the intensity line profile along the diagonals, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) represents the intensity line profile along the out-of-plane direction. The out-of-plane lattice parameters of the unit cell containing a bright Ni-O layer and dark Ni-O layer are 3.7 \u0026Aring; and 4.2 \u0026Aring;, respectively, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). The La-La distance or out-of-plane lattice parameter is observed to be larger in the darker Ni-O layer. The intensity of the La atom is nearly the same, which signifies no compositional change in the La atomic columns. The diagonal intensity line profiles of the 2\u0026times;2 unit cell of LNO marked with blue and orange rectangles are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c) and (d), respectively. The Ni and O atomic columns in the lower and upper-unit cells are represented by Ni\u0026thinsp;+\u0026thinsp;O(L) and Ni\u0026thinsp;+\u0026thinsp;O(U), respectively. Ni\u0026thinsp;+\u0026thinsp;O(U) 's intensity is lower than the Ni\u0026thinsp;+\u0026thinsp;O(L) atomic columns in both diagonals and has an average intensity ratio Ni\u0026thinsp;+\u0026thinsp;O(L) / Ni\u0026thinsp;+\u0026thinsp;O(U) of about 1.3. The out-of-plane lattice parameter modulation, lattice expansion in the Ni-O dark layer, and reduction in Ni\u0026thinsp;+\u0026thinsp;O atomic column intensity indicate that ordered oxygen vacancies are present in the LNO film.\u003c/p\u003e \u003cp\u003eThe ball-and-stick representation of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e configurations with different ordering of oxygen vacancies, C1-C7, respectively, are shown in Fig. S2 (a-g). In each case, the supercell total energy was minimized with respect to the internal coordinates and out-of-plane supercell parameter. Structures C1-C3 contain ordered oxygen vacancies in the La-O layer, and C4-C7 have ordered oxygen vacancies in the Ni-O layer. Then we performed the iDPC-STEM image simulation of all the structures C1-C7 and measured the out-of-plane lattice parameter (La-La distance) and intensity ratio of Ni\u0026thinsp;+\u0026thinsp;O atoms in different Ni-O layers.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(e) presents a simulated iDPC-STEM image of LaNiO\u003csub\u003e2.5\u003c/sub\u003e, generated from the unit cell structure C4. The green rectangle represents the intensity line profile along the out-of-plane direction, and the orange and blue rectangles represent the intensity line profile along the diagonals, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(f) shows the intensity profile of La atoms along the out-of-plane direction. In contrast, Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(g) and 3(h) display the intensity profiles of La and Ni\u0026thinsp;+\u0026thinsp;O atoms along the diagonals marked by blue and orange rectangles in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(e), respectively. The out-of-plane lattice parameters of the unit cell containing a dark Ni-O layer and bright Ni-O layer of the simulated iDPC image are 3.6 \u0026Aring; and 3.8 \u0026Aring;, respectively, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(f). The unit cell containing a dark Ni-O layer has a larger out-of-plane lattice parameter than the bright Ni-O layer, as shown in the experimental results in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). Also, it is observed that the intensity of Ni\u0026thinsp;+\u0026thinsp;O(U) is lower than the Ni\u0026thinsp;+\u0026thinsp;O(L) atomic columns in both diagonals, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(g) and (h), respectively. It has an average intensity ratio of about 1.28, which matches well with the average intensity ratio of about 1.3 in the experimental iDPC image in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c) and (d), respectively. Figure S3(a) and S3(b) illustrate the LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures (C4) with an oxygen vacancy in the Ni-O plane, viewed along the [010] direction and the simulated diffraction pattern of the C4 structure along the [010] direction and contains superlattice reflection at (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) like the experimental power spectrum shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). The out-of-plane lattice parameter measurement, the intensity of the Ni\u0026thinsp;+\u0026thinsp;O atomic columns, and the superlattice reflection in the diffraction pattern of the simulated C4 structure of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase match well with the experimental results.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe results of the iDPC-STEM image simulation of other LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures C1-C3 and C5-C7 with ordered oxygen vacancy are shown in Fig. S4 and S5, respectively. Figure S4 shows the cropped region of LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures C1-C3 viewed along the [010] direction in panels (a), (d), and (g), respectively. Panels (b), (e), and (h) display simulated iDPC-STEM images of the C1-C3 structures. Panels (c), (f), and (i) present the intensity line profiles of La and Ni\u0026thinsp;+\u0026thinsp;O atoms along the diagonals marked with orange rectangles in Fig. S3(b), (e), and (h), respectively. The simulated structures C1-C3 do not show any modulation in the out-of-plane lattice parameter, and the intensity ratio of Ni\u0026thinsp;+\u0026thinsp;O(L) and Ni\u0026thinsp;+\u0026thinsp;O(U) is about 1.0, which does not match with the experimental results in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b-d).\u003c/p\u003e \u003cp\u003eFigure S5(a), (f), and (k) depict the LaNiO\u003csub\u003e2.5\u003c/sub\u003e structures (C5-C7) viewed along the [010] direction, respectively. Figures S5(b), (g), and (i) show the simulated iDPC-STEM images of LaNiO\u003csub\u003e2.5\u003c/sub\u003e generated from the structures (C5-C7) shown in Figures S5(a), (f), and (k), with intensity line profiles along the out-of-plane and diagonals marked with green, orange, and blue rectangles. Figures S5(c), (h), and (m) present the intensity profiles of La atoms along the out-of-plane direction, while Figures S5(d), (e), (i), (j), (n), and (o) display the intensity profiles of La and Ni\u0026thinsp;+\u0026thinsp;O atoms along the diagonals marked with blue and orange rectangles for structures C5-C7, respectively. The structure C5-C7 shows out-of-plane lattice parameter modulation, but the intensity ratio of Ni\u0026thinsp;+\u0026thinsp;O(L) and Ni\u0026thinsp;+\u0026thinsp;O(U) does not match the experimental results in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c-d). Therefore, it can be stated that the LNO film contains OOV in the Ni-O layer and has a composition of LaNiO\u003csub\u003e2.5\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo understand strain's effect on OOV's stability, the strain map across the interface is generated using GPA on the iDPC-STEM image of the fault-free region of LNO, considering STO as a reference. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) illustrates the cropped region of the iDPC-STEM image of the fault-free region of the LNO film, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The out-of-plane strain component inset the iDPC-STEM image is represented by e\u003csub\u003eyy\u003c/sub\u003e. The STO substrate is taken as a reference to measure the strain variation in the LNO film. The color bar represents the strain variation between \u0026minus;\u0026thinsp;0.15 to 0.15 values. The result shows that e\u003csub\u003eyy\u003c/sub\u003e component within the fault-free LNO film is compressive, with a maximum strain of about 3.5% and an average strain of about 2.5%. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) shows the out-of-plane La-La atomic distance within the fault-free region of the STO-LNO film. As the compressive strain increases away from the interface, the modulation in the out-of-plane lattice parameter increases, which signifies that the out-of-plane compressive strain stabilizes the OOV.\u003c/p\u003e \u003cp\u003eThe iDPC-STEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c) shows the defective LNO film with a RP fault, 4 unit cells away from the STO-LNO interface, which occurs due to the relative displacement of two perfect perovskite blocks by half a unit cell. Strain component e\u003csub\u003eyy\u003c/sub\u003e across the interface is shown as an inset. The e\u003csub\u003eyy\u003c/sub\u003e strain component map shows that the defective LNO film near the RP fault has compressive strain with a maximum value of 2.5%, which reduces to zero along the out-of-plane direction. The defective LNO film with RP fault has a less compressive strain than the fault-free LNO region of 3.5%, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) shows the variation in out-of-plane La-La atomic distance beyond the RP fault. The out-of-plane lattice modulation is observed near the RP fault region, like Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), which indicates the presence of OOV. Fig. S6 shows the power spectrum of the iDPC-STEM image of LNO film taken from the purple rectangular region in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c). The power spectrum shows superlattice reflection at (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) like Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). It shows that the film near the RP fault has the compressive strain and OOV LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase. As we move away from the film, the compressive strain diminishes and eventually reaches zero at a thickness of 20 unit cells away from the RP fault. Correspondingly, the modulation in the out-of-plane lattice parameter also decreases and gradually ceases. In comparison, at the same film thickness, the compressive strain and the modulation in the out-of-plane lattice parameter are still present in the fault-free region of LNO, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a).\u003c/p\u003e"},{"header":"4. Discussion:","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Strain-dependent oxygen vacancy ordering:\u003c/h2\u003e \u003cp\u003eStudies have shown a significant relationship between the strain and the energy required for oxygen vacancy formation \u003csup\u003e54\u003c/sup\u003e. Due to lattice parameter mismatch between the film and the substrate, the tensile or compressive strain can be introduced in the film \u003csup\u003e55\u003c/sup\u003e, which can influence the ordering of oxygen vacancy in the film, as it is considered a strain relaxation mechanism \u003csup\u003e54\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cspan\u003e \u003cp\u003e4.1.1. Unstrained conditions: The LNO structure lacks ordered oxygen vacancies, as indicated by the absence of superlattice reflections in the diffraction pattern. Figure S7(a-c) illustrates the unit cell and the simulated diffraction pattern of the LNO phase along the [010] and [001] directions, respectively.\u003c/p\u003e \u003c/span\u003e \u003cspan\u003e \u003cp\u003e4.1.2. Tensile strain: The substrates, such as LAO, having smaller lattice parameters, introduce out-of-plane tensile strain to the LNO film. Lo\u0026acute;pez-Conesa et al. investigated the presence of monolithic LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase in the LNO thin film grown on LAO substrate \u003csup\u003e23\u003c/sup\u003e. The substrate introduced\u0026thinsp;+\u0026thinsp;2% tensile strain on the film and ordered oxygen vacancy was observed perpendicular to the interface. The LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase shows vacancy ordering along [110] direction, forming chains of NiO\u003csub\u003e6\u003c/sub\u003e octahedra and NiO\u003csub\u003e4\u003c/sub\u003e square planes aligned when viewed along the [001] zone axis. They also observed the contrast modulation along the [110] direction, which resembles a checkerboard pattern in the HRTEM image with [001] zone axis.\u003c/p\u003e \u003c/span\u003e \u003c/p\u003e \u003cp\u003eRawat et al. performed a first principle calculation examining how biaxial strain influences the ordering and disordering of oxygen vacancies in pseudocubic LaNiO\u003csub\u003e2.5\u003c/sub\u003e, employing STO as the substrate \u003csup\u003e56\u003c/sup\u003e. They introduced 2% tensile strain on the LaNiO\u003csub\u003e2.5\u003c/sub\u003e supercell to determine the stable structure with different arrangement of oxygen vacancy ordering. They observed that under tensile strain conditions, LaNiO\u003csub\u003e2.5\u003c/sub\u003e featuring NiO\u003csub\u003e4\u003c/sub\u003e square planar units oriented parallel to the c-axis exhibited greater stability, consistent with findings from Lo\u0026acute;pez-Conesa et al. and previous first-principle studies \u003csup\u003e22 23\u003c/sup\u003e. The schematic of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase under tensile strain and simulated diffraction patterns along the [010] and [001] direction is shown in fig. S7(d-f). No superlattice reflection is observed in the simulated diffraction pattern when viewed along [010] direction, as shown in Fig. S7(e). The presence of superlattice reflection at (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e0) corresponds to the doubling of the (110) interplanar distances, as shown in Fig. S7(f). The lattice parameters of the LaNiO\u003csub\u003e2.5\u003c/sub\u003e are double those of the pseudocubic LNO unit cell, which are also observed by Moriga et al. \u003csup\u003e57\u003c/sup\u003e from X-ray diffraction analysis and Alonso et al. \u003csup\u003e58\u003c/sup\u003e from high-resolution neutron diffraction experiments.\u003c/p\u003e \u003cp\u003eWe observed superlattice reflection at (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) in the power spectrum along [010] zone axis as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). It indicates that the ordered oxygen vacancy in LaNiO\u003csub\u003e2.5\u003c/sub\u003e under tensile strain is different as compared to compressive strain.\u003c/p\u003e \u003cp\u003e4.1.3. Compressive strain: The substrates, such as STO and LSAT, have larger lattice parameters and introduce out-of-plane compressive strain to the LNO film. We observed a compressive strain of about 3.5% of the fault-free region of the LNO film, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The application of compressive strains resulted in the organization of ordered oxygen vacancies, aligning perpendicularly with respect to the substrate-film interface. The LNO film shows superlattice reflection at (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) in the power spectrum along [010] zone axis, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d), which does not match with the LaNiO\u003csub\u003e2.5\u003c/sub\u003e structure under tensile strain condition, as shown in Fig. S7(e-f). It indicates that the ordered oxygen vacancy in LaNiO\u003csub\u003e2.5\u003c/sub\u003e under tensile strain is different as compared to compressive strain. The simulated diffraction pattern from the structure C4 with ordered oxygen vacancy in Ni-O layer shows superlattice reflection at (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) as shown in Fig. S4(b), which matches well with the experimental power spectrum in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). The C4 structure exhibits oxygen vacancy ordering, with NiO₄ square planar units oriented perpendicular to the c-axis (or [001] direction). Rawat et al. observed similar outcomes in their first-principles calculations, applying a 2% compressive strain to the LaNiO\u003csub\u003e2.5\u003c/sub\u003e supercell.\u003csup\u003e56\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Effect of RP fault on the strain and oxygen vacancy ordering:\u003c/h2\u003e \u003cp\u003eWe observed fault-free and RP fault regions in the iDPC-STEM images of the STO-LNO thin film, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) and 4(c), respectively. We observed that the out-of-plane compressive strain in the LNO film with the RP fault was approximately 2.5%, lower than the 3.5% strain measured in the fault-free region, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b).\u003c/p\u003e \u003cp\u003eIn LNO perovskite oxides, the formation of RP faults occurs through inserting [LaO]\u0026ndash;[LaO] layers, which increases the effective lattice parameter of the fault-containing material compared to fault-free LNO. This insertion of additional crystallographic monolayers leads to an overall expansion of the crystal lattice \u003csup\u003e25\u003c/sup\u003e. In contrast, the lattice parameter of the STO substrate is larger than that of LNO. Consequently, the presence of RP faults reduces the lattice mismatch between the LNO film and the STO substrate, thereby alleviating the misfit strain in the film. Hence, the presence of RP fault reduces the strain compared to the fault-free region and relaxes the LNO film.\u003c/p\u003e \u003cp\u003eThe LNO film with RP fault still maintains a significant compressive strain of about 2.5%, which promotes the formation of the ordered oxygen-vacant phase LaNiO\u003csub\u003e2.5\u003c/sub\u003e. However, as the distance from the RP fault increases, the compressive strain gradually decreases, leading to a corresponding reduction in the OOV and gradually transforming to the LNO phase. This observation underscores the critical role of compressive strain in driving the ordering of oxygen vacancies in LNO thin films.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusion:","content":"\u003cp\u003eIn this study, we investigated the effect of strain and RP faults on ordering oxygen vacancies in LNO thin films using atomic-scale HAADF-STEM and iDPC-STEM imaging, complemented by iDPC-STEM simulations and first-principles calculations. The iDPC-STEM imaging provided a much clearer depiction of compositional modulation in the Ni-O layer than HAADF-STEM. This modulation, caused by ordering oxygen vacancies within the dark Ni-O layers, resulted in variations in the out-of-plane lattice parameter and the emergence of superlattice reflections at the (00\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{2}\\)\u003c/span\u003e\u003c/span\u003e) Bragg plane along the [010] zone axis.\u003c/p\u003e \u003cp\u003eThe simulated LaNiO\u003csub\u003e2.5\u003c/sub\u003e structure, characterized by NiO\u003csub\u003e4\u003c/sub\u003e square planar units aligned perpendicular to the [001] direction obtained from DFT calculations, aligned well with the experimental iDPC-STEM observations. An out-of-plane compressive strain of about 2% was shown to stabilize the ordered oxygen-vacant LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase. The presence of RP faults mitigates the lattice mismatch between the film and the substrate, reducing the misfit strain in the film. As the compressive strain diminishes, leading to a corresponding reduction in the OOV. These findings emphasize the critical role of compressive strain and RP faults in stabilizing the ordered oxygen-vacant phase in LNO thin films.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eAuthor contribution statement\u003c/h2\u003e \u003cp\u003eConceptualization: P.B, P.O, and R.C; methodology: thin-film growth by P.O, TEM experiment conducted by A.M, S.L and R.C; formal analysis: P.B, P.O, and R.C; S.R.S. contributed to STEM data interpretation; P.V.S contributed to DFT calculations; All authors contributed to the data interpretation and preparation of the article.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003ePB acknowledges the receipt of a fellowship from the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. This work was partly authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. The views expressed in the presentation do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains, and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work or allow others to do so for the U.S. Government purposes. Ab initio modeling was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, Synthesis and Processing Science program (FWP 10122). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0028636.\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eThe data shown will be available upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e1. Catalan, G. Progress in perovskite nickelate research. \u003cem\u003ePhase Transitions\u003c/em\u003e \u003cb\u003e81\u003c/b\u003e, 729\u0026ndash;749 (2008).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e2. Zubko, P., Gariglio, S., Gabay, M., Ghosez, P. \u0026amp; Triscone, J.-M. Interface Physics in Complex Oxide Heterostructures. \u003cem\u003eAnnu. Rev. Condens. Matter Phys.\u003c/em\u003e \u003cb\u003e2\u003c/b\u003e, 141\u0026ndash;165 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e3. Alonso, J. A., Mart\u0026iacute;nez-Lope, M. J., Casais, M. T., Aranda, M. A. G. \u0026amp; Fern\u0026aacute;ndez-D\u0026iacute;az, M. T. Metal\u0026thinsp;\u0026minus;\u0026thinsp;Insulator Transitions, Structural and Microstructural Evolution of RNiO 3 (R\u0026thinsp;=\u0026thinsp;Sm, Eu, Gd, Dy, Ho, Y) Perovskites: Evidence for Room-Temperature Charge Disproportionation in Monoclinic HoNiO 3 and YNiO 3. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cb\u003e121\u003c/b\u003e, 4754\u0026ndash;4762 (1999).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e4. Park, H., Millis, A. J. \u0026amp; Marianetti, C. A. Site-Selective Mott Transition in Rare-Earth-Element Nickelates. \u003cem\u003ePhys. Rev. Lett.\u003c/em\u003e \u003cb\u003e109\u003c/b\u003e, 156402 (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e5. Liao, X., Singh, V. \u0026amp; Park, H. Oxygen vacancy induced site-selective Mott transition in LaNiO3. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e103\u003c/b\u003e, 1\u0026ndash;10 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e6. Catalano, S. \u003cem\u003eet al.\u003c/em\u003e Rare-earth nickelates R NiO 3 : thin films and heterostructures. \u003cem\u003eReports Prog. Phys.\u003c/em\u003e \u003cb\u003e81\u003c/b\u003e, 046501 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e7. Di Pietro, P. \u003cem\u003eet al.\u003c/em\u003e Spectroscopic Evidence of a Dimensionality-Induced Metal-to-Insulator Transition in the Ruddlesden\u0026ndash;Popper La n\u0026thinsp;+\u0026thinsp;1 Ni n O 3 n\u0026thinsp;+\u0026thinsp;1 Series. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e, 6813\u0026ndash;6819 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e8. Boris, A. V. \u003cem\u003eet al.\u003c/em\u003e Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices. \u003cem\u003eScience (80-. ).\u003c/em\u003e \u003cb\u003e332\u003c/b\u003e, 937\u0026ndash;940 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e9. Golalikhani, M. \u003cem\u003eet al.\u003c/em\u003e Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cb\u003e9\u003c/b\u003e, 2206 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e10. Gunkel, F., Christensen, D. V., Chen, Y. Z. \u0026amp; Pryds, N. Oxygen vacancies: The (in)visible friend of oxide electronics. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cb\u003e116\u003c/b\u003e, (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e11. Qiao, L. \u0026amp; Bi, X. Direct observation of Ni 3\u0026thinsp;+\u0026thinsp;and Ni 2\u0026thinsp;+\u0026thinsp;in correlated LaNiO 3\u0026thinsp;\u0026minus;\u0026thinsp;δ films. \u003cem\u003eEPL (Europhysics Lett.\u003c/em\u003e \u003cb\u003e93\u003c/b\u003e, 57002 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e12. Kaneko, D., Yamagishi, K., Tsukada, A., Manabe, T. \u0026amp; Naito, M. Synthesis of infinite-layer LaNiO2 films by metal organic decomposition. \u003cem\u003ePhys. C Supercond. its Appl.\u003c/em\u003e (2009) doi:10.1016/j.physc.2009.05.104.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e13. Ikeda, A., Manabe, T. \u0026amp; Naito, M. Improved conductivity of infinite-layer LaNiO2 thin films by metal organic decomposition. \u003cem\u003ePhys. C Supercond.\u003c/em\u003e \u003cb\u003e495\u003c/b\u003e, 134\u0026ndash;140 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e14. Guo, H. \u003cem\u003eet al.\u003c/em\u003e Antiferromagnetic correlations in the metallic strongly correlated transition metal oxide LaNiO3. \u003cem\u003eNat. Commun.\u003c/em\u003e (2018) doi:10.1038/s41467-017-02524-x.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e15. Zhang, J., Zheng, H., Ren, Y. \u0026amp; Mitchell, J. F. High-Pressure Floating-Zone Growth of Perovskite Nickelate LaNiO 3 Single Crystals. \u003cem\u003eCryst. Growth Des.\u003c/em\u003e \u003cb\u003e17\u003c/b\u003e, 2730\u0026ndash;2735 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e16. Wang, B.-X. \u003cem\u003eet al.\u003c/em\u003e Antiferromagnetic defect structure in LaNiO3\u0026thinsp;\u0026minus;\u0026thinsp;δ single crystals. \u003cem\u003ePhys. Rev. Mater.\u003c/em\u003e \u003cb\u003e2\u003c/b\u003e, 064404 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e17. McBride, P. M., Janotti, A., Dreyer, C. E., Himmetoglu, B. \u0026amp; Van de Walle, C. G. Effects of biaxial stress and layer thickness on octahedral tilts in LaNiO3. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cb\u003e107\u003c/b\u003e, (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e18. Ziese, M., Semmelhack, H. C. \u0026amp; Han, K. H. Strain-induced orbital ordering in thin La0.7Ca0.3MnO3 films on SrTiO3. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e68\u003c/b\u003e, 134444 (2003).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e19. S\u0026aacute;nchez, R. D. \u003cem\u003eet al.\u003c/em\u003e Metal-insulator transition in oxygen-deficient LaNiO 3-x perovskites. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e54\u003c/b\u003e, 16574\u0026ndash;16578 (1996).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e20. Misra, D. \u0026amp; Kundu, T. K. Oxygen vacancy induced metal-insulator transition in LaNiO3. \u003cem\u003eEur. Phys. J. B\u003c/em\u003e \u003cb\u003e89\u003c/b\u003e, 1\u0026ndash;8 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e21. Liu, J. \u003cem\u003eet al.\u003c/em\u003e Effect of polar discontinuity on the growth of LaNiO3/LaAlO3 superlattices. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cb\u003e96\u003c/b\u003e, (2010).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e22. Tung, I.-C. \u003cem\u003eet al.\u003c/em\u003e Polarity-driven oxygen vacancy formation in ultrathin LaNiO3 films on SrTiO3. \u003cem\u003ePhys. Rev. Mater.\u003c/em\u003e \u003cb\u003e1\u003c/b\u003e, 053404 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e23. L\u0026oacute;pez-Conesa, L. \u003cem\u003eet al.\u003c/em\u003e Evidence of a minority monoclinic LaNiO 2.5 phase in lanthanum nickelate thin films. \u003cem\u003ePhys. Chem. Chem. Phys.\u003c/em\u003e \u003cb\u003e19\u003c/b\u003e, 9137\u0026ndash;9142 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e24. Detemple, E. \u003cem\u003eet al.\u003c/em\u003e Ruddlesden-Popper faults in LaNiO 3/LaAlO 3 superlattices. \u003cem\u003eJ. Appl. Phys.\u003c/em\u003e (2012) doi:10.1063/1.4731249.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e25. Bak, J. \u0026amp; Chung, S. Y. Observation of fault-free coherent layer during Ruddlesden\u0026ndash;Popper faults generation in LaNiO3 thin films. \u003cem\u003eJ. Korean Ceram. Soc.\u003c/em\u003e \u003cb\u003e58\u003c/b\u003e, 169\u0026ndash;177 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e26. Hirai, K. \u003cem\u003eet al.\u003c/em\u003e Melting of Oxygen Vacancy Order at Oxide\u0026ndash;Heterostructure Interface. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cb\u003e9\u003c/b\u003e, 30143\u0026ndash;30148 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e27. Gazquez, J. \u003cem\u003eet al.\u003c/em\u003e Lattice mismatch accommodation via oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3-δ thin films. \u003cem\u003eAPL Mater.\u003c/em\u003e \u003cb\u003e1\u003c/b\u003e, 0\u0026ndash;7 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e28. Jeen, H. \u003cem\u003eet al.\u003c/em\u003e Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge. \u003cem\u003eNat. Mater.\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 1057\u0026ndash;1063 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e29. Ivanov, Y. P. \u003cem\u003eet al.\u003c/em\u003e Strain-induced structure and oxygen transport interactions in epitaxial La0.6Sr0.4CoO3\u0026thinsp;\u0026minus;\u0026thinsp;δ thin films. \u003cem\u003eCommun. Mater.\u003c/em\u003e (2020) doi:10.1038/s43246-020-0027-0.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e30. Khare, A. \u003cem\u003eet al.\u003c/em\u003e Directing Oxygen Vacancy Channels in SrFeO 2.5 Epitaxial Thin Films. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cb\u003e10\u003c/b\u003e, 4831\u0026ndash;4837 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e31. Shin, Y., Poeppelmeier, K. R. \u0026amp; Rondinelli, J. M. Informatics-Based Learning of Oxygen Vacancy Ordering Principles in Oxygen-Deficient Perovskites. \u003cem\u003eInorg. Chem.\u003c/em\u003e \u003cb\u003e63\u003c/b\u003e, 12785\u0026ndash;12802 (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e32. Jin, L. \u003cem\u003eet al.\u003c/em\u003e Understanding Structural Incorporation of Oxygen Vacancies in Perovskite Cobaltite Films and Potential Consequences for Electrocatalysis. \u003cem\u003eChem. Mater.\u003c/em\u003e \u003cb\u003e34\u003c/b\u003e, 10373\u0026ndash;10381 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e33. Yang, Z. \u003cem\u003eet al.\u003c/em\u003e Guided anisotropic oxygen transport in vacancy ordered oxides. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e, 1\u0026ndash;9 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e34. Yao, L., Inkinen, S., Komsa, H. P. \u0026amp; van Dijken, S. Structural Phase Transitions to 2D and 3D Oxygen Vacancy Patterns in a Perovskite Film Induced by Electrical and Mechanical Nanoprobing. \u003cem\u003eSmall\u003c/em\u003e \u003cb\u003e17\u003c/b\u003e, 1\u0026ndash;9 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e35. Krivanek, O. L. \u003cem\u003eet al.\u003c/em\u003e Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy. \u003cem\u003eNature\u003c/em\u003e (2010) doi:10.1038/nature08879.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e36. Bosch, E. G. T. \u0026amp; Lazić, I. Analysis of HR-STEM theory for thin specimen. \u003cem\u003eUltramicroscopy\u003c/em\u003e \u003cb\u003e156\u003c/b\u003e, 59\u0026ndash;72 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e37. Majert, S. \u0026amp; Kohl, H. High-resolution STEM imaging with a quadrant detector-Conditions for differential phase contrast microscopy in the weak phase object approximation. \u003cem\u003eUltramicroscopy\u003c/em\u003e (2015) doi:10.1016/j.ultramic.2014.09.009.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e38. Y\u0026uuml;celen, E., Lazić, I. \u0026amp; Bosch, E. G. T. Phase contrast scanning transmission electron microscopy imaging of light and heavy atoms at the limit of contrast and resolution. \u003cem\u003eSci. Rep.\u003c/em\u003e \u003cb\u003e8\u003c/b\u003e, 2676 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e39. Goossens, A. S. \u003cem\u003eet al.\u003c/em\u003e Memristive Memory Enhancement by Device Miniaturization for Neuromorphic Computing. \u003cem\u003eAdv. Electron. Mater.\u003c/em\u003e (2023) doi:10.1002/aelm.202201111.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e40. Nukala, P. \u003cem\u003eet al.\u003c/em\u003e In situ heating studies on temperature-induced phase transitions in epitaxial Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 heterostructures. \u003cem\u003eAppl. Phys. Lett.\u003c/em\u003e \u003cb\u003e118\u003c/b\u003e, (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e41. Jiang, H. \u003cem\u003eet al.\u003c/em\u003e Atomic-resolution characterization on the structure of strontium doped barium titanate nanoparticles. \u003cem\u003eNano Res.\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e, 4802\u0026ndash;4807 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e42. Penn, A. N. \u003cem\u003eet al.\u003c/em\u003e Explaining the Magnetic Properties of Oxygen Deficient LSMO Thin Films by iDPC. \u003cem\u003eMicrosc. Microanal.\u003c/em\u003e \u003cb\u003e25\u003c/b\u003e, 1748\u0026ndash;1749 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e43. Penn, A., Koohfar, S., Kumah, D. \u0026amp; LeBeau, J. Combined iDPC and EELS analyses for quantifying oxygen vacancy concentration in LSMO. \u003cem\u003eMicrosc. Microanal.\u003c/em\u003e \u003cb\u003e27\u003c/b\u003e, 1196\u0026ndash;1197 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e44. Nukala, P. \u003cem\u003eet al.\u003c/em\u003e Reversible oxygen migration and phase transitions in hafnia-based ferroelectric devices. \u003cem\u003eScience (80-. ).\u003c/em\u003e (2021) doi:10.1126/science.abf3789.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e45. Kresse, G. \u0026amp; Furthm\u0026uuml;ller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. \u003cem\u003ePhys. Rev. B - Condens. Matter Mater. Phys.\u003c/em\u003e \u003cb\u003e54\u003c/b\u003e, 11169\u0026ndash;11186 (1996).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e46. Kresse, G. \u0026amp; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e59\u003c/b\u003e, 1758\u0026ndash;1775 (1999).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e47. Perdew, J. P. \u003cem\u003eet al.\u003c/em\u003e Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. \u003cem\u003ePhys. Rev. Lett.\u003c/em\u003e \u003cb\u003e100\u003c/b\u003e, 136406 (2008).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e48. P.E., B. Projector augmented-wave method. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e50\u003c/b\u003e, 17953\u0026ndash;17979 (1994).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e49. H\u0026yuml;tch, M. J., Snoeck, E. \u0026amp; Kilaas, R. Quantitative measurement of displacement and strain fields from HREM micrographs. \u003cem\u003eUltramicroscopy\u003c/em\u003e \u003cb\u003e74\u003c/b\u003e, 131\u0026ndash;146 (1998).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e50. Peters, J. JJPPeters/Strainpp: v1.8. at https://doi.org/10.5281/zenodo.7422773 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e51. Momma, K. \u0026amp; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e \u003cb\u003e44\u003c/b\u003e, 1272\u0026ndash;1276 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e52. Seto, Y. \u0026amp; Ohtsuka, M. ReciPro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e (2022) doi:10.1107/S1600576722000139.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e53. Yuan, P. J. \u003cem\u003eet al.\u003c/em\u003e ToTEM: A software for fast TEM image simulation. \u003cem\u003eJ. Microsc.\u003c/em\u003e \u003cb\u003e287\u003c/b\u003e, 93\u0026ndash;104 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e54. Aidhy, D. S. \u0026amp; Rawat, K. Coupling between interfacial strain and oxygen vacancies at complex-oxides interfaces. \u003cem\u003eJ. Appl. Phys.\u003c/em\u003e \u003cb\u003e129\u003c/b\u003e, (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e55. Weber, M. C. \u003cem\u003eet al.\u003c/em\u003e Multiple strain-induced phase transitions in LaNiO 3 thin films. \u003cem\u003ePhys. Rev. B\u003c/em\u003e \u003cb\u003e94\u003c/b\u003e, 014118 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e56. Rawat, K., Fong, D. D. \u0026amp; Aidhy, D. S. Breaking atomic-level ordering via biaxial strain in functional oxides: A DFT study. \u003cem\u003eJ. Appl. Phys.\u003c/em\u003e \u003cb\u003e129\u003c/b\u003e, 095301 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e57. Moriga, T. \u003cem\u003eet al.\u003c/em\u003e Synthesis, Crystal Structure, and Properties of Oxygen-Deficient Lanthanum Nickelate LaNiO 3\u0026thinsp;\u0026minus;\u0026thinsp;x (0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;0.5). \u003cem\u003eBull. Chem. Soc. Jpn.\u003c/em\u003e \u003cb\u003e67\u003c/b\u003e, 687\u0026ndash;693 (1994).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e58. Alonso, J. A., Mart\u0026iacute;nez-Lope, M. J., Garc\u0026iacute;a-Mu\u0026ntilde;oz, J. L. \u0026amp; Fern\u0026aacute;ndez-D\u0026iacute;az, M. T. A structural and magnetic study of the defect perovskite LaNiO2.5 from high-resolution neutron diffraction data. \u003cem\u003eJ. Phys. Condens. Matter\u003c/em\u003e (1997) doi:10.1088/0953-8984/9/30/010.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5883878/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5883878/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe study of rare-earth nickelates, such as LaNiO\u003csub\u003e3\u003c/sub\u003e (LNO), is significant due to their complex electronic properties. Ordered oxygen vacancies (OOV) in LaNiO\u003csub\u003e3\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003e decrease conductivity, converting it from metallic to insulating state as 'x' approaches 0.5, and semiconducting behavior near x\u0026thinsp;=\u0026thinsp;0.75. These OOV also influence magnetic properties, causing LNO to exhibit anti-ferromagnetic and ferromagnetic behavior instead of its usual paramagnetic state. Interfacial strain in thin-film heterostructures is utilized to regulate the creation of oxygen vacancies and Ruddlesden-Popper (RP) faults, leading to notable impacts on materials' structural and electronic phases. The effect of strain on the formation of RP faults and the critical thickness of a fault-free layer in LNO has been studied, but atomic-scale insights into the relationship between strain, OOV, and RP faults are still limited. In this paper, we systematically investigated the effect of strain and RP faults on the formation of OOV in LNO thin films grown on SrTiO\u003csub\u003e3\u003c/sub\u003e (STO) substrates. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM imaging, we conducted atomic-scale structural and compositional analyses of OOV. Geometric phase analysis (GPA) was employed to measure the strain in fault-free and RP fault regions, while density functional theory (DFT) calculations explored different OOV arrangements in the LNO phase. Simulated iDPC-STEM imaging of energy-stabilized structures was performed to correlate with experimental results. Our findings reveal superstructure modulation in the chemical composition and atomic-scale lattice structure in LNO, primarily due to the formation of the OOV in Ni-O layer of LaNiO\u003csub\u003e2.5\u003c/sub\u003e phase. The out-of-plane compressive strain of about 2% stabilizes this phase, reducing the strain, diminishing OOV, and transforming them into LNO.\u003c/p\u003e","manuscriptTitle":"Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-06 21:21:47","doi":"10.21203/rs.3.rs-5883878/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-materials","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsmat","sideBox":"Learn more about [Communications Materials](https://www.nature.com/commsmat/)","snPcode":"","submissionUrl":"","title":"Communications Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"72fe236e-34cd-49b3-9c29-09bab9eedfb6","owner":[],"postedDate":"February 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":43447324,"name":"Physical sciences/Nanoscience and technology/Techniques and instrumentation/Microscopy/Transmission electron microscopy"},{"id":43447325,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Structural properties"}],"tags":[],"updatedAt":"2025-03-04T11:11:53+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-06 21:21:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5883878","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5883878","identity":"rs-5883878","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}