Structural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia (m-ZrO₂)

Structural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia (m-ZrO₂) | 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 Structural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia ( m -ZrO₂) Gabriel L. Murphy, Sara Gilson, Karin Popa, Damien Prieur, Sven Schenk, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7027845/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Dec, 2025 Read the published version in Communications Chemistry → Version 1 posted You are reading this latest preprint version Abstract Monoclinic zirconia ( m- ZrO 2 ) has been long of fundamental chemical interest and crucially it serves as a key barrier to radionuclide release at the fuel–zircaloy interface in spent nuclear fuel. However, the incorporation of transplutonic elements like americium in m- ZrO₂ remains poorly understood. Using a combination of microscopy, diffraction and high resolution X-ray spectroscopic techniques we have examined the doping of m- ZrO 2 with 5 mol % Am. We show Am enters m- ZrO 2 tetravalently, where its solubility is less than 1.05 mol %, m- (Am 4 + 0.0105 Zr 0.9895 )O 2 , attributed to the large Am 4+ cation coupled with its preference under the synthesis conditions to revert to its trivalent state, where excess Am adopts a C-type (Am 4+ / 3+ 1−x Zr x ) 2 O 3+x phase in space group Ia -3. The known reversible high temperature phase transformation of m- ZrO 2 to tetragonal is further shown to be reduced from 1150 o C to 1050 o C via Am 4+ incorporation. The investigation provides critical insight into behaviour of transplutonic elements with m- ZrO₂. Physical sciences/Chemistry/Materials chemistry Physical sciences/Materials science/Materials for energy and catalysis/Corrosion Physical sciences/Energy science and technology/Nuclear energy/Nuclear waste Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The chemistry of zirconia materials has been prolifically studied due to their superb properties which have found application in a variety of topics ranging from advanced ceramics, 1 oxygen censors, 2 optical fibres 3 to fuel cells 4 among others. Core to these applications, is the well-defined phase relationships among the zirconia polymorphs, crystalline monoclinic, tetragonal and cubic, in addition to the amorphous phase. 5 The stabilities of these phases have been well established in literature to arise from factors including particle size, gas phase pressure, temperature, or doping level. 5 , 6 In the case of monoclinic ZrO 2 ( m- ZrO 2 ), the structure can be readily stabilised to its cubic form through doping with Y 3+ or via heating where the transformation temperature to either cubic or tetragonal forms is reduced with progressive Y 3+ incorporation. Y 3+ -stabilised ZrO 2 (YSZ) is a prominent and topical ion conductor for oxide fuel cells due the oxygen vacancies induced from doping. 5 The success and application of YSZ has subsequently inspired many further studies to more broadly examine trivalent cation doping of ZrO 2 in order to build upon the achievements and properties of YSZ whilst providing understanding for m- ZrO 2 . 7 – 11 Despite the exceptional literature available on m- ZrO 2 based compounds and doping, a notable exception is the role minor actinides and 5 f element chemistry have in influencing the stability of zirconia materials. Unlike lanthanide ( Ln ) elements where relatively similar trends are observed between different Ln ’s when doping ZrO 2 due to the similar chemical properties of individual elements, actinide elements exhibit increasingly more diversity, due the variation in 5 f bonding of specific actinide elements. 12 This is well exemplified by the chemistry of neighbouring Pu and Am, where in the oxide state, the former can adopt oxidations states ranging from 3 + to 6 + in comparison to Am which is typically confined to 3 + and 4+. The difference is linked to the transition from itinerant to localised behaviour of the 5 f electrons in Pu and Am respectively. Critically, this prevents direct extrapolation of chemical behaviour occurring for lighter actinide elements like Th, U and Pu to heavier and more difficult to handle transuranic elements including Am and Cm. Practically, researchers have often turned to other non-active analogues, particularly Ln based, to help probe the chemistry of minor actinides like Am, for example with Nd, due to more similar chemical trends that have been observed. 13 However, recent comparative studies have highlighted associated shortcomings in inferring chemical behaviour through such surrogate studies of Am and Nd, 14 pointing to the pertinence of utilising actual Am materials in experimental investigations. Considering the known chemistry of m -ZrO 2 , the understanding of the role of Am doping on the m -ZrO 2 polymorph is essentially not established. For instance, the level of solubility in the structure, the effect on stabilising the higher temperature tetragonal form in addition to the overall redox chemistry are not defined. Pertinently, there is no known Am-Zr-O phase diagram to help provide insight unto these questions. If the chemistry of Nd is to be followed, previous work suggests the solubility is limited to 1 mol % where after a defect fluorite structure is quickly precipitated followed by eventual cubic pyrochlore structure in space group Fd-3m (No. 227). 15 Similarly, if the chemistry of neighbouring Pu is to be followed, the solubility is found to be less than 1 mol % with a similar progression to a F-type cubic structure. 16 It is noteworthy that both Am and Pu are capable of stabilizing in the tetravalent oxidation state, a feature shared with Ce 4+ , that is often used as a surrogate for Pu in oxide investigations. However, Ce 4+ can be incorporated into the ZrO 2 structure at much higher concentrations than Pu 4+ , up to 20 mol%. 10 Other larger tetravalent actinides, such as Th 4+ , have a solubility limit of approximately 7 mol% before phase separation occurs. Moreover, despite the other quoted similar chemistry of certain actinides and lanthanides, in the case of m -ZrO 2 little translational comparison can be made and direct investigation is necessary to properly determine specific chemical behaviour in the case of Am. The inclusion of Am in zirconium-based materials has been the firm focus of studies in relation to its disposal in the context of nuclear waste form science. For instance, a good body of literature exists for pyrochlore bearing americium oxides, Am 2 Zr 2 O 7 . 17 – 19 This is due to the exceptional ceramic and radiation resistance properties offered by pyrochlores as materials for the immobilisation of radionuclides. 20 In the case of incorporation in other zirconia-based materials such as monoclinic and cubic stabilized zirconia far less is known. Raison and Haire 21 , 22 examined the incorporation of americium into ZrO 2 a part of the ternary AmO 2 -Cm 2 O 3 -ZrO 2 AmO 2 -Y 2 O 3 -ZrO 2 systems finding Am 4+ could be stabilised within these structures. However, there appears to be no available literature on the incorporation of Am into m- ZrO 2 as a part of the binary Am 2 O 3 -ZrO 2 system besides for pyrochlore. The extensive literature available on YSZ suggests that the similar ionic radii between Y 3+ and Am 3+ will favour incorporation of Am 3+ into ZrO 2 and induce stabilisation towards the cubic form. However, there are no experimental studies attesting to this nor examining the high temperature behaviour and stability. This is particularly relevant since it is known that on the surface of UO 2 based spent nuclear fuel (SNF) zircalloy cladding, a surface layer of ZrO 2 forms arsing during nuclear reactor operations which can readily interact with the fuel elements and incorporate critical radionuclides like Am. 23 Considering what is described as a global nuclear renaissance in the use of nuclear energy involving the use of higher burn up fuels, 24 – 28 the inevitable increased pellet-cladding interactions that occur will convey enhanced actinide ZrO 2 interactions and derived phases forming. Consequently, understanding the solid-state chemistry and chemical behaviour of ZrO 2 derived phases with minor actinides, particularly Am, is a pertinent endeavour. Due to the relevance of zirconia-based materials to SNF disposal but also to the considerable general interest zirconia materials science research, 29 we have examined the incorporation of 5% mol Am-doped ZrO 2 . The structural, microstructural and local structure of the synthesized material, particularly Am incorporated m -ZrO 2 , is examined using powder X-ray diffraction, electron microscopy, Raman, Am L 3 -edge and Zr K-edge X-ray absorption fine structure (X-ray absorption near edge and extended X-ray absorption fine structure) as well as Am M 5 -edge high-energy resolution XANES (HR-XANES) spectroscopic techniques. The thermal dependence and stability of identified phases is further examined using high temperature in situ powder X-ray diffraction. The results of this investigation are discussed with respect to the literature of monoclinic and cubic stabilised ZrO 2 , highlighting the role and comparative action of minor actinide chemistry in stabilising specific ZrO 2 polymorphs. Experimental Synthesis This work involved the use of Am-241 which decays primarily via α emission with a t 1/2 of 432.2 years. For this, the Minor Actinide Laboratory available at the European Commission Joint Research Centre, Karlsruhe, Germany were used for safe and correct handling. m- ZrO 2 was synthesized following a high temperature solid state method. AmO 2 was weighed out in a hot cell-glovebox and mixed with pre-acquired ZrO 2 powder in stoichiometric quantities targeting 5 mol % addition of Am. The two reagents were homogenised using a mortar and pestle and ground into a fine powder. This powder was then compacted into a green pellet using a hydraulic press that applied 8 kN of force. Then, the pellet was transferred into a Mo crucible and placed in a furnace. The furnace was heated at a ramp rate of 200°C/h to 1600°C, held at 1600°C for 48 hours, and then cooled at a rate of 200°C/hr. In order to retain the Am in its tetravalent oxidation state and allow it to better react with the m- ZrO 2, the furnace used an Ar atmosphere that possessed 1000 ppm H 2 O. The sintered ceramic pellet was then removed for subsequent analysis. Powder X-ray Diffraction To safely and routinely study the synthesized Am 4+ incorporated m- ZrO 2, a piece of the sintered pellet was separated, ground to a fine powder and imbedded in embedded in Loctite® Double Bubble 2-part epoxy adhesive (5 minutes at 20 o C) resin matrix mounted to a standard powder X-ray diffraction (PXRD) holder. Measurements were performed using a Bruker D8 powder X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA). Measurements were recorded from 15 to 100 2θ in step sizes of 0.0017 o with counting time of 2s. Collected data was analysed using the Rietveld method as implemented in the program GSAS-II. 30 The peak shapes were modelled using a pseudo-Voigt function and the background was estimated using a 6–12 term shifted Chebyshev function. The atomic parameters of phases examined were fixed based on determined chemistries. The scale factor, detector zero-point and lattice parameters were refined together with the peak profile parameters. High Temperature X-ray Powder Diffraction To study the thermal dependence of 5% mol Am doped ZrO 2 , high temperature X-ray powder diffraction measurements (HT-PXRD) were performed on a Bruker D8 powder X-ray diffractometer. Approximately 15 mg of sample was loaded on a Pt heating stript and placed in an Anton Paar HTK2000 heating chamber. The small amount of sample was loaded to minimise radiation exposure risks. The sample was heated from 30 to 1100 o C with the sample kept under vacuum conditions. SEM The microstructure and homogeneity of the sintered pellet was investigated directly following synthesis with scanning electron microscopy adapted for the analysis of radioactive samples. For this purpose, a Philips XL40 microscope equipped with secondary and back-scattered electron detectors (SE and BSE) for the imaging and energy dispersive X-ray spectroscopy (EDX) for the analysis of the elemental distribution in the samples was used. A small fragment of non-coated ceramic pellets was deposited on a carbon sticker for the measurements. The sample was not polished and measured neat. Raman Spectroscopy Raman spectroscopic measurements of the sintered pellet were conducted using customized α-tight capsules as described previously. 14 Measurements were conducted with a Jobin Yvon T64000 Raman spectroscope, equipped with a 1800 grooves/mm holographic grating and a 50×magnification long focal (~ 1 cm) objective with an aperture of 0.5. A low-background, liquid nitrogen–cooled detector (HoribaSymphony) was used to acquire the signal. The instrument was calibrated using the T 2g band of a monocrystalline Si chip at 520.5 cm − 1 . Raman measurements were conducted using a 660 nm laser. The data acquisition and data treatment were done with Labspec 6. Additional fitting of the Raman peaks was conducted with Origin Pro, using Lorentzian peak functions to describe the peak profile. High-Energy Resolution X-ray Absorption Near Edge Structure Spectroscopy (HRXANES) For the determination of the oxidation states of Am present within the 5 mol % doped ZrO 2 material, Am M 5 -edge HR-XANES experiments were conducted at the ACT station of the beamline for catalysis and actinide research (CAT-ACT beamline) of the KIT Light source at the Karlsruhe Research Accelerator (KARA). 31 A Si(111) double-crystal monochromator (DCM) was used to monochromatize the incident beam. The beam was focused to 1000 × 1000 µm and further narrowed down by slits leading to a sample spot size of about 500 ×200 µm (vertical × horizontal). HR-XANES spectra were acquired with a Johann-type X-ray emission spectrometer, using up to four Si(220) (Saint-Gobain, France) analyzer crystals with 1.0 m bending radius and an AXAS-M silicon drift detector (SDD, KETEK GmbH), which together with the sample were arranged in a Rowland circle geometry. 31 The spectrometer and sample were encased in a He glove box, to minimize the influence of atmospheric O 2 and N 2 during measurements. For the sample itself, a about 1 mg of sample was diluted in BN, pressed into a pellet, placed in a double containment and sealed twice by 13 µm Kapton foil respectively. The HR-XANES spectra were measured with a step size of 0.1 eV from − 5 to + 20 eV from the white line (WL) of the respective edge and 0.5 eV in all other parts of the spectra. Collected spectra were calibrated against a Am 3+ -nPr-BTP reference sample. This was done by fixing the measured WL position of the reference sample to known literature values for Am 3+ (3888.5 eV). 32 The spectra were thereafter normalized to have a WL intensity of close to one. Four spectra were averaged for 5 mol % Am-doped ZrO 2 . Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) Am L 3 - and Zr K-edge extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) measurements of the 5 mol % Am-doped ZrO 2 were conducted at the KIT Light Source at the INE-beamline 33 using the KARA accelerator. Measurements were conducted at the Am L 3 - and Zr K-edges. The Larch software was used to extract EXAFS spectra from the raw absorption data. 15 Experimental Zr K-edge EXAFS spectra were Fourier-transformed using a Hanning window over the full k space range available 3.5–10.5 A˚ −1 Curve fitting was performed in k 3 for R values in the range 1.2–4.3 Å. Phases and amplitudes for the interatomic scattering paths were calculated with the ab initio code FEFF8L. 16 Coordination numbers for cation-cation shells were fixed. Once satisfactory results were obtained, the constraints were removed with no significant variation Results and Discussion Synthesis, Structural and Spectroscopic Characterization The synthesis of 5 mol % Am-doped ZrO 2 was attempted using conditions of a flowing Ar atmosphere at 1600 o C. However, subsequent PXRD analysis failed to reveal the targeted m -ZrO 2 phase with Am incorporation. To ameliorate this, more oxidising conditions involving 1000 ppm H 2 O in Ar atmosphere were employed. This was targeted to help retain Am in its tetravalent oxidation state which is more conducive to incorporation into the m -ZrO 2 structure. Subsequent collected PXRD data on the 5% mol Am-doped ZrO 2 from this resulting synthesis was analysed using the Rietveld method, where a monoclinic structure in space group P 2 1 / c consistent with m -ZrO 2 could be well refined against the diffraction pattern. The Rietveld profile is provided in Fig. 1 with determined lattice parameters in Table 1 . The determined lattice volume was determined to be 141.714(4) Å 3 , 1.063 Å 3 larger than the expected value for pure m -ZrO 2 of 140.651(3) Å 3 . 34 Such a lattice expansion is consistent with the inclusion of the Am cation within the crystal structure of m -ZrO 2 , due to its larger size than Zr. 35 Table 1 Refined lattice parameters for 5 mol % Am-doped ZrO 2 for determined phases m- (Am 0.0105 Zr 0.9895 )O 2 and C-type (Am 1 − x Zr x ) 2 O 3+x in addition reference data for m -ZrO 2 , 34 Am 2 O 3 and Am 2 Zr 2 O 7 . Phase m- (Am 0.0105 Zr 0.9895 )O 2 m -ZrO 2 34 (Am 1 − x Zr x ) 2 O 3+x Am 2 O 3 + x 36 Am 2 Zr 2 O 7 17 SG P 2 1 / c P 2 1 / c Ia-3 Ia-3 Fd -3 m a (Å) 5.1626(4) 5.14604(6) 10.3914(18) 10.92(2) 10.66849(4) b (Å) 5.2156 (2) 5.21162(7) - - - c (Å) 5.3314(4) 5.31308(7) - - - Β ( o ) 99.1744(2) 99.222(1) - - - V (Å 3 ) 141.714(4) Å 3 140.651(3) 1122.1(6) 1302.171(5) 1214.252(4) Additional to the main phase m -ZrO 2 structure, a minor secondary phase could be observed with reflections at approximately near 30 o and 34 o . By considering the position of these reflections against possible phases that may occur in the Zr-Am-O system, it was determined that the best candidate matching phases would be a cubic structure of origin, particularly pyrochlore type in space group Fd -3 m or C-type Am 2 O 3 sesquioxide in space group Ia -3. Other potential phases such as AmO 2 or a secondary phase m -ZrO 2 with depleted Am content were ruled out based on position of reflections. Subsequently, refinements were performed using pyrochlore and C-type Am 2 O 3 sesquioxide type models to determine the origin of this secondary phase. To these phases, unit cell volumes of 1124.2(3) Å 3 and 1122.1(6) Å 3 were determined respectively. Supplementary Information Note 1, provides details of the subsequent refinements performed. However, from the fitting values alone it was not definitively clear which of these phases better describes the secondary phase occurring, although a more consistent fit could be observed with the C-type Am 2 O 3 structure used. To shed light on the chemical phase assemblage within 5 mol % Am-doped ZrO 2 , SEM-EDS measurements were performed, as illustrated in Fig. 2 . These measurements indicated the inclusion of Am within the ZrO 2 matrix, however due to resolution issues could not discern nor clearly identify the minor secondary phase observed in PXRD measurements. Nevertheless, it could be observed that regions of the EDS maps showed variable areas that are low in quantity which are Am rich and more commonly regions that are Am poor. These results suggest that the respective phase composition is of a secondary minor phase that is Am rich type in addition to a major phase that is low in Am, like low Am-doped m -ZrO 2 . These observations are consistent with the results of the Rietveld PXRD analysis. Raman data were acquired for a pristine m- ZrO 2 specimen and a fragment of the sintered 5 mol % Am doped ZrO 2 pellet. The data were analysed using the Origin Pro 2025 software. The fitted spectrum of the pristine m- ZrO 2 sample, along with the measured spectrum of the Am-doped m-ZrO 2 sample, are displayed in Fig. 3 . Within the measured spectral range, 13 characteristic Raman bands are clearly distinguishable. Compared to the pristine material, the Raman bands of the Am-containing sample exhibit an average shift of 6 cm − 1 to lower frequencies. This downshift suggests bond lengthening due to the substitution of smaller Zr 4+ cations with larger Am 4+ cations. The observed shift is smaller than that reported for monoclinic ZrO 2 doped with 10 mol% Ce 4+ , which shows an average downshift of 9 cm − 1 relative to our pristine m-ZrO 2 . This is consistent with the lower concentration of Am 4+ in our sample, while still clearly supporting its incorporation into the monoclinic structure. A full list of observed peak positions and their corresponding Raman mode assignments is provided in Supplementary Information Note 2, together with the fitted Raman spectrum of the Am-doped sample. Despite probing eight different locations on the Am-doped ZrO 2 sample, Raman spectroscopy did not reveal any signature of the secondary C-type cubic phase identified in our PXRD investigations. Given its low phase fraction (~ 5%) and the surface-sensitive nature of the Raman method, it is conceivable that this phase remained undetected due to limited spatial distribution, low Raman scattering efficiency, or overlap with stronger signals from the monoclinic ZrO 2 matrix. Am M 5 -edge HR-XANES To understand the redox chemistry of Am within the 5 mol % Am-doped ZrO 2 sample and particularly understand redox speciation between separate phases, Am M 5 -edge HR-XANES experiments were performed. The normalised spectra are plotted in Fig. 4 in addition to the standards Am 4+ O 2 and Am 3+ VO 4 . Evident from Fig. 4 is the’ mixed spectrum of the 5 mol % Am-doped ZrO 2 sample which shows contributions from both Am 4+ and Am 3+ , where the Am 3+ dominates The HR-XANES spectra suggesting that the Am redox states are likely associated between Am 4+ and the m -ZrO 2 structure due to know chemistries of the material preferring tetravalent dopants 1 , 11 whereas the secondary phase possesses Am 3+ and likely some Am 4+ . Note that Am 3+ in a cubic structure exhibits double peak structure with not negligible peak intensity at the energy position of the Am 4+ main absorption peak. This peak gains additional intensity due to the Am 4+ contribution. This is shown in Fig. 4 through the U 0.80 Am 3+/4+ 0.20 O 2 reference which suggests mixed Am 3+/4+ is found within a cubic environment. This experimental evidence supports the previous arguments from PXRD analysis that it is potentially C-type Am 2 O 3 or pyrochlore-type Am 2 Zr 2 O 7 . Zr K-edge XANES and EXAFS Both Zr K-edge XANES and EXAFS analyses indicate that the local environment of the Zr atoms corresponds to a monoclinic structure. As shown in Fig. 5 , the normalized Zr K-edge XANES spectrum of the 5 mol % Am-doped ZrO 2 sample closely resembles that of a m- Zr 4+ O₂ reference. Furthermore, the Zr K-edge k 3 .χ(k) EXAFS spectrum was successfully fitted using a structural model based on the monoclinic phase. The structural parameters obtained from the fit (Table 2 ) are consistent with the monoclinic space group and suggest the incorporation of americium into the structure, as evidenced by slightly increased interatomic distances compared to pure m- ZrO₂. 37 Considering both the quality of the EXAFS fitting and the detection limits of the technique, the data support the conclusion that zirconium is present predominantly in the monoclinic ZrO₂ phase, indicating that the secondary phase is mainly americium-based. Table 2 Structural parameters derived from the analysis of the EXAFS signal of the Zr K-edge for 5 mol % Am-doped ZrO 2 . Sample Shell R(A˚) CN σ 2 (A˚ 2 ) ZrK Rf = 1.5% Zr – O 2.083 (5) 2 0.012 (1) Zr - O 2.176 (5) 3 0.011 (2) Zr - O 2.285 (5) 2 0.011 (2) Zr - M 3.52 (1) 7 0.007 (1) Am L 3 -edge XANES The oxidation state of Am within the 5 mol % Am-doped ZrO 2 sample was investigated using Am L 3 -edge XANES spectroscopy. As shown in Fig. 6 , the XANES spectrum of the 5 mol % Am-doped ZrO 2 sample lies between those of Am 3+ and Am 4+ reference compounds, indicating the presence of mixed-valence states. Linear combination fitting reveals that approximately 80 ± 1% of the americium is present in the trivalent state, with the remaining 20 ± 1% in the tetravalent state. This result is in accordance with the qualitative fingerprint analyses of the Am M 5 -edge HR-XANES spectra. Phase Composition Determination via Vegard’s Law From the experiments and analysis thus far, it is shown the synthesis of 5 mol % Am-doped ZrO 2 under mildly oxidising conditions results in the formation of m -ZrO 2 with an additional secondary phase at low amount (< 5 phase %) in which the Am oxidation state is mixed, occurring as trivalent and tetravalent mixed. Am L 3 -edge XANES analysis shows that 80% of the Am occurs as trivalent whereas as the 20% as tetravalent; this result agrees with the results of the Am M 5 -edge HR-XANES spectroscopic technique, which is more sensitive to small variations of the actinide oxidation states but quantitative analyses are generally more difficult. When the phase diagram of ZrO 2 is considered, 5 m -ZrO 2 is not expected to host oxygen defects which would occur when Am 3+ is incorporated within it. Rather, it is more expected Am 4+ would incorporate into m -ZrO 2 and the identified secondary phase accordingly contains Am 3+ . Based on the XANES analysis, it implies that of the 5 mol % Am that was used in the synthesis, only 1 mol % enters m -ZrO 2 and it follows this is tetravalent Am. The subsequent 80% extra is subsequently attributed primarily to trivalent and some tetravalent Am that reports to the secondary phase, which Rietveld analysis suggests is likely C-type Am 2 O 3 or pyrochlore type Am 2 Zr 2 O 7 , i.e. an Am rich phase consistent with the EXAFS and EDS measurements. Invariably, the observed phenomena of phase separation is likely associated with limited solubility of Am 4+ within m -ZrO 2 . Subsequently, in order to identify the origin of the secondary cubic phase occurring whilst simultaneously determining the solubility of Am with m -ZrO 2 and its composition, calculations were performed using Vegard’s Law (Eq. 1.) with determined lattice parameters from Rietveld analysis employed. The observed lattice expansion of the m -ZrO 2 phase with Am 4+ doping compared to the non-doped state will depend on the specific amount of Am that enters the lattice. Whereby, the observation of the secondary phases implies that the solubility limit has been reached in the main phase. Since Am was used in its tetravalent state during synthesis, further supported by XANES and particularly EXAFS analysis, the expansion of the m -ZrO 2 structure can be assumed to follow a linear expansion via substitution of Zr 4+ for Am 4+ as given in Eqs. 1 and 2. V = V o ​+x⋅ΔV (1) Zr 1 − x Am x O 2 (2) Using the determined lattice volume from the Rietveld method in Table 1 . and comparing it to reference data from Gualtieri et al. 34 provided also in Table 1 , by Eq. 1. the necessary amount of Am 4+ required to induce the observed 0.75% lattice expansion would be x = 0.0105 when using ionic radii of 0.90 Å and 0.78 Å for Am 4+ and Zr 4+ respectively in CN = 7. 35 Accordingly, for the m -ZrO 2 structure identified, this will correspond to a formula of (Am 0.0105 Zr 0.9895 )O 2 , i.e. only 1.05 mol % incorporation into m -ZrO 2 . This value is considerably lower than the 5 mol % addition used in the synthesis which will subsequently result in precipitation of a secondary Am rich phase, as is described and observed. Notably, this determined value from Vegard’s law is consistent with the Am L 3 -edge XANES analysis and appears to corroborate both the amount of phase separation but also difference in Am valence between the phases, namely m -ZrO 2 contains only Am 4+ and the secondary Am rich phase is predominantly Am 3+ . When the Zr K-edge XANES results are considered, they show that the Zr appears to be predominantly associated with the tetragonal m -ZrO 2 structure. This implies that the secondary phase is likely very poor in the amount of Zr contained and rather rich in Am. Accordingly, it is suspected then the secondary phase is more likely C-type Am 2 O 3 + x compared to pyrochlore-type Am 2 Zr 2 O 7 . Nevertheless, this can be further confirmed and understood when examining determined lattice parameters from Rietveld refinements as was previously performed. For the pyrochlore-type Am 2 Zr 2 O 7 a unit cell volume of 1124.2(3) Å 3 was determined, this can be compared against the determined lattice volume of Am 2 Zr 2 O 7 synthesized by Belin et al. 17 of 1214.252(4) Å 3 . Since the unit cell of volume of 1124.2(3) Å 3 is considerably smaller than that determined by Bellin et al. , this would imply that the amount of Am with the pyrochlore structure is significantly less than the Am:Zr 1:1 ratio. Such an argument is difficult to support considering the noted significance ejection of Am from m -ZrO 2 which should coincide with only some minor Zr incorporation. In the case of C-type Am 2 O 3 + x a lattice volume of 1122.1(6) Å 3 was determined that is smaller than the reference C-type Am 2 O 3 + x value provided by Epifano et al. 36 of 1302.171 Å. For a C-type Am 2 O 3 structure occurring as a secondary phase it is likely that some Zr will be incorporated in the structure when it is ejected from the m -ZrO 2 phase. Since XANES measurements show Zr occurs as Zr 4+ this will lead to a contraction of the C-type Am 2 O 3 structure which is consistent with the trend in lattice parameters when the end member value of Epifano et al. 36 is compared to the solid solution determined here. Naturally, variations in oxygen stoichiometry, although difficult to quantify, will further influence the lattice parameters when comparing the investigations, nevertheless the results are still very consistent. Subsequently it is argued that the significant excess Am 3 that does not enter m -ZrO 2 results in the formation of a C-type (Am 1 − x Zr x ) 2 O 3+2x structure where x is small. Interestingly, the amounts of Am deposited between the phases observed is strikingly consistent with what is seen in the Nd 2 O 3 -ZrO 2 phase diagram, 15 suggesting in this instance there is good congruency between the behaviour of Am and Nd within ZrO 2 . High Temperature X-ray Diffraction A notable known behaviour of m -ZrO 2 is the ability to stabilise it to its tetragonal and cubic forms with increasing temperature where the transition temperature can be reduced via doping. An appropriate test of confirming incorporating Am 4+ within m -ZrO 2 can be achieved subsequently through HT-PXRD measurements and showing a reduced phase transformation temperature with doping. Accordingly, HT-PXRD measurements were performed on a small amount of 5 mol % Am-doped ZrO 2 material (15 mg) where the sample was heated sequentially to 1100 o C before cooling to RT. As shown in Fig. 7 which provides a portion of the collected data, the phase transformation the tetragonal form from the monoclinic can be observed between 900–1050 o C. This is significantly less than that known to occur for pure m -ZrO 2 which transforms to the tetragonal polymorph usually at 1100 o C. 5 This indicates the inclusion of the Am 4+ within m -ZrO 2 lowers the transition temperature as expected but corroborates its occurrence within the structure. Conclusions The solubility and structural chemistry of 5 mol % Am doped m -ZrO 2 has been determined through a combination of structure and spectroscopic techniques. Synthesis of a 5 mol % Am-doped ZrO 2 under mildly oxidising conditions is found to result in the formation of major phase m -(Am 4 + 0.0105 Zr 0.9895 )O 2 and a minor phase that is attributed to C-type (Am 3 + 1−x Zr x ) 2 O 3 structure, where x is small, via PXRD, HR-XANES, EXAFS measurements and supported by calculations using Vegard’s law. The known HT transformation from m -ZrO 2 to t -ZrO 2 occurring above 1100 o C is found to be reduced to below 1050 o C via HT-XRD in situ measurements, due to the inclusion of the Am 4+ cation in the lattice. The limited solubility of Am within m -ZrO 2 is attributed to its size in its trivalent form in addition to its trivalent stability at HT. Nevertheless, m -ZrO 2 is able to successfully immobilize the tetravalent form under prescribed conditions which otherwise would result in tetravalent occurrence, albeit with low solubility. When the results of the investigation are considered with the limited literature available on the Zr-Am-O system, despite there being no ZrO 2 -Am 2 O 3 phase diagram existing it appears that the behavior of Am appears to follow well with that of the ZrO 2 -Nd 2 O 3 phase diagram, given the observations made, and the results of this investigation can be used potentially with Nd data to infer the chemical behavior of Am within zirconia matrices. Such results are pertinent and relevant to the understanding of Am within waste form environments arising from the direct storage and reprocessing of SNF materials arising from nuclear energy production. Declarations Acknowledgments The authors are grateful to funding and support from the German Federal Ministry of Education and Research (BMBF), Project No. 02NUK060 that enabled this research. The experimental data used in this research were generated through access to the ActUsLab/FMR under the Framework of access to the Joint Research Centre Physical Research Infrastructures of the European Commission (RISE-241, Research Infrastructure Access Agreement N°36344/02). The authors give thanks to the Institute for Beam Physics and Technology (IBPT, KIT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA), and the KIT Light Source for provision of beamtime. The authors acknowledge funding from the European Research Council (ERC) Consolidator Grant 2020 under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 101003292). The authors thank Cedric Reitz (KIT-INE) for support during the experiments. The authors are also grateful for HT-XRD and SEM measurement support by Eckhard Dahms and Ramon Carlos Marquez. Author Contributions The project was conceived and developed by Gabriel L. Murphy, Nina Huittinen, Sara Gilson and Karin Popa . The research methodology, experimental planning, and formal analysis was conducted by Gabriel L. Murphy, Nina Huittinen, Sara Gilson and Karin Popa . The materials were synthesized by Karin Popa and Octavian Valu . PXRD measurements and analysis were performed by Gabriel L. Murphy and Olaf Walter . Raman measurements were performed by Jean-Yves Colle and Nina Huittinen . HR-XANES experiments were conducted by Sven M. Schenk, Harry Ramanantoanina, Tim Prüßmann, Tonya Vitova . EXAFS were recorded by Tim Prüßmann, Kathy Dardenne and Jörg Rothe . Manuscript writing, review and editing was performed by Gabriel L. Murphy and Nina Huittinen with input from all authors. References Clough, D. J. in Proceedings of the Conference on Raw Materials for Advanced and Engineered Ceramics: Ceramic Engineering and Science Proceedings. 1244-1260 (Wiley Online Library). Fidelus, J. D., Łojkowski, W., Millers, D., Smits, K. & Grigorjeva, L. in SENSORS, 2009 IEEE. 1268-1272 (IEEE). Tong, L. Growth of high-quality Y2O3–ZrO2 single-crystal optical fibers for ultra-high-temperature fiber-optic sensors. Journal of crystal growth 217 , 281-286 (2000). Ishihara, T., Sato, K. & Takita, Y. Electrophoretic deposition of Y2O3‐stabilized ZrO2 electrolyte films in solid oxide fuel cells. Journal of the American Ceramic Society 79 , 913-919 (1996). Bannister, M., Badwal, S. & Hannink, R. H. Science and technology of zirconia V . 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Communications Materials 5 , 274 (2024). https://doi.org/10.1038/s43246-024-00714-x Murphy, G. L. et al. Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies. Nature Communications 14 , 2455 (2023). https://doi.org/10.1038/s41467-023-38109-0 Gamble, K. A., Pastore, G., Andersson, D. & D Cooper, M. W. Bison Capability And Validation For U 3 Si 2 , Cr 2 O 3 -Doped UO 2 , Fecral, And Cr-Coated Zircaloy Atf Concepts. (Idaho National Lab.(INL), Idaho Falls, ID (United States), 2020). Kitano, K. & Akiyama, H. Research on the properties of high-burnup and high plutonium content mixed-oxide fuels. Journal of Nuclear Materials 572 , 154075 (2022). https://doi.org/https://doi.org/10.1016/j.jnucmat.2022.154075 Murphy, G. L. et al. The lattice contraction of UO2 from Cr doping as determined via high resolution synchrotron X-ray powder diffraction. Journal of Nuclear Materials 595 , 155046 (2024). https://doi.org/https://doi.org/10.1016/j.jnucmat.2024.155046 Solomon, A. P. et al. Atomic-scale structure of ZrO2: Formation of metastable polymorphs. Science Advances 11 , eadq5943 (2025). https://doi.org/doi:10.1126/sciadv.adq5943 Toby, B. H. & Von Dreele, R. B. GSAS-II: The Genesis Of A Modern Open-Source All Purpose Crystallography Software Package. Journal of Applied Crystallography 46 , 544-549 (2013). https://doi.org/doi:10.1107/S0021889813003531 Zimina, A. et al. CAT-ACT—A new highly versatile x-ray spectroscopy beamline for catalysis and radionuclide science at the KIT synchrotron light facility ANKA. Review of Scientific Instruments 88 (2017). https://doi.org/10.1063/1.4999928 Vigier, J.-F. et al. Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4. Inorganic Chemistry 62 , 9350-9359 (2023). https://doi.org/10.1021/acs.inorgchem.3c00251 Rothe, J. et al. The INE-Beamline for actinide science at ANKA. Review of scientific instruments 83 (2012). Gualtieri, A., Norby, P., Hanson, J. & Hriljac, J. Rietveld Refinement using Synchrotron X-ray Powder Diffraction Data Collected in Transmission Geometry using an Imaging-Plate Detector: Application to Standard m-ZrO2. Journal of Applied Crystallography 29 , 707-713 (1996). https://doi.org/doi:10.1107/S0021889896008199 Shannon, R. Revised Effective Ionic Radii And Systematic Studies Of Interatomic Distances In Halides And Chalcogenides. Acta Crystallographica Section A 32 , 751-767 (1976). Epifano, E. et al. Insight into the Am–O Phase Equilibria: A Thermodynamic Study Coupling High-Temperature XRD and CALPHAD Modeling. Inorganic Chemistry 56 , 7416-7432 (2017). https://doi.org/10.1021/acs.inorgchem.7b00572 Bondars, B. et al. Powder diffraction investigations of plasma sprayed zirconia. Journal of Materials Science 30 , 1621-1625 (1995). https://doi.org/10.1007/BF00375275 Additional Declarations There is NO Competing Interest. Supplementary Files AmZrO2SIFinal.docx Supplementary Information File TOC.docx Cite Share Download PDF Status: Published Journal Publication published 26 Dec, 2025 Read the published version in Communications Chemistry → 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-7027845","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":482987246,"identity":"444a8641-b248-4fe7-87c8-bb772b860f92","order_by":0,"name":"Gabriel L. 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represent collected data, refined model and the difference curve. The vertical purple and blue markers respectively represent the \u003cem\u003em-\u003c/em\u003e(Am\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.09895\u003c/sub\u003e)O\u003csub\u003e2\u003c/sub\u003e monoclinic (SG \u003cem\u003eP\u003c/em\u003e2\u003csub\u003e1\u003c/sub\u003e/\u003cem\u003ec\u003c/em\u003e) and C-type (Am\u003csub\u003e1-x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)O\u003csub\u003e3+x\u003c/sub\u003e (SG \u003cem\u003eIa\u003c/em\u003e-3) structures determined. Aberrations in the background are associated with the resin holder that the sample was mounted in. Rwp (%) = 1.95 % and Rp (%) = 2.70 %. Justification and detail for final determine structures is provided in the section “\u003cem\u003ePhase Composition Determination via Vegards Law”.\u0026nbsp;\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/15905ef37298dc34a56c796d.png"},{"id":86501133,"identity":"74816dd2-b192-41a7-9fa9-b15ce9a2abe6","added_by":"auto","created_at":"2025-07-11 11:08:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":146711,"visible":true,"origin":"","legend":"\u003cp\u003eSEM-BSE and -EDS on the Am L, Zr L and O K edges images of synthesized 5 % mol Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e. Samples were not polished and measured neat, hence the EDS signal is not observed in some regions of the sample due to the variable height.\u0026nbsp; \u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/eeaa2d90ca92abd187c22ddc.png"},{"id":86500616,"identity":"33623f42-cfb7-4d40-b7d8-c5a58e7df5a4","added_by":"auto","created_at":"2025-07-11 11:00:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":64558,"visible":true,"origin":"","legend":"\u003cp\u003eFitted Raman spectrum of pristine (undoped) m-ZrO\u003csup\u003e2\u003c/sup\u003e and the measured Raman spectrum of a small fragment of the sintered 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e pellet, both collected at 660 nm.\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/9a6e6640aecd5123c15dce0d.png"},{"id":86500618,"identity":"8d3c07e2-fa0e-4183-9558-8e1a45852b85","added_by":"auto","created_at":"2025-07-11 11:00:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":142603,"visible":true,"origin":"","legend":"\u003cp\u003eAm M\u003csub\u003e5\u003c/sub\u003e-edge HR-XANES spectra of the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample performed on the Am M\u003csub\u003e5\u003c/sub\u003e-edge with the standards Am\u003csup\u003e4+\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and Am\u003csup\u003e3+\u003c/sup\u003eVO\u003csub\u003e4\u003c/sub\u003e, as well as U\u003csub\u003e0.80\u003c/sub\u003eAm\u003csup\u003e3+/4+\u003c/sup\u003e\u003csub\u003e0.20\u003c/sub\u003eO\u003csub\u003e2-x \u003c/sub\u003e(sintered).\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage41.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/052220480c3eb5d992137c5c.png"},{"id":86501130,"identity":"b834486c-db01-4ca2-8421-9f2710f3f24e","added_by":"auto","created_at":"2025-07-11 11:08:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":279247,"visible":true,"origin":"","legend":"\u003cp\u003eNormalised Zr K XANES spectra 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample with a \u003cem\u003em-\u003c/em\u003eZr\u003csup\u003e+4\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e standard.\u003c/p\u003e","description":"","filename":"floatimage51.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/e764f81d33d2d3b96b334939.png"},{"id":86500626,"identity":"947d0c80-3549-41b7-8dd4-e80d3ca823cd","added_by":"auto","created_at":"2025-07-11 11:00:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":317022,"visible":true,"origin":"","legend":"\u003cp\u003eNormalised Am L\u003csub\u003e3\u003c/sub\u003e-edge spectra of the 5 mol % Am-doped ZrO\u003csub\u003e2 \u003c/sub\u003esample measured with the standards Am\u003csup\u003e4+\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and (U\u003csub\u003e0.85\u003c/sub\u003eAm\u003csup\u003e3+\u003c/sup\u003e\u003csub\u003e0.15\u003c/sub\u003e)O\u003csub\u003e1.99.\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage61.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/e0b21b41ffae1351e6e7ab86.png"},{"id":86501137,"identity":"731a0f2a-9a44-4dbb-93cd-b32e4ed645bb","added_by":"auto","created_at":"2025-07-11 11:08:10","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":381581,"visible":true,"origin":"","legend":"\u003cp\u003eHT-XRD of 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e where the transition from the \u003cem\u003em\u003c/em\u003e-(Am\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e)O\u003csub\u003e2 \u003c/sub\u003ephase (labelled M) can be observed to its tetragonal form (labelled T) reversibly between 900-1050 C\u003csup\u003eo\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/2f90248a3487a49459d9c1df.png"},{"id":101391495,"identity":"be40673f-0abe-479e-9a22-afbc9a8091d6","added_by":"auto","created_at":"2026-01-29 08:32:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3164933,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/d343d6cd-9756-43af-9980-986625692c92.pdf"},{"id":86500617,"identity":"42b23f8b-fbdf-4635-9586-0631989bef4f","added_by":"auto","created_at":"2025-07-11 11:00:10","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1508702,"visible":true,"origin":"","legend":"Supplementary Information File","description":"","filename":"AmZrO2SIFinal.docx","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/7339c333123fbebd5a9633b4.docx"},{"id":86500619,"identity":"6a0b61c9-8964-43bb-a82b-76799edba0d9","added_by":"auto","created_at":"2025-07-11 11:00:10","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":558836,"visible":true,"origin":"","legend":"","description":"","filename":"TOC.docx","url":"https://assets-eu.researchsquare.com/files/rs-7027845/v1/e424da5d380350f0b815795d.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eStructural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia (\u003cem\u003em\u003c/em\u003e-ZrO₂)\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe chemistry of zirconia materials has been prolifically studied due to their superb properties which have found application in a variety of topics ranging from advanced ceramics,\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e oxygen censors,\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e optical fibres\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e to fuel cells\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e among others. Core to these applications, is the well-defined phase relationships among the zirconia polymorphs, crystalline monoclinic, tetragonal and cubic, in addition to the amorphous phase.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The stabilities of these phases have been well established in literature to arise from factors including particle size, gas phase pressure, temperature, or doping level.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e In the case of monoclinic ZrO\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e), the structure can be readily stabilised to its cubic form through doping with Y\u003csup\u003e3+\u003c/sup\u003e or via heating where the transformation temperature to either cubic or tetragonal forms is reduced with progressive Y\u003csup\u003e3+\u003c/sup\u003e incorporation. Y\u003csup\u003e3+\u003c/sup\u003e-stabilised ZrO\u003csub\u003e2\u003c/sub\u003e (YSZ) is a prominent and topical ion conductor for oxide fuel cells due the oxygen vacancies induced from doping.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The success and application of YSZ has subsequently inspired many further studies to more broadly examine trivalent cation doping of ZrO\u003csub\u003e2\u003c/sub\u003e in order to build upon the achievements and properties of YSZ whilst providing understanding for \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan additionalcitationids=\"CR8 CR9 CR10\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e–\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eDespite the exceptional literature available on \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e based compounds and doping, a notable exception is the role minor actinides and 5\u003cem\u003ef\u003c/em\u003e element chemistry have in influencing the stability of zirconia materials. Unlike lanthanide (\u003cem\u003eLn\u003c/em\u003e) elements where relatively similar trends are observed between different \u003cem\u003eLn\u003c/em\u003e’s when doping ZrO\u003csub\u003e2\u003c/sub\u003e due to the similar chemical properties of individual elements, actinide elements exhibit increasingly more diversity, due the variation in 5\u003cem\u003ef\u003c/em\u003e bonding of specific actinide elements.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e This is well exemplified by the chemistry of neighbouring Pu and Am, where in the oxide state, the former can adopt oxidations states ranging from 3 + to 6 + in comparison to Am which is typically confined to 3 + and 4+. The difference is linked to the transition from itinerant to localised behaviour of the 5\u003cem\u003ef\u003c/em\u003e electrons in Pu and Am respectively. Critically, this prevents direct extrapolation of chemical behaviour occurring for lighter actinide elements like Th, U and Pu to heavier and more difficult to handle transuranic elements including Am and Cm. Practically, researchers have often turned to other non-active analogues, particularly \u003cem\u003eLn\u003c/em\u003e based, to help probe the chemistry of minor actinides like Am, for example with Nd, due to more similar chemical trends that have been observed.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e However, recent comparative studies have highlighted associated shortcomings in inferring chemical behaviour through such surrogate studies of Am and Nd,\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e pointing to the pertinence of utilising actual Am materials in experimental investigations.\u003c/p\u003e \u003cp\u003eConsidering the known chemistry of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e, the understanding of the role of Am doping on the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e polymorph is essentially not established. For instance, the level of solubility in the structure, the effect on stabilising the higher temperature tetragonal form in addition to the overall redox chemistry are not defined. Pertinently, there is no known Am-Zr-O phase diagram to help provide insight unto these questions. If the chemistry of Nd is to be followed, previous work suggests the solubility is limited to 1 mol % where after a defect fluorite structure is quickly precipitated followed by eventual cubic pyrochlore structure in space group Fd-3m (No. 227).\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Similarly, if the chemistry of neighbouring Pu is to be followed, the solubility is found to be less than 1 mol % with a similar progression to a F-type cubic structure.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e It is noteworthy that both Am and Pu are capable of stabilizing in the tetravalent oxidation state, a feature shared with Ce\u003csup\u003e4+\u003c/sup\u003e, that is often used as a surrogate for Pu in oxide investigations. However, Ce\u003csup\u003e4+\u003c/sup\u003e can be incorporated into the ZrO\u003csub\u003e2\u003c/sub\u003e structure at much higher concentrations than Pu\u003csup\u003e4+\u003c/sup\u003e, up to 20 mol%.\u003csup\u003e10\u003c/sup\u003e Other larger tetravalent actinides, such as Th\u003csup\u003e4+\u003c/sup\u003e, have a solubility limit of approximately 7 mol% before phase separation occurs. Moreover, despite the other quoted similar chemistry of certain actinides and lanthanides, in the case of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e little translational comparison can be made and direct investigation is necessary to properly determine specific chemical behaviour in the case of Am.\u003c/p\u003e \u003cp\u003eThe inclusion of Am in zirconium-based materials has been the firm focus of studies in relation to its disposal in the context of nuclear waste form science. For instance, a good body of literature exists for pyrochlore bearing americium oxides, Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.\u003csup\u003e\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e–\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e This is due to the exceptional ceramic and radiation resistance properties offered by pyrochlores as materials for the immobilisation of radionuclides.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e In the case of incorporation in other zirconia-based materials such as monoclinic and cubic stabilized zirconia far less is known. Raison and Haire\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e examined the incorporation of americium into ZrO\u003csub\u003e2\u003c/sub\u003e a part of the ternary AmO\u003csub\u003e2\u003c/sub\u003e-Cm\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e AmO\u003csub\u003e2\u003c/sub\u003e-Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e systems finding Am\u003csup\u003e4+\u003c/sup\u003e could be stabilised within these structures. However, there appears to be no available literature on the incorporation of Am into \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e as a part of the binary Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e system besides for pyrochlore. The extensive literature available on YSZ suggests that the similar ionic radii between Y\u003csup\u003e3+\u003c/sup\u003e and Am\u003csup\u003e3+\u003c/sup\u003e will favour incorporation of Am\u003csup\u003e3+\u003c/sup\u003e into ZrO\u003csub\u003e2\u003c/sub\u003e and induce stabilisation towards the cubic form. However, there are no experimental studies attesting to this nor examining the high temperature behaviour and stability. This is particularly relevant since it is known that on the surface of UO\u003csub\u003e2\u003c/sub\u003e based spent nuclear fuel (SNF) zircalloy cladding, a surface layer of ZrO\u003csub\u003e2\u003c/sub\u003e forms arsing during nuclear reactor operations which can readily interact with the fuel elements and incorporate critical radionuclides like Am.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Considering what is described as a global nuclear renaissance in the use of nuclear energy involving the use of higher burn up fuels,\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26 CR27\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e–\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e the inevitable increased pellet-cladding interactions that occur will convey enhanced actinide ZrO\u003csub\u003e2\u003c/sub\u003e interactions and derived phases forming. Consequently, understanding the solid-state chemistry and chemical behaviour of ZrO\u003csub\u003e2\u003c/sub\u003e derived phases with minor actinides, particularly Am, is a pertinent endeavour.\u003c/p\u003e \u003cp\u003eDue to the relevance of zirconia-based materials to SNF disposal but also to the considerable general interest zirconia materials science research,\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e we have examined the incorporation of 5% mol Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e. The structural, microstructural and local structure of the synthesized material, particularly Am incorporated \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e, is examined using powder X-ray diffraction, electron microscopy, Raman, Am L\u003csub\u003e3\u003c/sub\u003e-edge and Zr K-edge X-ray absorption fine structure (X-ray absorption near edge and extended X-ray absorption fine structure) as well as Am M\u003csub\u003e5\u003c/sub\u003e-edge high-energy resolution XANES (HR-XANES) spectroscopic techniques. The thermal dependence and stability of identified phases is further examined using high temperature \u003cem\u003ein situ\u003c/em\u003e powder X-ray diffraction. The results of this investigation are discussed with respect to the literature of monoclinic and cubic stabilised ZrO\u003csub\u003e2\u003c/sub\u003e, highlighting the role and comparative action of minor actinide chemistry in stabilising specific ZrO\u003csub\u003e2\u003c/sub\u003e polymorphs.\u003c/p\u003e "},{"header":"Experimental","content":"\u003cp\u003e \u003cem\u003eSynthesis\u003c/em\u003e \u003c/p\u003e\u003cp\u003e \u003cem\u003eThis work involved the use of Am-241 which decays primarily via α emission with a t\u003c/em\u003e \u003csub\u003e \u003cem\u003e1/2\u003c/em\u003e \u003c/sub\u003e \u003cem\u003eof 432.2 years. For this, the Minor Actinide Laboratory available at the European Commission Joint Research Centre, Karlsruhe, Germany were used for safe and correct handling.\u003c/em\u003e\u003c/p\u003e\u003cp\u003e \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e was synthesized following a high temperature solid state method. AmO\u003csub\u003e2\u003c/sub\u003e was weighed out in a hot cell-glovebox and mixed with pre-acquired ZrO\u003csub\u003e2\u003c/sub\u003e powder in stoichiometric quantities targeting 5 mol % addition of Am. The two reagents were homogenised using a mortar and pestle and ground into a fine powder. This powder was then compacted into a green pellet using a hydraulic press that applied 8 kN of force. Then, the pellet was transferred into a Mo crucible and placed in a furnace. The furnace was heated at a ramp rate of 200°C/h to 1600°C, held at 1600°C for 48 hours, and then cooled at a rate of 200°C/hr. In order to retain the Am in its tetravalent oxidation state and allow it to better react with the \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2,\u003c/sub\u003e the furnace used an Ar atmosphere that possessed 1000 ppm H\u003csub\u003e2\u003c/sub\u003eO. The sintered ceramic pellet was then removed for subsequent analysis.\u003c/p\u003e\u003cp\u003e \u003cem\u003ePowder X-ray Diffraction\u003c/em\u003e \u003c/p\u003e\u003cp\u003eTo safely and routinely study the synthesized Am\u003csup\u003e4+\u003c/sup\u003e incorporated \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2,\u003c/sub\u003e a piece of the sintered pellet was separated, ground to a fine powder and imbedded in embedded in Loctite® Double Bubble 2-part epoxy adhesive (5 minutes at 20 \u003csup\u003eo\u003c/sup\u003eC) resin matrix mounted to a standard powder X-ray diffraction (PXRD) holder. Measurements were performed using a Bruker D8 powder X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA). Measurements were recorded from 15 to 100 2θ in step sizes of 0.0017 \u003csup\u003eo\u003c/sup\u003e with counting time of 2s. Collected data was analysed using the Rietveld method as implemented in the program GSAS-II.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The peak shapes were modelled using a pseudo-Voigt function and the background was estimated using a 6–12 term shifted Chebyshev function. The atomic parameters of phases examined were fixed based on determined chemistries. The scale factor, detector zero-point and lattice parameters were refined together with the peak profile parameters.\u003c/p\u003e\u003cp\u003e \u003cem\u003eHigh Temperature X-ray Powder Diffraction\u003c/em\u003e \u003c/p\u003e\u003cp\u003eTo study the thermal dependence of 5% mol Am doped ZrO\u003csub\u003e2\u003c/sub\u003e, high temperature X-ray powder diffraction measurements (HT-PXRD) were performed on a Bruker D8 powder X-ray diffractometer. Approximately 15 mg of sample was loaded on a Pt heating stript and placed in an Anton Paar HTK2000 heating chamber. The small amount of sample was loaded to minimise radiation exposure risks. The sample was heated from 30 to 1100 \u003csup\u003eo\u003c/sup\u003eC with the sample kept under vacuum conditions.\u003c/p\u003e\u003cp\u003e \u003cem\u003eSEM\u003c/em\u003e \u003c/p\u003e\u003cp\u003eThe microstructure and homogeneity of the sintered pellet was investigated directly following synthesis with scanning electron microscopy adapted for the analysis of radioactive samples. For this purpose, a Philips XL40 microscope equipped with secondary and back-scattered electron detectors (SE and BSE) for the imaging and energy dispersive X-ray spectroscopy (EDX) for the analysis of the elemental distribution in the samples was used. A small fragment of non-coated ceramic pellets was deposited on a carbon sticker for the measurements. The sample was not polished and measured neat.\u003c/p\u003e\u003cp\u003e \u003cem\u003eRaman Spectroscopy\u003c/em\u003e \u003c/p\u003e\u003cp\u003eRaman spectroscopic measurements of the sintered pellet were conducted using customized α-tight capsules as described previously.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Measurements were conducted with a Jobin Yvon T64000 Raman spectroscope, equipped with a 1800 grooves/mm holographic grating and a 50×magnification long focal (~ 1 cm) objective with an aperture of 0.5. A low-background, liquid nitrogen–cooled detector (HoribaSymphony) was used to acquire the signal. The instrument was calibrated using the T\u003csub\u003e2g\u003c/sub\u003e band of a monocrystalline Si chip at 520.5 cm\u003csup\u003e− 1\u003c/sup\u003e. Raman measurements were conducted using a 660 nm laser. The data acquisition and data treatment were done with Labspec 6. Additional fitting of the Raman peaks was conducted with Origin Pro, using Lorentzian peak functions to describe the peak profile.\u003c/p\u003e\u003cp\u003e \u003cem\u003eHigh-Energy Resolution X-ray Absorption Near Edge Structure Spectroscopy (HRXANES)\u003c/em\u003e \u003c/p\u003e\u003cp\u003eFor the determination of the oxidation states of Am present within the 5 mol % doped ZrO\u003csub\u003e2\u003c/sub\u003e material, Am M\u003csub\u003e5\u003c/sub\u003e-edge HR-XANES experiments were conducted at the ACT station of the beamline for catalysis and actinide research (CAT-ACT beamline) of the KIT Light source at the Karlsruhe Research Accelerator (KARA).\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e A Si(111) double-crystal monochromator (DCM) was used to monochromatize the incident beam. The beam was focused to 1000 × 1000 µm and further narrowed down by slits leading to a sample spot size of about 500 ×200 µm (vertical × horizontal). HR-XANES spectra were acquired with a Johann-type X-ray emission spectrometer, using up to four Si(220) (Saint-Gobain, France) analyzer crystals with 1.0 m bending radius and an AXAS-M silicon drift detector (SDD, KETEK GmbH), which together with the sample were arranged in a Rowland circle geometry.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e The spectrometer and sample were encased in a He glove box, to minimize the influence of atmospheric O\u003csub\u003e2\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003e during measurements. For the sample itself, a about 1 mg of sample was diluted in BN, pressed into a pellet, placed in a double containment and sealed twice by 13 µm Kapton foil respectively. The HR-XANES spectra were measured with a step size of 0.1 eV from − 5 to + 20 eV from the white line (WL) of the respective edge and 0.5 eV in all other parts of the spectra. Collected spectra were calibrated against a Am\u003csup\u003e3+\u003c/sup\u003e-nPr-BTP reference sample. This was done by fixing the measured WL position of the reference sample to known literature values for Am\u003csup\u003e3+\u003c/sup\u003e (3888.5 eV).\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e The spectra were thereafter normalized to have a WL intensity of close to one. Four spectra were averaged for 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\u003cp\u003e \u003cem\u003eExtended X-ray Absorption Fine Structure Spectroscopy (EXAFS)\u003c/em\u003e \u003c/p\u003e\u003cp\u003eAm L\u003csub\u003e3\u003c/sub\u003e- and Zr K-edge extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) measurements of the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e were conducted at the KIT Light Source at the INE-beamline\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e using the KARA accelerator. Measurements were conducted at the Am L\u003csub\u003e3\u003c/sub\u003e- and Zr K-edges. The Larch software was used to extract EXAFS spectra from the raw absorption data.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Experimental Zr K-edge EXAFS spectra were Fourier-transformed using a Hanning window over the full k space range available 3.5–10.5 A˚\u003csup\u003e−1\u003c/sup\u003e Curve fitting was performed in k\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e for R values in the range 1.2–4.3 Å. Phases and amplitudes for the interatomic scattering paths were calculated with the ab initio code FEFF8L.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e Coordination numbers for cation-cation shells were fixed. Once satisfactory results were obtained, the constraints were removed with no significant variation\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e \u003cem\u003eSynthesis, Structural and Spectroscopic Characterization\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe synthesis of 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e was attempted using conditions of a flowing Ar atmosphere at 1600 \u003csup\u003eo\u003c/sup\u003eC. However, subsequent PXRD analysis failed to reveal the targeted \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e phase with Am incorporation. To ameliorate this, more oxidising conditions involving 1000 ppm H\u003csub\u003e2\u003c/sub\u003eO in Ar atmosphere were employed. This was targeted to help retain Am in its tetravalent oxidation state which is more conducive to incorporation into the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure. Subsequent collected PXRD data on the 5% mol Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e from this resulting synthesis was analysed using the Rietveld method, where a monoclinic structure in space group \u003cem\u003eP\u003c/em\u003e2\u003csub\u003e1\u003c/sub\u003e/\u003cem\u003ec\u003c/em\u003e consistent with \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e could be well refined against the diffraction pattern. The Rietveld profile is provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e with determined lattice parameters in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The determined lattice volume was determined to be 141.714(4) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, 1.063 \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e larger than the expected value for pure \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e of 140.651(3) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Such a lattice expansion is consistent with the inclusion of the Am cation within the crystal structure of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e, due to its larger size than Zr.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRefined lattice parameters for 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e for determined phases \u003cem\u003em-\u003c/em\u003e(Am\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e )O\u003csub\u003e2\u003c/sub\u003e and C-type (Am\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3+x\u003c/sub\u003e in addition reference data for \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e,\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003em-\u003c/em\u003e(Am\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e )O\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e34\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Am\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3+x\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAm\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003e\u003csup\u003e\u003cem\u003e36\u003c/em\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAm\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e17\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e2\u003csub\u003e1\u003c/sub\u003e/\u003cem\u003ec\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e2\u003csub\u003e1\u003c/sub\u003e/\u003cem\u003ec\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIa-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eIa-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eFd\u003c/em\u003e-3\u003cem\u003em\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003ea\u003c/em\u003e (\u0026Aring;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.1626(4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.14604(6)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10.3914(18)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.92(2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e10.66849(4)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eb\u003c/em\u003e (\u0026Aring;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.2156 (2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.21162(7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003ec\u003c/em\u003e (\u0026Aring;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.3314(4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.31308(7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eΒ\u003c/em\u003e (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.1744(2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e99.222(1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eV\u003c/em\u003e (\u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e141.714(4) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e140.651(3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1122.1(6)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1302.171(5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1214.252(4)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAdditional to the main phase \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure, a minor secondary phase could be observed with reflections at approximately near 30 \u003csup\u003eo\u003c/sup\u003e and 34 \u003csup\u003eo\u003c/sup\u003e. By considering the position of these reflections against possible phases that may occur in the Zr-Am-O system, it was determined that the best candidate matching phases would be a cubic structure of origin, particularly pyrochlore type in space group \u003cem\u003eFd\u003c/em\u003e-3\u003cem\u003em\u003c/em\u003e or C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sesquioxide in space group \u003cem\u003eIa\u003c/em\u003e-3. Other potential phases such as AmO\u003csub\u003e2\u003c/sub\u003e or a secondary phase \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e with depleted Am content were ruled out based on position of reflections. Subsequently, refinements were performed using pyrochlore and C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sesquioxide type models to determine the origin of this secondary phase. To these phases, unit cell volumes of 1124.2(3) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e and 1122.1(6) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e were determined respectively. Supplementary Information Note 1, provides details of the subsequent refinements performed. However, from the fitting values alone it was not definitively clear which of these phases better describes the secondary phase occurring, although a more consistent fit could be observed with the C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure used.\u003c/p\u003e \u003cp\u003eTo shed light on the chemical phase assemblage within 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e, SEM-EDS measurements were performed, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. These measurements indicated the inclusion of Am within the ZrO\u003csub\u003e2\u003c/sub\u003e matrix, however due to resolution issues could not discern nor clearly identify the minor secondary phase observed in PXRD measurements. Nevertheless, it could be observed that regions of the EDS maps showed variable areas that are low in quantity which are Am rich and more commonly regions that are Am poor. These results suggest that the respective phase composition is of a secondary minor phase that is Am rich type in addition to a major phase that is low in Am, like low Am-doped \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e. These observations are consistent with the results of the Rietveld PXRD analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRaman data were acquired for a pristine \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e specimen and a fragment of the sintered 5 mol % Am doped ZrO\u003csub\u003e2\u003c/sub\u003e pellet. The data were analysed using the Origin Pro 2025 software. The fitted spectrum of the pristine \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e sample, along with the measured spectrum of the Am-doped m-ZrO\u003csub\u003e2\u003c/sub\u003e sample, are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Within the measured spectral range, 13 characteristic Raman bands are clearly distinguishable. Compared to the pristine material, the Raman bands of the Am-containing sample exhibit an average shift of 6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to lower frequencies. This downshift suggests bond lengthening due to the substitution of smaller Zr\u003csup\u003e4+\u003c/sup\u003e cations with larger Am\u003csup\u003e4+\u003c/sup\u003e cations. The observed shift is smaller than that reported for monoclinic ZrO\u003csub\u003e2\u003c/sub\u003e doped with 10 mol% Ce\u003csup\u003e4+\u003c/sup\u003e, which shows an average downshift of 9 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e relative to our pristine m-ZrO\u003csub\u003e2\u003c/sub\u003e. This is consistent with the lower concentration of Am\u003csup\u003e4+\u003c/sup\u003e in our sample, while still clearly supporting its incorporation into the monoclinic structure. A full list of observed peak positions and their corresponding Raman mode assignments is provided in Supplementary Information Note 2, together with the fitted Raman spectrum of the Am-doped sample.\u003c/p\u003e \u003cp\u003eDespite probing eight different locations on the Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample, Raman spectroscopy did not reveal any signature of the secondary C-type cubic phase identified in our PXRD investigations. Given its low phase fraction (~\u0026thinsp;5%) and the surface-sensitive nature of the Raman method, it is conceivable that this phase remained undetected due to limited spatial distribution, low Raman scattering efficiency, or overlap with stronger signals from the monoclinic ZrO\u003csub\u003e2\u003c/sub\u003e matrix.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eAm M\u003c/em\u003e \u003csub\u003e \u003cem\u003e5\u003c/em\u003e \u003c/sub\u003e \u003cem\u003e-edge HR-XANES\u003c/em\u003e \u003c/p\u003e \u003cp\u003eTo understand the redox chemistry of Am within the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample and particularly understand redox speciation between separate phases, Am M\u003csub\u003e5\u003c/sub\u003e-edge HR-XANES experiments were performed. The normalised spectra are plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e in addition to the standards Am\u003csup\u003e4+\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and Am\u003csup\u003e3+\u003c/sup\u003eVO\u003csub\u003e4\u003c/sub\u003e. Evident from Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e is the\u0026rsquo; mixed spectrum of the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample which shows contributions from both Am\u003csup\u003e4+\u003c/sup\u003e and Am\u003csup\u003e3+\u003c/sup\u003e, where the Am\u003csup\u003e3+\u003c/sup\u003e dominates The HR-XANES spectra suggesting that the Am redox states are likely associated between Am\u003csup\u003e4+\u003c/sup\u003e and the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure due to know chemistries of the material preferring tetravalent dopants\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e whereas the secondary phase possesses Am\u003csup\u003e3+\u003c/sup\u003e and likely some Am\u003csup\u003e4+\u003c/sup\u003e. Note that Am\u003csup\u003e3+\u003c/sup\u003e in a cubic structure exhibits double peak structure with not negligible peak intensity at the energy position of the Am\u003csup\u003e4+\u003c/sup\u003e main absorption peak. This peak gains additional intensity due to the Am\u003csup\u003e4+\u003c/sup\u003e contribution. This is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e through the U\u003csub\u003e0.80\u003c/sub\u003eAm\u003csup\u003e3+/4+\u003c/sup\u003e\u003csub\u003e0.20\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e reference which suggests mixed Am\u003csup\u003e3+/4+\u003c/sup\u003e is found within a cubic environment. This experimental evidence supports the previous arguments from PXRD analysis that it is potentially C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or pyrochlore-type Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eZr K-edge XANES and EXAFS\u003c/em\u003e \u003c/p\u003e \u003cp\u003eBoth Zr K-edge XANES and EXAFS analyses indicate that the local environment of the Zr atoms corresponds to a monoclinic structure. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the normalized Zr K-edge XANES spectrum of the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample closely resembles that of a \u003cem\u003em-\u003c/em\u003eZr\u003csup\u003e4+\u003c/sup\u003eO₂ reference. Furthermore, the Zr K-edge k\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e.χ(k) EXAFS spectrum was successfully fitted using a structural model based on the monoclinic phase. The structural parameters obtained from the fit (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) are consistent with the monoclinic space group and suggest the incorporation of americium into the structure, as evidenced by slightly increased interatomic distances compared to pure \u003cem\u003em-\u003c/em\u003eZrO₂.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e Considering both the quality of the EXAFS fitting and the detection limits of the technique, the data support the conclusion that zirconium is present predominantly in the monoclinic ZrO₂ phase, indicating that the secondary phase is mainly americium-based.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eStructural parameters derived from the analysis of the EXAFS signal of the Zr K-edge for 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eShell\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eR(A˚)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCN\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eσ\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e (A˚\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eZrK\u003c/p\u003e \u003cp\u003eRf\u0026thinsp;=\u0026thinsp;1.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZr \u0026ndash; O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.083 (5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.012 (1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZr - O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.176 (5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.011 (2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZr - O\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.285 (5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.011 (2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eZr - M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.52 (1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.007 (1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eAm L\u003c/em\u003e \u003csub\u003e \u003cem\u003e3\u003c/em\u003e \u003c/sub\u003e \u003cem\u003e-edge XANES\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe oxidation state of Am within the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample was investigated using Am L\u003csub\u003e3\u003c/sub\u003e-edge XANES spectroscopy. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the XANES spectrum of the 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e sample lies between those of Am\u003csup\u003e3+\u003c/sup\u003e and Am\u003csup\u003e4+\u003c/sup\u003e reference compounds, indicating the presence of mixed-valence states. Linear combination fitting reveals that approximately 80\u0026thinsp;\u0026plusmn;\u0026thinsp;1% of the americium is present in the trivalent state, with the remaining 20\u0026thinsp;\u0026plusmn;\u0026thinsp;1% in the tetravalent state. This result is in accordance with the qualitative fingerprint analyses of the Am M\u003csub\u003e5\u003c/sub\u003e-edge HR-XANES spectra.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003ePhase Composition Determination via Vegard\u0026rsquo;s Law\u003c/em\u003e \u003c/p\u003e \u003cp\u003eFrom the experiments and analysis thus far, it is shown the synthesis of 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e under mildly oxidising conditions results in the formation of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e with an additional secondary phase at low amount (\u0026lt;\u0026thinsp;5 phase %) in which the Am oxidation state is mixed, occurring as trivalent and tetravalent mixed. Am L\u003csub\u003e3\u003c/sub\u003e-edge XANES analysis shows that 80% of the Am occurs as trivalent whereas as the 20% as tetravalent; this result agrees with the results of the Am M\u003csub\u003e5\u003c/sub\u003e-edge HR-XANES spectroscopic technique, which is more sensitive to small variations of the actinide oxidation states but quantitative analyses are generally more difficult. When the phase diagram of ZrO\u003csub\u003e2\u003c/sub\u003e is considered,\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e is not expected to host oxygen defects which would occur when Am\u003csup\u003e3+\u003c/sup\u003e is incorporated within it. Rather, it is more expected Am\u003csup\u003e4+\u003c/sup\u003e would incorporate into \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e and the identified secondary phase accordingly contains Am\u003csup\u003e3+\u003c/sup\u003e. Based on the XANES analysis, it implies that of the 5 mol % Am that was used in the synthesis, only 1 mol % enters \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e and it follows this is tetravalent Am. The subsequent 80% extra is subsequently attributed primarily to trivalent and some tetravalent Am that reports to the secondary phase, which Rietveld analysis suggests is likely C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or pyrochlore type Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, i.e. an Am rich phase consistent with the EXAFS and EDS measurements. Invariably, the observed phenomena of phase separation is likely associated with limited solubility of Am\u003csup\u003e4+\u003c/sup\u003e within \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e. Subsequently, in order to identify the origin of the secondary cubic phase occurring whilst simultaneously determining the solubility of Am with \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e and its composition, calculations were performed using Vegard\u0026rsquo;s Law (Eq.\u0026nbsp;1.) with determined lattice parameters from Rietveld analysis employed. The observed lattice expansion of the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e phase with Am\u003csup\u003e4+\u003c/sup\u003e doping compared to the non-doped state will depend on the specific amount of Am that enters the lattice. Whereby, the observation of the secondary phases implies that the solubility limit has been reached in the main phase. Since Am was used in its tetravalent state during synthesis, further supported by XANES and particularly EXAFS analysis, the expansion of the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure can be assumed to follow a linear expansion via substitution of Zr\u003csup\u003e4+\u003c/sup\u003e for Am\u003csup\u003e4+\u003c/sup\u003e as given in Eqs.\u0026nbsp;1 and 2.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eV\u0026thinsp;=\u0026thinsp;V\u003c/em\u003e\u003csub\u003e\u003cem\u003eo\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e​+x\u0026sdot;ΔV\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e(1)\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eZr\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eAm\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e(2)\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eUsing the determined lattice volume from the Rietveld method in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. and comparing it to reference data from Gualtieri \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e provided also in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, by Eq.\u0026nbsp;1. the necessary amount of Am\u003csup\u003e4+\u003c/sup\u003e required to induce the observed 0.75% lattice expansion would be \u003cem\u003ex\u0026thinsp;=\u003c/em\u003e\u0026thinsp;0.0105 when using ionic radii of 0.90 \u0026Aring; and 0.78 \u0026Aring; for Am\u003csup\u003e4+\u003c/sup\u003e and Zr\u003csup\u003e4+\u003c/sup\u003e respectively in CN\u0026thinsp;=\u0026thinsp;7.\u003csup\u003e35\u003c/sup\u003e Accordingly, for the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure identified, this will correspond to a formula of (Am\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e)O\u003csub\u003e2\u003c/sub\u003e, i.e. only 1.05 mol % incorporation into \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e. This value is considerably lower than the 5 mol % addition used in the synthesis which will subsequently result in precipitation of a secondary Am rich phase, as is described and observed. Notably, this determined value from Vegard\u0026rsquo;s law is consistent with the Am L\u003csub\u003e3\u003c/sub\u003e-edge XANES analysis and appears to corroborate both the amount of phase separation but also difference in Am valence between the phases, namely \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e contains only Am\u003csup\u003e4+\u003c/sup\u003e and the secondary Am rich phase is predominantly Am\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWhen the Zr K-edge XANES results are considered, they show that the Zr appears to be predominantly associated with the tetragonal \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e structure. This implies that the secondary phase is likely very poor in the amount of Zr contained and rather rich in Am. Accordingly, it is suspected then the secondary phase is more likely C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003e compared to pyrochlore-type Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e. Nevertheless, this can be further confirmed and understood when examining determined lattice parameters from Rietveld refinements as was previously performed. For the pyrochlore-type Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e a unit cell volume of 1124.2(3) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e was determined, this can be compared against the determined lattice volume of Am\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e synthesized by Belin \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e of 1214.252(4) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Since the unit cell of volume of 1124.2(3) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e is considerably smaller than that determined by Bellin \u003cem\u003eet al.\u003c/em\u003e, this would imply that the amount of Am with the pyrochlore structure is significantly less than the Am:Zr 1:1 ratio. Such an argument is difficult to support considering the noted significance ejection of Am from \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e which should coincide with only some minor Zr incorporation. In the case of C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003e a lattice volume of 1122.1(6) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e was determined that is smaller than the reference C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003e value provided by Epifano \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e of 1302.171 \u0026Aring;. For a C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure occurring as a secondary phase it is likely that some Zr will be incorporated in the structure when it is ejected from the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e phase. Since XANES measurements show Zr occurs as Zr\u003csup\u003e4+\u003c/sup\u003e this will lead to a contraction of the C-type Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure which is consistent with the trend in lattice parameters when the end member value of Epifano \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e is compared to the solid solution determined here. Naturally, variations in oxygen stoichiometry, although difficult to quantify, will further influence the lattice parameters when comparing the investigations, nevertheless the results are still very consistent. Subsequently it is argued that the significant excess Am\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e that does not enter \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e results in the formation of a C-type (Am\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3+2x\u003c/sub\u003e structure where x is small. Interestingly, the amounts of Am deposited between the phases observed is strikingly consistent with what is seen in the Nd\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e phase diagram,\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e suggesting in this instance there is good congruency between the behaviour of Am and Nd within ZrO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003cem\u003eHigh Temperature X-ray Diffraction\u003c/em\u003e \u003c/p\u003e \u003cp\u003eA notable known behaviour of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e is the ability to stabilise it to its tetragonal and cubic forms with increasing temperature where the transition temperature can be reduced via doping. An appropriate test of confirming incorporating Am\u003csup\u003e4+\u003c/sup\u003e within \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e can be achieved subsequently through HT-PXRD measurements and showing a reduced phase transformation temperature with doping. Accordingly, HT-PXRD measurements were performed on a small amount of 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e material (15 mg) where the sample was heated sequentially to 1100 \u003csup\u003eo\u003c/sup\u003eC before cooling to RT. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e which provides a portion of the collected data, the phase transformation the tetragonal form from the monoclinic can be observed between 900\u0026ndash;1050 \u003csup\u003eo\u003c/sup\u003eC. This is significantly less than that known to occur for pure \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e which transforms to the tetragonal polymorph usually at 1100 \u003csup\u003eo\u003c/sup\u003eC.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e This indicates the inclusion of the Am\u003csup\u003e4+\u003c/sup\u003e within \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e lowers the transition temperature as expected but corroborates its occurrence within the structure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe solubility and structural chemistry of 5 mol % Am doped \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e has been determined through a combination of structure and spectroscopic techniques. Synthesis of a 5 mol % Am-doped ZrO\u003csub\u003e2\u003c/sub\u003e under mildly oxidising conditions is found to result in the formation of major phase \u003cem\u003em\u003c/em\u003e-(Am\u003csup\u003e4\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e)O\u003csub\u003e2\u003c/sub\u003e and a minor phase that is attributed to C-type (Am\u003csup\u003e3\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003e1\u0026minus;x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure, where x is small, via PXRD, HR-XANES, EXAFS measurements and supported by calculations using Vegard\u0026rsquo;s law. The known HT transformation from \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e to \u003cem\u003et\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e occurring above 1100 \u003csup\u003eo\u003c/sup\u003eC is found to be reduced to below 1050 \u003csup\u003eo\u003c/sup\u003eC via HT-XRD \u003cem\u003ein situ\u003c/em\u003e measurements, due to the inclusion of the Am\u003csup\u003e4+\u003c/sup\u003e cation in the lattice. The limited solubility of Am within \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e is attributed to its size in its trivalent form in addition to its trivalent stability at HT. Nevertheless, \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e is able to successfully immobilize the tetravalent form under prescribed conditions which otherwise would result in tetravalent occurrence, albeit with low solubility. When the results of the investigation are considered with the limited literature available on the Zr-Am-O system, despite there being no ZrO\u003csub\u003e2\u003c/sub\u003e-Am\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase diagram existing it appears that the behavior of Am appears to follow well with that of the ZrO\u003csub\u003e2\u003c/sub\u003e-Nd\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase diagram, given the observations made, and the results of this investigation can be used potentially with Nd data to infer the chemical behavior of Am within zirconia matrices. Such results are pertinent and relevant to the understanding of Am within waste form environments arising from the direct storage and reprocessing of SNF materials arising from nuclear energy production.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are grateful to funding and support from the German Federal Ministry of Education and Research (BMBF), Project No. 02NUK060 that enabled this research. The experimental data used in this research were generated through access to the ActUsLab/FMR under the Framework of access to the Joint Research Centre Physical Research Infrastructures of the European Commission (RISE-241, \u0026nbsp;Research Infrastructure Access Agreement N\u0026deg;36344/02). The authors give thanks to the Institute for Beam Physics and Technology (IBPT, KIT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA), and the KIT Light Source for provision of beamtime. The authors acknowledge funding from the European Research Council (ERC) Consolidator Grant 2020 under the European Union\u0026rsquo;s Horizon 2020 research and innovation programme (grant agreement No. 101003292). The authors thank Cedric Reitz (KIT-INE) for support during the experiments. The authors are also grateful for HT-XRD and SEM measurement support by \u0026nbsp;Eckhard Dahms and Ramon Carlos Marquez.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe project was conceived and developed by \u003cstrong\u003eGabriel L. Murphy, Nina Huittinen, Sara Gilson and Karin Popa\u003c/strong\u003e. The research methodology, experimental planning, and formal analysis was conducted by \u003cstrong\u003eGabriel L. Murphy, Nina Huittinen, Sara Gilson and Karin Popa\u003c/strong\u003e. The materials were synthesized by \u003cstrong\u003eKarin Popa and Octavian Valu\u003c/strong\u003e. PXRD measurements and analysis were performed by \u003cstrong\u003eGabriel L. Murphy\u003c/strong\u003e and \u003cstrong\u003eOlaf Walter\u003c/strong\u003e. Raman measurements were performed by \u003cstrong\u003eJean-Yves Colle\u003c/strong\u003e\u0026nbsp; and \u003cstrong\u003eNina Huittinen\u003c/strong\u003e. HR-XANES experiments were conducted by \u003cstrong\u003eSven M. Schenk, Harry Ramanantoanina, Tim Pr\u0026uuml;\u0026szlig;mann, Tonya Vitova\u003c/strong\u003e. EXAFS were recorded by \u003cstrong\u003eTim Pr\u0026uuml;\u0026szlig;mann, Kathy Dardenne and J\u0026ouml;rg Rothe\u003c/strong\u003e. Manuscript writing, review and editing was performed by \u003cstrong\u003eGabriel L. 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GSAS-II: The Genesis Of A Modern Open-Source All Purpose Crystallography Software Package. \u003cem\u003eJournal of Applied Crystallography\u003c/em\u003e \u003cstrong\u003e46\u003c/strong\u003e, 544-549 (2013). https://doi.org/doi:10.1107/S0021889813003531\u003c/li\u003e\n\u003cli\u003eZimina, A.\u003cem\u003e et al.\u003c/em\u003e CAT-ACT\u0026mdash;A new highly versatile x-ray spectroscopy beamline for catalysis and radionuclide science at the KIT synchrotron light facility ANKA. \u003cem\u003eReview of Scientific Instruments\u003c/em\u003e \u003cstrong\u003e88\u003c/strong\u003e (2017). https://doi.org/10.1063/1.4999928\u003c/li\u003e\n\u003cli\u003eVigier, J.-F.\u003cem\u003e et al.\u003c/em\u003e Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4. \u003cem\u003eInorganic Chemistry\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, 9350-9359 (2023). https://doi.org/10.1021/acs.inorgchem.3c00251\u003c/li\u003e\n\u003cli\u003eRothe, J.\u003cem\u003e et al.\u003c/em\u003e The INE-Beamline for actinide science at ANKA. \u003cem\u003eReview of scientific instruments\u003c/em\u003e \u003cstrong\u003e83\u003c/strong\u003e (2012). \u003c/li\u003e\n\u003cli\u003eGualtieri, A., Norby, P., Hanson, J. \u0026amp; Hriljac, J. 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Revised Effective Ionic Radii And Systematic Studies Of Interatomic Distances In Halides And Chalcogenides. \u003cem\u003eActa Crystallographica Section A\u003c/em\u003e \u003cstrong\u003e32\u003c/strong\u003e, 751-767 (1976). \u003c/li\u003e\n\u003cli\u003eEpifano, E.\u003cem\u003e et al.\u003c/em\u003e Insight into the Am\u0026ndash;O Phase Equilibria: A Thermodynamic Study Coupling High-Temperature XRD and CALPHAD Modeling. \u003cem\u003eInorganic Chemistry\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 7416-7432 (2017). https://doi.org/10.1021/acs.inorgchem.7b00572\u003c/li\u003e\n\u003cli\u003eBondars, B.\u003cem\u003e et al.\u003c/em\u003e Powder diffraction investigations of plasma sprayed zirconia. \u003cem\u003eJournal of Materials Science\u003c/em\u003e\u003cstrong\u003e30\u003c/strong\u003e, 1621-1625 (1995). https://doi.org/10.1007/BF00375275\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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-7027845/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7027845/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMonoclinic zirconia (\u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e) has been long of fundamental chemical interest and crucially it serves as a key barrier to radionuclide release at the fuel\u0026ndash;zircaloy interface in spent nuclear fuel. However, the incorporation of transplutonic elements like americium in \u003cem\u003em-\u003c/em\u003eZrO₂ remains poorly understood. Using a combination of microscopy, diffraction and high resolution X-ray spectroscopic techniques we have examined the doping of \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e with 5 mol % Am. We show Am enters \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e tetravalently, where its solubility is less than 1.05 mol %, \u003cem\u003em-\u003c/em\u003e(Am\u003csup\u003e4\u0026thinsp;+\u003c/sup\u003e\u0026thinsp;\u003csub\u003e0.0105\u003c/sub\u003eZr\u003csub\u003e0.9895\u003c/sub\u003e)O\u003csub\u003e2\u003c/sub\u003e, attributed to the large Am\u003csup\u003e4+\u003c/sup\u003e cation coupled with its preference under the synthesis conditions to revert to its trivalent state, where excess Am adopts a C-type (Am\u003csup\u003e4+\u003c/sup\u003e/\u003csup\u003e3+\u003c/sup\u003e\u003csub\u003e1\u0026minus;x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3+x\u003c/sub\u003e phase in space group \u003cem\u003eIa\u003c/em\u003e-3. The known reversible high temperature phase transformation of \u003cem\u003em-\u003c/em\u003eZrO\u003csub\u003e2\u003c/sub\u003e to tetragonal is further shown to be reduced from 1150 \u003csup\u003eo\u003c/sup\u003eC to 1050 \u003csup\u003eo\u003c/sup\u003eC via Am\u003csup\u003e4+\u003c/sup\u003e incorporation. The investigation provides critical insight into behaviour of transplutonic elements with \u003cem\u003em-\u003c/em\u003eZrO₂.\u003c/p\u003e","manuscriptTitle":"Structural and Chemical Insights on the Incorporation of Americium into Monoclinic Zirconia (m-ZrO₂)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-11 11:00:05","doi":"10.21203/rs.3.rs-7027845/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commschem","sideBox":"Learn more about [Communications Chemistry](http://www.nature.com/commschem/)","snPcode":"","submissionUrl":"","title":"Communications Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1bebf92f-9f4e-4b49-ac61-4e8d9d8fbced","owner":[],"postedDate":"July 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":51392299,"name":"Physical sciences/Chemistry/Materials chemistry"},{"id":51392300,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Corrosion"},{"id":51392301,"name":"Physical sciences/Energy science and technology/Nuclear energy/Nuclear waste"}],"tags":[],"updatedAt":"2026-01-29T08:32:27+00:00","versionOfRecord":{"articleIdentity":"rs-7027845","link":"https://doi.org/10.1038/s42004-025-01857-9","journal":{"identity":"communications-chemistry","isVorOnly":false,"title":"Communications Chemistry"},"publishedOn":"2025-12-26 05:00:00","publishedOnDateReadable":"December 26th, 2025"},"versionCreatedAt":"2025-07-11 11:00:05","video":"","vorDoi":"10.1038/s42004-025-01857-9","vorDoiUrl":"https://doi.org/10.1038/s42004-025-01857-9","workflowStages":[]},"version":"v1","identity":"rs-7027845","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7027845","identity":"rs-7027845","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}