Anomalous polarization enhancement in a vdW ferroelectric material under pressure

Anomalous polarization enhancement in a vdW ferroelectric material under pressure | 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 Anomalous polarization enhancement in a vdW ferroelectric material under pressure Jinlong Zhu, Xiaodong Yao, Yinxin Bai, Cheng Jin, Xinyu Zhang, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2620145/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Jul, 2023 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract CuInP 2 S 6 with robust room-temperature ferroelectricity has recently attracted much attention due to the spatial instability of its Cu cations and the van der Waals (vdW) layered structure. Herein, we report a significant enhancement of its remanent polarization by more than 50% from 4.06 to 6.36 µC cm − 2 under a small pressure between 0.26 to 1.40 GPa. Comprehensive analysis suggests that even though the hydrostatic pressure suppresses the crystal distortion, it initially forces Cu cations to largely occupy the interlayer site, causing the spontaneous polarization to increase. Under intermediate pressure, the condensation of Cu cations to the ground state and the polarization increase due cell volume reduction compensate each other, resulting in a constant polarization. Under high pressure, the migration of Cu cations to the center of the S octahedron dominates. These findings improve our understanding of this fascinating vdW ferroelectric material, and suggest new ways to improve its properties. Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics Physical sciences/Materials science/Materials for devices/Information storage Figures Figure 1 Figure 2 Figure 3 Introduction Ferroelectricity is characterized by a spontaneous polarization below the Curie temperature, which can be switched by external electric fields. While the Landau–Ginzburg–Devonshire (LGD) phenomenological theory is simple and useful to describe ferroelectric phase transitions, 1,2 modern theory based on the Berry phase allows for accurate calculation of the spontaneous polarization. 3,4 Previously, perovskite oxide ferroelectrics, such as PbTiO 3 and BaTiO 3 , were mostly studied and widely used. To facilitate integration with modern electronics, thin films (both on a substrate and free-standing) were extensively investigated. However, structural and chemical incompatibility at the interface/surface often hamper the ferroelectric properties and device performances. Recently, the development of two-dimensional (2D) van der Waals (vdW) ferroelectric materials opens a new paradigm in the field. 2D vdW ferroelectrics, such as SnTe, In 2 Se 3 , CuInP 2 S 6 (CIPS), and transition metal chalcogenides (WTe 2 and MoTe 2 ), have been reported. Chang et al. 5 discovered a stable in-plane spontaneous polarization in atomic-thick SnTe down to a 1-unit cell (UC) limit in 2016. Subsequent ab initio calculations suggested a polarization of 2.3×10 − 6 µC cm − 1 for both 2- and 6-atomic-layer γ-SnTe films. 6 Room-temperature ferroelectricity was discovered in CIPS with a transition temperature of ∼320 K (Liu et al. , 2016), and switchable out-of-plane polarization was observed in thin CIPS of ∼ 4 nm. 7,8 In 2018, Xue et al. reported that room-temperature ferroelectricity exists in hexagonal layered α-In 2 Se 3 nanoflakes down to the monolayer limit. 9 The calculated out-of-plane and in-plane spontaneous polarizations were 0.97 and 8.0 µC cm − 2 , respectively. 10 Furthermore, two- or three-layer WTe 2 was shown to exhibit spontaneous out-of-plane polarization of ∼ 0.03 µC cm − 2 . 11 It was also reported that AB-stacked bilayer BN possesses a spontaneous polarization of 0.68 µC cm − 2 . 12 In general, the out-of-plane spontaneous polarization of 2D vdW ferroelectrics is relatively small, which may hinder their applications in electronic devices. Thus, polarization enhancement in vdW ferroelectrics is of crucial importance for both fundamental study and technological development of new ferroelectricity-based electronic devices. Herein, we report a hydrostatic-pressure-driven 56.5% enhancement of the spontaneous polarization of CIPS at room temperature, which is completely opposite to the usual pressure induced suppression of ferroelectricity. 13–16 Detailed Raman analysis suggests that the anomalous behavior is due to an increase in the interlayer coupling upon reducing the vdW gaps, promoting the Cu occupancy at the interlayer site. Results Polarization-electric field ( P-E ) hysteresis loops of CIPS under pressure. The crystal structure of CIPS is defined by the sulfur framework as shown in Fig. 1 a, where the octahedral voids are filled with the triangularly arranged Cu, In, and P-P cations. Bulk crystals are composed of vertically stacked layers weakly linked by vdW interactions. Because of the site exchange between Cu and P-P pairs from one layer to another, a complete unit cell consists of two adjacent layers, which is required to describe the material’s symmetry. 17 When the temperature drops below T C , a first-order phase transition occurs, and the symmetry reduces from C 2/ c to Cc . 18 The ferroelectricity of CIPS originates from the spatial instability of Cu cations. Monovalent Cu cations favor lower coordination because of the second-order Jahn-Teller coupling between the filled 3 d 10 manifold and the empty 4 s orbital. The electric dipoles produced by Cu cations deviating from the center of the S octahedron lead to the macroscopic polarization. You et al. 19 and Brehm et al. 20 discovered an unusual ferroelectric characteristic of CIPS, i.e., a uniaxial quadruple-potential well for Cu 1+ displacements. They theoretically and experimentally demonstrated that the low-polarization (LP) and high-polarization (HP) states correspond to the Cu 1+ displacement within and between vdW layers, respectively. In the metastable HP phase, the c -axis decreases from 13.4834 to 12.9305 Å, with an energy of 14 meV per Cu higher than the LP state. In the HP phase, the spontaneous polarization is greatly enhanced (∼12.24 µC cm − 2 ) compared to the ground state (∼3.34 µC cm − 2 with full intralayer sites occupancy of Cu 1+ ). On the other hand, giant negative piezoelectricity in CIPS also suggests a strong correlation between spontaneous polarization and strain. 21,22 These earlier reports prompted us to investigate the spontaneous polarization of CIPS under pressure. Figure 1 b shows a schematic of the experimental setup. Single-crystal CIPS of ∼10 µm thick is placed on a plastic holder and electrically connected to the ferroelectric tester through copper wires. Figure 1 c illustrates the macroscopic polarization-electric field ( P-E ) hysteresis loops measured at different frequencies under ambient conditions, and the horizontal shift of the P-E loops with increasing frequency indicates defect dipoles in CIPS. 23 Figs. 1 d and e show the P-E loops measured at 1 kHz under different pressures, and the change in remanent polarization ( P r ) is summarized in Fig. 1 f. A clear increase of P r up to 0.26 GPa is observed, which is then maintained at about 6.30 µC cm − 2 between 0.26 and 1.40 GPa. Subsequently, the remanent polarization gradually decreases upon further pressure increases, eventually disappearing at about 2.68 GPa. Figures S1 - S4 show the P-E loops measured at 500, 200, 100, and 50 Hz, respectively, which reveal the same behavior. Understanding the enhancement of P r from lattice vibration. Previous studies 24,25 reported that CIPS maintains the ambient conditions phase up to 4–5 GPa then a phase transition from the monoclinic ( Cc) to centrosymmetric trigonal ( P -31 m ) structure occurs. However, the sharp increase in spontaneous polarization up to about 0.26 GPa has not been reported. To elucidate the origin of this polarization enhancement, we explored the structural evolution under pressure using Raman spectroscopy. The high-pressure Raman spectroscopy results for CIPS are shown in Fig. 2 a. The Raman modes can be divided into several regions (as shown in Fig. 2 d): (i) modes < 100 cm − 1 , which can be attributed to cation translations (Cu 1+ and In 3+ ) and out-of-plane Cu vibration (black arrows); (ii) 100 cm − 1 < modes < 140 cm − 1 , which are related to anion (P 2 S 6 4− ) deformation; (iii) 150 cm − 1 < modes < 300 cm − 1 , which are contributed by S-P-P bending motion (blue arcs) and S-P-S deformation, changing the S-P-S angles (orange arrows); (iv) the mode around 315 cm − 1 , originating from cation vibrations of Cu 1+ ions (black arrows) 26 , and (v) modes > 350 cm − 1 , corresponding to P-P stretching (plum arrows) and P-S oscillations (purple arrows). 18,27 Because of the highly dispersed Cu cations, the FWHM of the peak at 315 cm − 1 is relatively large. Figure 2 b shows the logarithms of relative intensity ratios of representative Raman modes as a function of pressure. Below 0.26 GPa, the intensities of the Cu 1+ vibrations modes at 72 and 315 cm − 1 gradually increase with pressure, while their FWHM values gradually decrease, indicating the location dispersion of Cu cation becomes narrower. 28–30 The Raman modes at 549 and 556 cm − 1 , which are sensitive to the change in the cation position, merge into one peak > 0.26 GPa (as shown in Figure S5). The increase in the intensity of 104 cm − 1 indicates that the distortion of P 2 S 6 4− decreases and becomes more uniform throughout the sample. As the P 2 S 6 4− cluster has a relatively rigid structural frame, its ordering drives Cu cations to the interlayer site. 18 Fig. 2 c shows the evolution of the relative Raman shift of representative modes. All modes are generally blue-shifted, except for modes at 216 and 238 cm − 1 . The softening of these modes indicates that the bond angle deformation of S-P-S is weakened, which can be explained by the stiffness increase of the vibration through the enhanced interaction between Cu cations and adjacent S atoms, similar to what happens during the paraelectric-ferroelectric transition in CIPS. 27 As a result, the Cu cation gradually moves toward the sulfur plane with increasing pressure. The schematic diagram of the Cu cations change at high pressures is shown in Fig. 2 e. To summarize, the Raman spectra show that the uniformity of CIPS framework improves with pressure up to 0.26 GPa and the Cu cations move toward interlayer site, leading to the increase in spontaneous polarization. The competition between cell volume reduction and migration of Cu cations. Another possible contribution to the observed polarization enhancement is the cell volume reduction under pressure. To evaluate this effect quantitively, we performed in-situ high-pressure single-crystal X-ray diffraction (SCXRD) measurements (Figures S6 and S7). Figure 3 a shows the evolution of unit cell volume with pressure, a third-order Birch-Murnaghan equation of state was used to fit the data. 31 The fitting results give that K 0 (the isothermal bulk modulus) is 7.90(3) GPa, the V 0 (the initial volume) is 846.44(1) Å 3 , and the K 0 ' is 7.65(4). The results show that, if we assume that the dipoles do not change under pressure, the volume reduction would increase the macroscopic polarization by only about 5% (0.12 µC cm − 2 ) at 0.26 GPa. The SCXRD results also reveal that c -axis compression mainly originates from the reduction of the vdW gaps, consistent with previous report. 19 Due to the enhanced coupling between Cu 4 s and S sp orbitals in the adjacent layer as the interlayer distance reduces, it would be expected that the interlayer site are more energetically favored 20 with the pressure increase, consistent with the Raman results shown in Fig. 2 . Based on our SCXRD results and P-E loops obtained under pressure, we can also estimate the macroscopic negative piezoelectric coefficient ( e 33 = \(\frac{\partial P}{\partial \epsilon }\) , where \(P\) is the spontaneous polarization, and ε is the c -axis strain) of CIPS upto 0.26 GPa, which is \(\frac{2.30 {\mu }\text{C} {\text{c}\text{m}}^{-2}}{-0.00869}\) = -264 µC cm − 2 . This value is close to that reported in Ref. [ 19 ] (-272 µC cm − 2 ). Since the polarization enhancement is mainly due to increased occupancy of Cu cations at the interlayer site, which is caused by the enhanced coupling between Cu 4 s and S sp orbitals in the adjacent layer, we thus tried to estimate the percentage of Cu cations at different sites under pressure. We took the sites reported in Ref [ 19 ] as the possible locations of Cu cations. For Cu 1+ at different sites, the values of the corresponding dipoles were first calculated as described in the Methods. Under ambient conditions, detailed XRD analysis suggested (Fig. 3 b inset) 32% Cu 1+ at site Cu(1), 37.3% at site Cu(2), 7.9% at site Cu(3), 12% at site Cu(4), there was also 8% at site Cu(6). 19 Starting from this distribution, we obtain a spontaneous polarization of 3.50 µC cm − 2 . Considering that polarization back-switching occurred in part of the sample before XRD test, this value is consistent with our experimental result ( P-E loops) under ambient conditions. As pressure increases, we suggest that the Cu cations migrate from the original sites (proportionally) into the interlayer site Cu(4). As shown in Fig. 3 b, the occupancy of interlayer site reaches the maximum at 0.26 GPa and the change of Cu(4) site occupancy is ~ 30%. At 2.00 GPa, the polarization is 2.47 µC cm − 2 with a zero occupancy of interlayer site. The evolution of Cu(4) site occupancy is summarized in Fig. 3 b and schematically shown in Fig. 3 c-f. Discussion The experimental Raman and SCXRD results show that polarization enhancement below 0.26 GPa is related to the displacement of Cu cations with a small contribution from pure cell volume reduction effect. At high pressures (> 1.4 GPa), ferroelectricity is gradually suppressed and completely disappears at 2.68 GPa, similar to traditional perovskite oxide ferroelectrics. 32 The off-center Cu cations displacements in CIPS result from the coupling of the chemically-active valence d band with the s -like conduction band. 33 However, the increased optical band gap under high pressure indicates that the coupling is weakened, and the migration of Cu cations to the center of the S octahedron becomes the dominant factor. 25 The competition between the two factors likely give rise to the plateau of remanent polarization between 0.26 and 1.4 GPa. To conclude, we have quantitatively determined the evolution of the remanent polarization of the vdW layered ferroelectric CIPS under pressure. An anomalous polarization enhancement was observed under low pressure, which stems from the spatial instability of Cu cations and the vdW layered structure. This result is beneficial for improving the giant negative piezoelectricity and the performance of non-volatile memory devices. Methods Sample preparation High-quality single crystals of CIPS were grown by the chemical vapor transport method without a transport agent. Copper powder, indium powder, red phosphorus, and sulfur were placed in a quartz tube according to stoichiometric ratios. The temperature of the evaporation and crystallization zones was set to 750 ℃ and 650 ℃, respectively, and the entire growth process lasted for 5 days. 34 Ferroelectric Measurements Under Pressure To facilitate the testing of P-E loops, Au electrodes were deposited through direct current sputtering. The areas of electrodes range from 0.0095 to 0.0113 mm 2 . The standard two-probe ferroelectric measurements were carried out using a piston-cylinder pressure cell. Ferroelectric hysteresis loops were recorded using a commercial ferroelectric tester (Precision Multiferroic, Radiant Technologies). In Situ Raman In Raman experiments, a pair of diamond anvils with a flat top of 1000 µm was used, and a CuBe gasket was pre-indented to ∼ 80 µm in thickness. A sample hole of ∼ 500 µm was drilled in the center of the pre-indented gasket. Silicone oil was used as a pressure-transmitting medium (PTM) to ensure hydrostatic pressure conditions. The pressure was gauged at room temperature by monitoring the shift of the ruby R1 fluorescence line. 35 In-situ high-pressure Raman experiments were carried out using a SpectraPro HRS-500 spectrometer with excitation lasers with a wavelength of 532 nm. In situ SCXRD In SCXRD experiments, a pair of diamond anvils with a flat top of 500 µm was used, and a T-301 stainless-steel gasket was pre-indented to ∼ 40 µm in thickness. The PTM also was silicone oil. In-situ high-pressure SCXRD experiments were performed on a Bruker D8QUEST diffractometer with Mo Kα radiation (λ = 0.71073 Å). Diffraction data were collected by ω- and φ- scan methods, and lattice parameters and volumes were determined using APEX3 software. Numerical Calculation To calculate the polarization, we used the point charge model, 3 where \(P=\frac{1}{V}(-e\sum _{i}{N}_{i}{z}_{i}\) ), V is the unit cell volume, e is the electron charge, N i is the ionic valence state, z i is the projection of atomic position vector in an unit cell along the z direction. The reduced atom model is shown in Figure S8. The Cu 1+ shift downward and the In 3+ and P 4+ -P 4+ pair shift upward. The positive charge center is located at the atomic position ( z i ) of Cu, In and P-P pairs respectively and the negative charge center is located at the average position of six S atoms. We calculated the electric dipoles by using the relative negative and positive charge positions, and divided S charges proportionally to Cu, In and P forming dipoles respectively. The atomic positions are shown in Table 1. The negative charge center of S atoms is at (4.79+4.80+4.83+8.13+8.15+8.19)/6=6.48 Å and positive center of P-P pair is at (7.62+5.40)/2=6.51 Å. The macroscopic polarization is thus [(4.27–6.48)×1×occupancy (Cu(4)) + (4.72–6.48)×1×occupancy (Cu(1)) + (5.09–6.48)×1×occupancy (Cu(2)) + (5.50–6.48)×1×occupancy (Cu(3)) + (8.03–6.48)×1×occupancy (Cu(6)) + (6.69–6.48)×3×1 (In) + (6.51–6.48)×4×2 (P)]×1.602×10 −19 ×10 6 ×10 −8 ×4 /(unit cell volume), where the − 1.602×10 −19 is the electron charge, 4 indicates that there are four such reduced atomic models in an unit cell, the unit cell volume is 838.1931×10 −24 cm 3 . Declarations Acknowledgments The work was partly supported by the National Key R&D Program of China grant 2018YFA0305703, the National Natural Science Foundation of China grant No. 12274193, 2074164, 12004161 and 11904281, the Stable Support Plan Program of Shenzhen Natural Science Fund under grant No. 20200925152415003, the Guang dong Basic and Applied Basic Research Foundation of 2022A1515010044. J. L. Zhu and Y. L also acknowledged the Major Science and Technology Infrastructure Project of Material Genome Big-science Facilities Platform supported by Municipal Development and Reform Commission of Shenzhen. Some experiments were supported by the Synergic Extreme Condition User Facility. J.W. also acknowledges support from the Guangdong Provincial Key Laboratory Program (2021B1212040001) from the Department of Science and Technology of Guangdong Province, and the startup grant from the Southern University of Science and Technology (SUSTech), China. Author contributions J.Z. and J.W. initiated this work. Y.B. prepared the samples of CIPS. X.Y. carried out the ferroelectric and Raman experiments under pressure with help from Q.Z. and Y.L., C.J. performed SCXRD. Z.X., L.C. and S.W. provided experimental equipment support. X.Z. participated in the discussion of experimental results. X.Y. wrote the first draft, X.Y., J.W. and J.Z. co-rewrote the manuscript. Additional information Competing financial interests: The authors declare no competing financial interests. References L. H. Ong, J. Osman, D. R. Tilley, Landau theory of second-order phase transitions in ferroelectric films. Physical Review B 63 , 144109 (2001). P. Marton, I. Rychetsky, J. Hlinka, Domain walls of ferroelectric BaTiO 3 within the Ginzburg-Landau-Devonshire phenomenological model. Physical Review B 81 , 144125 (2010). R. Resta, Macroscopic polarization in crystalline dielectrics: the geometric phase approach. Reviews of modern physics 66 , 899 (1994). R. D. 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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-2620145","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":179136976,"identity":"70524d8b-77e3-41e2-a683-de4bcf13f707","order_by":0,"name":"Jinlong Zhu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3klEQVRIie3QMQqDMBSA4UggU6BrQMQrBAS7lJ7FIDh1cHQoIghOpXNLpV4hk3OKYBcPYRfngtC1jXbpFOtWaH4IvOF9kAQAne4XIwDQNnrPUB4j+Yp4jRzQHAJYNofYpx0N2TlmRZFe+hCsLC5g16qIkTeUsrJivEa+eQCBwwVaUhWBZDMQwTjCLsRAWoERURE0klxeLFs8JHlOEzySBLKkxkgSMU0ICULq1ZXD68AxMfWdY4VcJbEPfknv29gq0urW42ht7a9ppyTDcz7+ZxjhxP6w0k7v6HQ63V/3AgFpQ+acDQ9gAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-4062-0963","institution":"Department of Physics, Southern University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Jinlong","middleName":"","lastName":"Zhu","suffix":""},{"id":179136977,"identity":"5f5a99ab-5712-428d-bd24-33bfda5fe112","order_by":1,"name":"Xiaodong Yao","email":"","orcid":"","institution":"Department of Physics, Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiaodong","middleName":"","lastName":"Yao","suffix":""},{"id":179136978,"identity":"e8489c45-3f05-4f0c-aabd-762508c790f8","order_by":2,"name":"Yinxin Bai","email":"","orcid":"","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Yinxin","middleName":"","lastName":"Bai","suffix":""},{"id":179136979,"identity":"49a234ff-396f-44fc-bc8b-7e89c0dda784","order_by":3,"name":"Cheng Jin","email":"","orcid":"","institution":"Center for High Pressure Science and Technology Advanced Research (HPSTAR)","correspondingAuthor":false,"prefix":"","firstName":"Cheng","middleName":"","lastName":"Jin","suffix":""},{"id":179136980,"identity":"2a19dbee-3519-44ee-abe8-22b884df2a52","order_by":4,"name":"Xinyu Zhang","email":"","orcid":"","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xinyu","middleName":"","lastName":"Zhang","suffix":""},{"id":179136981,"identity":"37acee63-a865-4393-beca-93ae92871ab4","order_by":5,"name":"QunFei Zheng","email":"","orcid":"","institution":"Department of Physics, Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"QunFei","middleName":"","lastName":"Zheng","suffix":""},{"id":179136982,"identity":"2ffa45e6-637e-4c67-b2f7-8e18916e2ee9","order_by":6,"name":"Zedong Xu","email":"","orcid":"","institution":"Tiangong University","correspondingAuthor":false,"prefix":"","firstName":"Zedong","middleName":"","lastName":"Xu","suffix":""},{"id":179136983,"identity":"a3b4eb8b-61b0-4415-a484-002d87bd8e8d","order_by":7,"name":"Lang Chen","email":"","orcid":"https://orcid.org/0000-0003-2460-8232","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Lang","middleName":"","lastName":"Chen","suffix":""},{"id":179136984,"identity":"3ce34ab7-e833-4079-8ec1-5335f1f90570","order_by":8,"name":"Shanmin Wang","email":"","orcid":"https://orcid.org/0000-0001-7273-2786","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Shanmin","middleName":"","lastName":"Wang","suffix":""},{"id":179136985,"identity":"e5659db4-e64a-47ac-96ee-dd934cb1f938","order_by":9,"name":"Ying Liu","email":"","orcid":"","institution":"Department of Physics, Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Ying","middleName":"","lastName":"Liu","suffix":""},{"id":179136986,"identity":"7de25bf1-2abf-4bb2-991e-6af052f3dc81","order_by":10,"name":"Junling Wang","email":"","orcid":"https://orcid.org/0000-0003-3663-7081","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Junling","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2023-02-23 10:52:38","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2620145/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2620145/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-023-40075-6","type":"published","date":"2023-07-18T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":33584830,"identity":"9343e686-1aa5-45b7-8ab2-59e944eb0149","added_by":"auto","created_at":"2023-02-28 22:29:01","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":182741,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eP-E\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e loops under pressure.\u003c/strong\u003e \u003cstrong\u003e(a)\u003c/strong\u003e Crystal structure of CIPS viewed along the layer normal direction (\u003cem\u003ec\u003c/em\u003e\u003csup\u003e∗\u003c/sup\u003e axis) and \u003cem\u003eb\u003c/em\u003e axis, respectively, with an octahedral sulfur cage showing various copper sites at room temperature. The size of the Cu atom represents its occupation probability. \u003cstrong\u003e(b)\u003c/strong\u003e Schematic of the high-pressure experimental setup. A CIPS ferroelectric crystal is held in a piston pressure cell. The yellow line embedded in epoxy represents an enameled copper wire. \u003cstrong\u003e(c)\u003c/strong\u003e Polarization–electric field (\u003cem\u003eP-E\u003c/em\u003e) hysteresis loops of CIPS measured at different frequencies under ambient conditions. \u003cstrong\u003e(d)\u003c/strong\u003e and \u003cstrong\u003e(e)\u003c/strong\u003e \u003cem\u003eP-E\u003c/em\u003e loops of CIPS measured at 1 kHz under representative pressures. \u003cstrong\u003e(f)\u003c/strong\u003e Remanent polarization (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e) as a function of pressure, obtained from (d) and (e).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-2620145/v1/890e47fcc107368c0a83f3dc.png"},{"id":33584828,"identity":"190f180d-0ede-489d-9401-7d3a9b8117ff","added_by":"auto","created_at":"2023-02-28 22:29:01","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":483981,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eLattice vibration\u003c/strong\u003e \u003cstrong\u003eof CIPS.\u003c/strong\u003e \u003cstrong\u003e(a)\u003c/strong\u003e Pressure-dependent Raman spectra of CIPS from 0 to 1.90 GPa. \u003cstrong\u003e(b)\u003c/strong\u003e Pressure-evolution of the logarithms of the relative intensity ratio of the modes at 72, 104, 116, 162, 264, and 315 cm\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003e(c)\u003c/strong\u003e Relative Raman shifts of the modes at 72, 104, 216, 238, 264, and 315 cm\u003csup\u003e-1\u003c/sup\u003e under pressure. \u003cstrong\u003e(d)\u003c/strong\u003e The schematic diagram of the Raman modes for cation, anion (P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e4-\u003c/sup\u003e), S-P-P, S-P-S vibrations, P-P stretching, and P-S oscillations. \u003cstrong\u003e(e)\u003c/strong\u003e The schematic diagram of the evolution of Cu cations and S-P-S vibrations below 0.26 GPa.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-2620145/v1/8d9016d733267e3d1c8c0b87.png"},{"id":33584829,"identity":"c012de4e-9faa-4d85-9a2d-e10284039bbb","added_by":"auto","created_at":"2023-02-28 22:29:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":357299,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCopper migration under pressure.\u003c/strong\u003e Pressure-dependent evolution of unit cell volume \u003cstrong\u003e(a)\u003c/strong\u003e and the occupancies of interlayer site \u003cstrong\u003e(b)\u003c/strong\u003e from 0 to 2.00 GPa. The black line in (a) is the fitting of the pressure-cell volume curve and the green dots are the contribution of cell volume reduction to polarization increase. \u003cstrong\u003e(c)\u003c/strong\u003e Highly smeared distribution of Cu atoms at ambient conditions. The size of Cu atoms represents the occupancy of different sites. \u003cstrong\u003e(d)\u003c/strong\u003e Below 0.26 GPa, the Cu atoms move toward interlayer site according to the ferroelectric hysteresis loops measurements and Raman experimental results. \u003cstrong\u003e(e)\u003c/strong\u003e The atomic site occupation of Cu is close to the ground state at 2.00 GPa. \u003cstrong\u003e(f)\u003c/strong\u003e The Cu atoms are completely located in the center of the S octahedron at 2.68 GPa, and ferroelectricity completely disappears.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-2620145/v1/1d75a3e2c1257386adecac46.png"},{"id":40357127,"identity":"30a5dceb-5829-4813-b521-8dcea6965543","added_by":"auto","created_at":"2023-07-21 07:25:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1379354,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2620145/v1/78b8a2f8-2f35-4639-8e33-6d711b3d7e7d.pdf"},{"id":33584831,"identity":"1b026fd9-4264-41b9-b869-be15e630cf2e","added_by":"auto","created_at":"2023-02-28 22:29:01","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1667975,"visible":true,"origin":"","legend":"","description":"","filename":"S.M.docx","url":"https://assets-eu.researchsquare.com/files/rs-2620145/v1/f0f867087fc74bfaed42d8cd.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Anomalous polarization enhancement in a vdW ferroelectric material under pressure","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFerroelectricity is characterized by a spontaneous polarization below the Curie temperature, which can be switched by external electric fields. While the Landau\u0026ndash;Ginzburg\u0026ndash;Devonshire (LGD) phenomenological theory is simple and useful to describe ferroelectric phase transitions, \u003csup\u003e1,2\u003c/sup\u003e modern theory based on the Berry phase allows for accurate calculation of the spontaneous polarization. \u003csup\u003e3,4\u003c/sup\u003e Previously, perovskite oxide ferroelectrics, such as PbTiO\u003csub\u003e3\u003c/sub\u003e and BaTiO\u003csub\u003e3\u003c/sub\u003e, were mostly studied and widely used. To facilitate integration with modern electronics, thin films (both on a substrate and free-standing) were extensively investigated. However, structural and chemical incompatibility at the interface/surface often hamper the ferroelectric properties and device performances. Recently, the development of two-dimensional (2D) van der Waals (vdW) ferroelectric materials opens a new paradigm in the field. 2D vdW ferroelectrics, such as SnTe, In\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e, CuInP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e (CIPS), and transition metal chalcogenides (WTe\u003csub\u003e2\u003c/sub\u003e and MoTe\u003csub\u003e2\u003c/sub\u003e), have been reported. Chang \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e5\u003c/sup\u003e discovered a stable in-plane spontaneous polarization in atomic-thick SnTe down to a 1-unit cell (UC) limit in 2016. Subsequent \u003cem\u003eab initio\u003c/em\u003e calculations suggested a polarization of 2.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for both 2- and 6-atomic-layer γ-SnTe films. \u003csup\u003e6\u003c/sup\u003e Room-temperature ferroelectricity was discovered in CIPS with a transition temperature of \u0026sim;320 K (Liu \u003cem\u003eet al.\u003c/em\u003e, 2016), and switchable out-of-plane polarization was observed in thin CIPS of \u0026sim; 4 nm. \u003csup\u003e7,8\u003c/sup\u003e In 2018, Xue \u003cem\u003eet al.\u003c/em\u003e reported that room-temperature ferroelectricity exists in hexagonal layered α-In\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e nanoflakes down to the monolayer limit. \u003csup\u003e9\u003c/sup\u003e The calculated out-of-plane and in-plane spontaneous polarizations were 0.97 and 8.0 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, respectively. \u003csup\u003e10\u003c/sup\u003e Furthermore, two- or three-layer WTe\u003csub\u003e2\u003c/sub\u003e was shown to exhibit spontaneous out-of-plane polarization of \u0026sim; 0.03 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. \u003csup\u003e11\u003c/sup\u003e It was also reported that AB-stacked bilayer BN possesses a spontaneous polarization of 0.68 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. \u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn general, the out-of-plane spontaneous polarization of 2D vdW ferroelectrics is relatively small, which may hinder their applications in electronic devices. Thus, polarization enhancement in vdW ferroelectrics is of crucial importance for both fundamental study and technological development of new ferroelectricity-based electronic devices. Herein, we report a hydrostatic-pressure-driven 56.5% enhancement of the spontaneous polarization of CIPS at room temperature, which is completely opposite to the usual pressure induced suppression of ferroelectricity. \u003csup\u003e13\u0026ndash;16\u003c/sup\u003e Detailed Raman analysis suggests that the anomalous behavior is due to an increase in the interlayer coupling upon reducing the vdW gaps, promoting the Cu occupancy at the interlayer site.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003ePolarization-electric field (\u003c/b\u003e \u003cspan type=\"BoldItalic\" class=\"BoldItalic\" name=\"Emphasis\"\u003eP-E\u003c/span\u003e \u003cb\u003e) hysteresis loops of CIPS under pressure.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe crystal structure of CIPS is defined by the sulfur framework as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, where the octahedral voids are filled with the triangularly arranged Cu, In, and P-P cations. Bulk crystals are composed of vertically stacked layers weakly linked by vdW interactions. Because of the site exchange between Cu and P-P pairs from one layer to another, a complete unit cell consists of two adjacent layers, which is required to describe the material\u0026rsquo;s symmetry. \u003csup\u003e17\u003c/sup\u003e When the temperature drops below \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e, a first-order phase transition occurs, and the symmetry reduces from \u003cem\u003eC\u003c/em\u003e2/\u003cem\u003ec\u003c/em\u003e to \u003cem\u003eCc\u003c/em\u003e. \u003csup\u003e18\u003c/sup\u003e The ferroelectricity of CIPS originates from the spatial instability of Cu cations. Monovalent Cu cations favor lower coordination because of the second-order Jahn-Teller coupling between the filled 3\u003cem\u003ed\u003c/em\u003e\u003csup\u003e10\u003c/sup\u003e manifold and the empty 4\u003cem\u003es\u003c/em\u003e orbital. The electric dipoles produced by Cu cations deviating from the center of the S octahedron lead to the macroscopic polarization.\u003c/p\u003e \u003cp\u003eYou \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e19\u003c/sup\u003e and Brehm \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e20\u003c/sup\u003e discovered an unusual ferroelectric characteristic of CIPS, i.e., a uniaxial quadruple-potential well for Cu\u003csup\u003e1+\u003c/sup\u003e displacements. They theoretically and experimentally demonstrated that the low-polarization (LP) and high-polarization (HP) states correspond to the Cu\u003csup\u003e1+\u003c/sup\u003e displacement within and between vdW layers, respectively. In the metastable HP phase, the \u003cem\u003ec\u003c/em\u003e-axis decreases from 13.4834 to 12.9305 \u0026Aring;, with an energy of 14 meV per Cu higher than the LP state. In the HP phase, the spontaneous polarization is greatly enhanced (\u0026sim;12.24 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) compared to the ground state (\u0026sim;3.34 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e with full intralayer sites occupancy of Cu\u003csup\u003e1+\u003c/sup\u003e). On the other hand, giant negative piezoelectricity in CIPS also suggests a strong correlation between spontaneous polarization and strain. \u003csup\u003e21,22\u003c/sup\u003e These earlier reports prompted us to investigate the spontaneous polarization of CIPS under pressure.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb shows a schematic of the experimental setup. Single-crystal CIPS of \u0026sim;10 \u0026micro;m thick is placed on a plastic holder and electrically connected to the ferroelectric tester through copper wires. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec illustrates the macroscopic polarization-electric field (\u003cem\u003eP-E\u003c/em\u003e) hysteresis loops measured at different frequencies under ambient conditions, and the horizontal shift of the \u003cem\u003eP-E\u003c/em\u003e loops with increasing frequency indicates defect dipoles in CIPS. \u003csup\u003e23\u003c/sup\u003e Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and e show the \u003cem\u003eP-E\u003c/em\u003e loops measured at 1 kHz under different pressures, and the change in remanent polarization (\u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e) is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef. A clear increase of \u003cem\u003eP\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e up to 0.26 GPa is observed, which is then maintained at about 6.30 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e between 0.26 and 1.40 GPa. Subsequently, the remanent polarization gradually decreases upon further pressure increases, eventually disappearing at about 2.68 GPa. Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e - S4 show the \u003cem\u003eP-E\u003c/em\u003e loops measured at 500, 200, 100, and 50 Hz, respectively, which reveal the same behavior.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eUnderstanding the enhancement of\u003c/b\u003e \u003cspan type=\"BoldItalic\" class=\"BoldItalic\" name=\"Emphasis\"\u003eP\u003c/span\u003e \u003csub\u003e \u003cspan type=\"BoldItalic\" class=\"BoldItalic\" name=\"Emphasis\"\u003er\u003c/span\u003e \u003c/sub\u003e \u003cb\u003efrom lattice vibration.\u003c/b\u003e \u003c/p\u003e \u003cp\u003ePrevious studies \u003csup\u003e24,25\u003c/sup\u003e reported that CIPS maintains the ambient conditions phase up to 4\u0026ndash;5 GPa then a phase transition from the monoclinic (\u003cem\u003eCc)\u003c/em\u003e to centrosymmetric trigonal (\u003cem\u003eP\u003c/em\u003e-31\u003cem\u003em\u003c/em\u003e) structure occurs. However, the sharp increase in spontaneous polarization up to about 0.26 GPa has not been reported. To elucidate the origin of this polarization enhancement, we explored the structural evolution under pressure using Raman spectroscopy. The high-pressure Raman spectroscopy results for CIPS are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. The Raman modes can be divided into several regions (as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed): (i) modes\u0026thinsp;\u0026lt;\u0026thinsp;100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which can be attributed to cation translations (Cu\u003csup\u003e1+\u003c/sup\u003e and In\u003csup\u003e3+\u003c/sup\u003e) and out-of-plane Cu vibration (black arrows); (ii) 100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e \u0026lt; modes\u0026thinsp;\u0026lt;\u0026thinsp;140 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are related to anion (P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e4\u0026minus;\u003c/sup\u003e) deformation; (iii) 150 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e \u0026lt; modes\u0026thinsp;\u0026lt;\u0026thinsp;300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are contributed by S-P-P bending motion (blue arcs) and S-P-S deformation, changing the S-P-S angles (orange arrows); (iv) the mode around 315 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, originating from cation vibrations of Cu\u003csup\u003e1+\u003c/sup\u003e ions (black arrows) \u003csup\u003e26\u003c/sup\u003e, and (v) modes\u0026thinsp;\u0026gt;\u0026thinsp;350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to P-P stretching (plum arrows) and P-S oscillations (purple arrows). \u003csup\u003e18,27\u003c/sup\u003e Because of the highly dispersed Cu cations, the FWHM of the peak at 315 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is relatively large.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows the logarithms of relative intensity ratios of representative Raman modes as a function of pressure. Below 0.26 GPa, the intensities of the Cu\u003csup\u003e1+\u003c/sup\u003e vibrations modes at 72 and 315 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e gradually increase with pressure, while their FWHM values gradually decrease, indicating the location dispersion of Cu cation becomes narrower. \u003csup\u003e28\u0026ndash;30\u003c/sup\u003e The Raman modes at 549 and 556 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are sensitive to the change in the cation position, merge into one peak\u0026thinsp;\u0026gt;\u0026thinsp;0.26 GPa (as shown in Figure S5). The increase in the intensity of 104 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates that the distortion of P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e4\u0026minus;\u003c/sup\u003e decreases and becomes more uniform throughout the sample. As the P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e4\u0026minus;\u003c/sup\u003e cluster has a relatively rigid structural frame, its ordering drives Cu cations to the interlayer site. \u003csup\u003e18\u003c/sup\u003e Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the evolution of the relative Raman shift of representative modes. All modes are generally blue-shifted, except for modes at 216 and 238 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The softening of these modes indicates that the bond angle deformation of S-P-S is weakened, which can be explained by the stiffness increase of the vibration through the enhanced interaction between Cu cations and adjacent S atoms, similar to what happens during the paraelectric-ferroelectric transition in CIPS. \u003csup\u003e27\u003c/sup\u003e As a result, the Cu cation gradually moves toward the sulfur plane with increasing pressure. The schematic diagram of the Cu cations change at high pressures is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. To summarize, the Raman spectra show that the uniformity of CIPS framework improves with pressure up to 0.26 GPa and the Cu cations move toward interlayer site, leading to the increase in spontaneous polarization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eThe competition between cell volume reduction and migration of Cu cations.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAnother possible contribution to the observed polarization enhancement is the cell volume reduction under pressure. To evaluate this effect quantitively, we performed \u003cem\u003ein-situ\u003c/em\u003e high-pressure single-crystal X-ray diffraction (SCXRD) measurements (Figures S6 and S7). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the evolution of unit cell volume with pressure, a third-order Birch-Murnaghan equation of state was used to fit the data. \u003csup\u003e31\u003c/sup\u003e The fitting results give that \u003cem\u003eK\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e (the isothermal bulk modulus) is 7.90(3) GPa, the \u003cem\u003eV\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e (the initial volume) is 846.44(1) \u0026Aring;\u003csup\u003e3\u003c/sup\u003e, and the K\u003csub\u003e0\u003c/sub\u003e\u003csup\u003e'\u003c/sup\u003e is 7.65(4). The results show that, if we assume that the dipoles do not change under pressure, the volume reduction would increase the macroscopic polarization by only about 5% (0.12 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) at 0.26 GPa. The SCXRD results also reveal that \u003cem\u003ec\u003c/em\u003e-axis compression mainly originates from the reduction of the vdW gaps, consistent with previous report. \u003csup\u003e19\u003c/sup\u003e Due to the enhanced coupling between Cu 4\u003cem\u003es\u003c/em\u003e and S \u003cem\u003esp\u003c/em\u003e orbitals in the adjacent layer as the interlayer distance reduces, it would be expected that the interlayer site are more energetically favored \u003csup\u003e20\u003c/sup\u003e with the pressure increase, consistent with the Raman results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Based on our SCXRD results and \u003cem\u003eP-E\u003c/em\u003e loops obtained under pressure, we can also estimate the macroscopic negative piezoelectric coefficient (\u003cem\u003ee\u003c/em\u003e\u003csub\u003e33\u003c/sub\u003e = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{\\partial P}{\\partial \\epsilon }\\)\u003c/span\u003e\u003c/span\u003e, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(P\\)\u003c/span\u003e\u003c/span\u003e is the spontaneous polarization, and \u003cem\u003eε\u003c/em\u003e is the \u003cem\u003ec\u003c/em\u003e-axis strain) of CIPS upto 0.26 GPa, which is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{2.30 {\\mu }\\text{C} {\\text{c}\\text{m}}^{-2}}{-0.00869}\\)\u003c/span\u003e\u003c/span\u003e = -264 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. This value is close to that reported in Ref. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] (-272 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eSince the polarization enhancement is mainly due to increased occupancy of Cu cations at the interlayer site, which is caused by the enhanced coupling between Cu 4\u003cem\u003es\u003c/em\u003e and S \u003cem\u003esp\u003c/em\u003e orbitals in the adjacent layer, we thus tried to estimate the percentage of Cu cations at different sites under pressure. We took the sites reported in Ref [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] as the possible locations of Cu cations. For Cu\u003csup\u003e1+\u003c/sup\u003e at different sites, the values of the corresponding dipoles were first calculated as described in the Methods. Under ambient conditions, detailed XRD analysis suggested (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb inset) 32% Cu\u003csup\u003e1+\u003c/sup\u003e at site Cu(1), 37.3% at site Cu(2), 7.9% at site Cu(3), 12% at site Cu(4), there was also 8% at site Cu(6). \u003csup\u003e19\u003c/sup\u003e Starting from this distribution, we obtain a spontaneous polarization of 3.50 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. Considering that polarization back-switching occurred in part of the sample before XRD test, this value is consistent with our experimental result (\u003cem\u003eP-E\u003c/em\u003e loops) under ambient conditions. As pressure increases, we suggest that the Cu cations migrate from the original sites (proportionally) into the interlayer site Cu(4). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the occupancy of interlayer site reaches the maximum at 0.26 GPa and the change of Cu(4) site occupancy is ~\u0026thinsp;30%. At 2.00 GPa, the polarization is 2.47 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e with a zero occupancy of interlayer site. The evolution of Cu(4) site occupancy is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and schematically shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-f.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe experimental Raman and SCXRD results show that polarization enhancement below 0.26 GPa is related to the displacement of Cu cations with a small contribution from pure cell volume reduction effect. At high pressures (\u0026gt;\u0026thinsp;1.4 GPa), ferroelectricity is gradually suppressed and completely disappears at 2.68 GPa, similar to traditional perovskite oxide ferroelectrics. \u003csup\u003e32\u003c/sup\u003e The off-center Cu cations displacements in CIPS result from the coupling of the chemically-active valence \u003cem\u003ed\u003c/em\u003e band with the \u003cem\u003es\u003c/em\u003e-like conduction band. \u003csup\u003e33\u003c/sup\u003e However, the increased optical band gap under high pressure indicates that the coupling is weakened, and the migration of Cu cations to the center of the S octahedron becomes the dominant factor. \u003csup\u003e25\u003c/sup\u003e The competition between the two factors likely give rise to the plateau of remanent polarization between 0.26 and 1.4 GPa.\u003c/p\u003e \u003cp\u003eTo conclude, we have quantitatively determined the evolution of the remanent polarization of the vdW layered ferroelectric CIPS under pressure. An anomalous polarization enhancement was observed under low pressure, which stems from the spatial instability of Cu cations and the vdW layered structure. This result is beneficial for improving the giant negative piezoelectricity and the performance of non-volatile memory devices.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv class=\"Section2\" id=\"Sec5\"\u003e\n \u003ch2\u003eSample preparation\u003c/h2\u003e\n \u003cp\u003eHigh-quality single crystals of CIPS were grown by the chemical vapor transport method without a transport agent. Copper powder, indium powder, red phosphorus, and sulfur were placed in a quartz tube according to stoichiometric ratios. The temperature of the evaporation and crystallization zones was set to 750 ℃ and 650 ℃, respectively, and the entire growth process lasted for 5 days. \u003csup\u003e34\u003c/sup\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eFerroelectric Measurements Under Pressure\u003c/h3\u003e\n\u003cp\u003eTo facilitate the testing of \u003cem\u003eP-E\u003c/em\u003e loops, Au electrodes were deposited through direct current sputtering. The areas of electrodes range from 0.0095 to 0.0113 mm\u003csup\u003e2\u003c/sup\u003e. The standard two-probe ferroelectric measurements were carried out using a piston-cylinder pressure cell. Ferroelectric hysteresis loops were recorded using a commercial ferroelectric tester (Precision Multiferroic, Radiant Technologies).\u003c/p\u003e\n\u003ch3\u003eIn Situ Raman\u003c/h3\u003e\n\u003cp\u003eIn Raman experiments, a pair of diamond anvils with a flat top of 1000 \u0026micro;m was used, and a CuBe gasket was pre-indented to \u0026sim; 80 \u0026micro;m in thickness. A sample hole of \u0026sim; 500 \u0026micro;m was drilled in the center of the pre-indented gasket. Silicone oil was used as a pressure-transmitting medium (PTM) to ensure hydrostatic pressure conditions. The pressure was gauged at room temperature by monitoring the shift of the ruby R1 fluorescence line. \u003csup\u003e35\u003c/sup\u003e\u003cem\u003eIn-situ\u003c/em\u003e high-pressure Raman experiments were carried out using a SpectraPro HRS-500 spectrometer with excitation lasers with a wavelength of 532 nm.\u003c/p\u003e\n\u003ch3\u003eIn situ SCXRD\u003c/h3\u003e\n\u003cp\u003eIn SCXRD experiments, a pair of diamond anvils with a flat top of 500 \u0026micro;m was used, and a T-301 stainless-steel gasket was pre-indented to \u0026sim; 40 \u0026micro;m in thickness. The PTM also was silicone oil. \u003cem\u003eIn-situ\u003c/em\u003e high-pressure SCXRD experiments were performed on a Bruker D8QUEST diffractometer with Mo K\u0026alpha; radiation (\u0026lambda;\u0026thinsp;=\u0026thinsp;0.71073 \u0026Aring;). Diffraction data were collected by \u0026omega;- and \u0026phi;- scan methods, and lattice parameters and volumes were determined using APEX3 software.\u003c/p\u003e\n\u003ch3\u003eNumerical Calculation\u003c/h3\u003e\n\u003cp\u003eTo calculate the polarization, we used the point charge model, \u003csup\u003e3\u003c/sup\u003e where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(P=\\frac{1}{V}(-e\\sum _{i}{N}_{i}{z}_{i}\\)\u003c/span\u003e\u003c/span\u003e), \u003cem\u003eV\u003c/em\u003e is the unit cell volume, \u003cem\u003ee\u003c/em\u003e is the electron charge, \u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the ionic valence state, \u003cem\u003ez\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the projection of atomic position vector in an unit cell along the z direction. The reduced atom model is shown in Figure S8. The Cu\u003csup\u003e1+\u003c/sup\u003e shift downward and the In\u003csup\u003e3+\u003c/sup\u003e and P\u003csup\u003e4+\u003c/sup\u003e-P\u003csup\u003e4+\u003c/sup\u003e pair shift upward. The positive charge center is located at the atomic position (\u003cem\u003ez\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e) of Cu, In and P-P pairs respectively and the negative charge center is located at the average position of six S atoms. We calculated the electric dipoles by using the relative negative and positive charge positions, and divided S charges proportionally to Cu, In and P forming dipoles respectively. The atomic positions are shown in Table\u0026nbsp;1. The negative charge center of S atoms is at (4.79+4.80+4.83+8.13+8.15+8.19)/6=6.48 \u0026Aring; and positive center of P-P pair is at (7.62+5.40)/2=6.51 \u0026Aring;. The macroscopic polarization is thus [(4.27\u0026ndash;6.48)\u0026times;1\u0026times;occupancy (Cu(4)) + (4.72\u0026ndash;6.48)\u0026times;1\u0026times;occupancy (Cu(1)) + (5.09\u0026ndash;6.48)\u0026times;1\u0026times;occupancy (Cu(2)) + (5.50\u0026ndash;6.48)\u0026times;1\u0026times;occupancy (Cu(3)) + (8.03\u0026ndash;6.48)\u0026times;1\u0026times;occupancy (Cu(6)) + (6.69\u0026ndash;6.48)\u0026times;3\u0026times;1 (In) + (6.51\u0026ndash;6.48)\u0026times;4\u0026times;2 (P)]\u0026times;1.602\u0026times;10\u003csup\u003e\u0026minus;19\u003c/sup\u003e\u0026times;10\u003csup\u003e6\u003c/sup\u003e\u0026times;10\u003csup\u003e\u0026minus;8\u003c/sup\u003e\u0026times;4 /(unit cell volume), where the \u0026minus;\u0026thinsp;1.602\u0026times;10\u003csup\u003e\u0026minus;19\u003c/sup\u003e is the electron charge, 4 indicates that there are four such reduced atomic models in an unit cell, the unit cell volume is 838.1931\u0026times;10\u003csup\u003e\u0026minus;24\u003c/sup\u003e cm\u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe work was partly supported by the National Key R\u0026amp;D Program of China grant 2018YFA0305703, the National Natural Science Foundation of China grant No. 12274193,\u0026nbsp;2074164,\u0026nbsp;12004161 and 11904281, the Stable Support Plan Program of Shenzhen Natural Science Fund under grant No. 20200925152415003, the Guang dong Basic and Applied Basic Research Foundation of 2022A1515010044. J. L. Zhu and Y. L also acknowledged the Major Science and Technology Infrastructure Project of Material Genome Big-science Facilities Platform supported by Municipal Development and Reform Commission of Shenzhen. Some experiments were supported by the Synergic Extreme Condition User Facility.\u0026nbsp;J.W. also acknowledges support from the Guangdong Provincial Key Laboratory Program (2021B1212040001) from the Department of Science and Technology of Guangdong Province, and the startup grant from the Southern University of Science and Technology (SUSTech), China.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJ.Z. and J.W. initiated this work. Y.B. prepared the samples of CIPS. X.Y. carried out the ferroelectric and Raman experiments under pressure with help from Q.Z. and Y.L., C.J. performed SCXRD. Z.X., L.C. and S.W. provided experimental equipment support. X.Z. participated in the discussion of experimental results. X.Y. wrote the first draft, X.Y., J.W. and J.Z. co-rewrote the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting financial interests:\u003c/strong\u003e The authors declare no competing financial interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eL. H. Ong, J. Osman, D. R. Tilley, Landau theory of second-order phase transitions in ferroelectric films. \u003cem\u003ePhysical Review B\u003c/em\u003e \u003cstrong\u003e63\u003c/strong\u003e, 144109 (2001).\u003c/li\u003e\n\u003cli\u003eP. Marton, I. Rychetsky, J. Hlinka, Domain walls of ferroelectric BaTiO\u003csub\u003e3\u003c/sub\u003e within the Ginzburg-Landau-Devonshire phenomenological model. \u003cem\u003ePhysical Review B\u003c/em\u003e \u003cstrong\u003e81\u003c/strong\u003e, 144125 (2010).\u003c/li\u003e\n\u003cli\u003eR. 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Bell, Calibration of the ruby pressure gauge to 800 kbar under quasi‐hydrostatic conditions. \u003cem\u003eJournal of Geophysical Research: Solid Earth\u003c/em\u003e\u003cstrong\u003e91\u003c/strong\u003e, 4673-4676 (1986).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"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-2620145/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2620145/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCuInP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e with robust room-temperature ferroelectricity has recently attracted much attention due to the spatial instability of its Cu cations and the van der Waals (vdW) layered structure. Herein, we report a significant enhancement of its remanent polarization by more than 50% from 4.06 to 6.36 \u0026micro;C cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e under a small pressure between 0.26 to 1.40 GPa. Comprehensive analysis suggests that even though the hydrostatic pressure suppresses the crystal distortion, it initially forces Cu cations to largely occupy the interlayer site, causing the spontaneous polarization to increase. Under intermediate pressure, the condensation of Cu cations to the ground state and the polarization increase due cell volume reduction compensate each other, resulting in a constant polarization. Under high pressure, the migration of Cu cations to the center of the S octahedron dominates. These findings improve our understanding of this fascinating vdW ferroelectric material, and suggest new ways to improve its properties.\u003c/p\u003e","manuscriptTitle":"Anomalous polarization enhancement in a vdW ferroelectric material under pressure","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-02-28 22:28:56","doi":"10.21203/rs.3.rs-2620145/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6d86866e-1de9-4652-a0ce-0ba05c78e648","owner":[],"postedDate":"February 28th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":19500200,"name":"Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics"},{"id":19500201,"name":"Physical sciences/Materials science/Materials for devices/Information storage"}],"tags":[],"updatedAt":"2023-07-21T07:25:18+00:00","versionOfRecord":{"articleIdentity":"rs-2620145","link":"https://doi.org/10.1038/s41467-023-40075-6","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2023-07-18 04:00:00","publishedOnDateReadable":"July 18th, 2023"},"versionCreatedAt":"2023-02-28 22:28:56","video":"","vorDoi":"10.1038/s41467-023-40075-6","vorDoiUrl":"https://doi.org/10.1038/s41467-023-40075-6","workflowStages":[]},"version":"v1","identity":"rs-2620145","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2620145","identity":"rs-2620145","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}