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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Controlling Oxygen Defect Formation and Its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase Transformations in Nonstoichiometric Ternary Uranium Oxides Gabriel L. Murphy,†,‡,# Chun-Hai Wang,† Zhaoming Zhang,*,‡ Piotr M. Kowalski,∥,⊥ George Beridze,∥,⊥ Maxim Avdeev,†,‡ Ondrej Muransky,‡ Helen E.A. Brand,§ Qin-Fen Gu,§ and Brendan J. Kennedy*,† Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on April 9, 2019 at 13:38:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia § Australian Synchrotron, Australian Nuclear Science and Technology Organisation, 800 Blackburn Road, Clayton, Victoria 3168, Australia ∥ Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany ⊥ JARA High-Performance Computing, Schinkelstrasse 2, 52062 Aachen, Germany ‡

S Supporting Information *

ABSTRACT: In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4−x (A = alkaline earth metal cation) oxides CaUO4−x and α-Sr0.4Ca0.6UO4−x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅m) CaUO4−x and α-Sr0.4Ca0.6UO4−x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4−x and δSr0.4Ca0.6UO4−x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4−x and δ-Sr0.4Ca0.6UO4−x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4−x and α-Sr0.4Ca0.6UO4−x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4−x and α-Sr0.4Ca0.6UO4−x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect−defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect−defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4−x and Sr0.4Ca0.6UO4−x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δSrUO4−x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature.

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INTRODUCTION

performance through increasing defect concentrations against the loss of structural stability and/or decreased performance associated with increasing defect interactions.6−9 Understanding the thermal stability of defects in a crystal lattice is

The advent of defect engineering has seen a cascade of development in several technologically important materials ranging from semiconductors to solid oxide fuel cells to oxygen deficient high temperature superconductors.1−5 Modification of materials by enhancing their defect properties requires controlled approaches that carefully balance improved © XXXX American Chemical Society

Received: February 12, 2019

A

DOI: 10.1021/acs.inorgchem.9b00406 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

α-SrUO4 and CaUO4 have a rhombohedral structure and contain two crystallographically distinct oxygen sites, O(1) and O(2): the former corresponds to the out-of-plane collinear uranium−oxygen oxo bond, consistent with the uranyl group,20,34−36 and the latter to the in-plane coordinating oxygen, see Figure 1. Our previous investigations into the

a crucial component of materials design as this will ultimately dictate both a material’s performance and its suitability for the targeted application. The oxide ion conductor δ-Bi2O3 is a good example of this; it displays exceptional ionic conductivity at high temperatures which is inevitably lost at lower temperatures as the structure transforms to its lower temperature polymorphs.10,11 The inclusion of high valence transition metal cations has been demonstrated to stabilize the δ-Bi2O3 structure at lower temperatures.12 Indeed, the approach of structural stabilization through chemical modification leading to improved material properties has been effective and become quite topical in other avenues of materials science with several noteworthy recent successes.13−15 As a part of our focus on actinide solid state chemistry,16−22 we recently showed that when the two-dimensional (2D) layered substoichiometric oxide α-SrUO4−x obtains a critical amount of disordered oxygen defects located at the in-plane positions, heating results in a reversible first order phase transformation to a lower symmetry structure in which the inplane defects order δ-SrUO3.70(6).16 The transformation is apparently the first of its kind. Interest in the formation and clustering of defects in uranium oxides has traditionally been related to the understanding of nuclear fuel behavior and stability.23−25 It was reported that appreciable ionic conductivities could be achieved through the partial substitution of aliovalent cations for uranium in the UO2 lattice.26−28 However, the majority of structural and conductivity studies have largely been confined to tetravalent uranium containing materials with few reports of materials with higher U valence states. Bond valence sum (BVS) calculations indicated that the ability for AUO4 (A = Ca, Sr, and Ba) oxides to host oxygen defects is a synergetic structural consequence of both the preferred bonding of the A and U cations and the ability for uranium to undergo reduction.20 These calculations indicate that α-SrUO4 can support more defects than the isostructural Ca analogue CaUO4.20 Consequently, it was postulated, which is the focus of the present investigation, that the amount of oxygen defects that form at high temperatures and the potential ordering of these vacancy defects in CaUO4 can be modulated through partially exchanging Ca2+ with other alkaline earth metal cations. Surprisingly, α-SrUO4 readily forms an appreciable number of oxygen defects under oxidizing conditions, associated with the reduction of uranium, at high temperatures prior to its transformation to the orthorhombic β-SrUO4 polymorph.21 Under similar conditions, CaUO4 has been reported to form oxygen vacancy defects, but it does not undergo any thermally induced phase transitions29,30 (as CaUO4 has not been reported to exist in the orthorhombic β form, α-CaUO4 is abbreviated as CaUO4 throughout this paper). CaUO4 has been previously studied by thermogravimetric methods under reducing conditions where an apparent two-step reduction process was identified.29 Tagawa et al.29 argued that the two steps correspond to the initial reduction of CaUO4 to CaUO3.55 and then subsequent reduction to CaUO3.5. Prodan and Boswell31 postulated that these events are related to interactions between the defects leading to the formation of microdomains with variable concentrations of oxygen vacancies. Oxygen transport, associated with the reduction and oxidation of the uranium cations, occurs between these microdomains. However, the few available in situ structural studies do not provide much detail.32,33

Figure 1. Representation of the rhombohedral CaUO4 structure in R3̅m. The U and Ca cations are represented by navy and dark green spheres, respectively, whereas the O(1) out-of-plane and O(2) inplane oxygen anions are represented by small aqua and red spheres, respectively.

room temperature structure of α-SrUO4 and its transformation to the β and δ polymorphs showed that oxygen vacancies, or defects, form preferentially at the O(2) site and that ordering of these results in the transformation to the δ-polymorph.16,21 At room temperature, α-SrUO4 supports more oxygen defects on the O(2) site than CaUO4 due to the over bonded environment of the Sr2+ cation, which is alleviated through defect formation and consequent reduction of the uranium cation.20 The Ca2+ cation, being smaller than Sr2+, is better accommodated in the structure, and oxygen vacancies are not required to stabilize the structure. Considering this, it is not unreasonable to expect that the formation of oxygen vacancies may be enhanced through the substitution of Sr2+ for Ca2+ cations in CaUO 4 . This is examined in the present investigation using neutron and synchrotron X-ray powder diffraction measurements. Ab initio methods using density functional theory were employed to gain insight into the ability for these lattices to generate defects, and such results were supported by estimated oxygen vacancy formation energies based on thermodynamic considerations utilizing the experimentally determined amount of vacancies as a function of temperature. The variation in the oxygen defect formation, and how this impacts the formation of the defect-ordered δ phase, was examined for two members of the α-SryCa1−yUO4 solid solution with y = 0 and 0.4. This was undertaken using in situ synchrotron X-ray diffraction under a flowing H2 atmosphere. Interestingly subtle differences are observed both in the temperature at which the disordered δ phase initially forms and in the structural details of the ordered δ phase. These results point to the importance of the nature of A-site cations and variable defect−defect interactions in isostructural oxides and their influences on the long-range structural order.

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EXPERIMENTAL SECTION

Caution. The uranium used in this investigation was in a depleted state (DU), where the primary hazard arises from the decay of the majority U-238 isotope via α emission (4.2 MeV) with a half-life of 4.5 × 109 years. Appropriate radioactive material handling precautions were taken at all times. Synthesis and Room Temperature Characterization. A polycrystalline sample of CaUO4 was prepared using conventional B


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Inorganic Chemistry

solid complexes46 and applied in the computational studies of the α, β, and δ phases of SrUO4−x.16,21 The Hubbard U parameter used in the current study is 2.5 eV, which is the same value as derived for the α, β, and δ phases of SrUO4−x.16,21 This value is in the range expected for U(VI) and U(V) compounds.46 In addition to the oxygen deficient CaUO4−x phase, we computed defect formation energies for the isostructural α-Sr0.4Ca0.6UO4−x. The random distribution of Ca and Sr cations was modeled by a representative structure constructed using the special quasi-random structure approach.47,48 Super Structure Analysis. The structure of δ-CaUO4−x was analyzed using TOPAS Academic (TA)49,50 and Jana200651 against SXRD data collected at 1000 °C. The 1000 °C pattern was chosen to serve as a comparison to the previous study of δ-SrUO4−x,16 and the same protocol was followed in the current study. First, an average structure of δ-CaUO4−x at 1000 °C was developed that did not take the supercell reflections into account. Then, the modulation vector that accounted for the observed superlattice reflections was investigated. Attempts were initially made to assign a commensurate distortion vector k, but none could be found, and an incommensurate modulation vector k = ⟨∼0.27, ∼0.16, ∼0.28⟩ was obtained instead. The structure of δ-CaUO4−x was then refined using Jana2006. For each atomic position, one modulation wave was applied per site. There should be an additional occupancy modulation of oxygen; however, this could not be identified, reflecting the relatively much smaller scattering factor of oxygen in X-ray diffraction studies.

solid state synthesis methods as described previously.20 The series of oxides α-SryCa1−yUO4, 0 < y < 1 in steps of 0.1, was synthesized under reducing conditions (3.5%H2/N2) at 950 °C for 20 h with intermittent mixing. The oxides were then annealed at 200 °C in air for 20 h to ensure the final products were stoichiometric with respect to oxygen content. Room temperature structural characterization was performed at the powder diffraction (PD) beamline at the Australian Synchrotron37 and the high resolution neutron powder diffractometer (ECHIDNA)38 at ANSTO’s OPAL nuclear reactor facility. For neutron powder diffraction (NPD) measurements, a wavelength of 1.622 Å was used. The wavelength used in the synchrotron X-ray powder diffraction (S-XRD) measurements was determined using a NIST LaB6 reference as 0.7764 and 0.7755 Å for the structural studies of CaUO4 and α-Sr0.4Ca0.6UO4, respectively. The structures were refined by the Rietveld method as implemented in the program GSAS.39,40 The peak shapes were modeled using a pseudo-Voigt function, and the background was estimated by a shifted Chebyshev function. The scale factor, detector zero point, lattice parameters, atomic coordinates, and atomic displacement parameters were refined together with the peak profile parameters. In Situ Structural Studies. For high temperature in situ NPD measurements, CaUO4 was pelletized and placed in a thin walled vanadium can that was positioned in an ILL-type high vacuum furnace equipped with niobium elements and operating at 99.9% with moisture ≤25 ppm and N2 ≤500 ppm) were employed. Prior to variable temperature measurements in high purity hydrogen and subsequently oxygen, the sample was flushed with hydrogen or nitrogen gas, respectively, for 30 min to purge the system. Temperature was increased at a ramp rate of 5 °C per minute and decreased at a rate of 25 °C per minute, and the data collection was commenced after a 30 s delay at temperature to allow for thermal equilibration. Ab Initio Methods. The ab initio calculations were performed by applying density functional theory (DFT) using the QuantumESPRESSO simulations package.42 We applied the PBE exchangecorrelation functional, the plane-wave energy cutoff of 50 Ryd, and the ultrasoft pseudopotentials to mimic the presence of core electrons.43,44 The stoichiometric α and β structures of AUO4 (A = Ca and Sr0.4Ca0.6) were modeled in this study by a supercell containing 48 and 24 atoms, respectively. To account for electronic correlations, the DFT+U method was applied, but with the Hubbard model computed ab initio using the linear response method of Cococcioni and de Gironcoli.45 The same procedure has been successfully tested on large sets of uranium-bearing molecular and

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RESULTS AND DISCUSSION Structural Study 1: Variable Temperature S-XRD and NPD. As-prepared CaUO4 is essentially stoichiometric with the refined occupancy of the O(1) and O(2) sites equal to 1.00(3) and 0.984(7), respectively, at 60 °C from NPD data analyzed using the Rietveld method (Figure S1 in Supporting Information). S-XRD showed that heating CaUO4 in a sealed quartz capillary containing air resulted in linear thermal expansion up to approximately 700 °C. Continued heating (up to 1000 °C) resulted in an increase in the rate of thermal expansion of the CaUO4 unit cell volume, see Figure 2, reminiscent to that observed in α-SrUO4 under similar conditions.21 This is consistent with the formation of oxygen vacancies and partial reduction of U cations at temperatures above 700 °C.21,52,53 There is no evidence for any phase

Figure 2. Temperature dependence of the unit cell volume of CaUO4 and α-Sr0.4Ca0.6UO4 in sealed quartz capillaries containing air obtained from Rietveld refinements of in situ synchrotron X-ray powder diffraction patterns. The solid red lines are linear fits to the data between room temperature and 500 °C to guide the eyes. C


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Inorganic Chemistry

Figure 3. Temperature dependence of (a) the unit cell volume, (b) O(2) occupancy and isotropic atomic displacement parameter, (c) bond length U−O(2), and (d) bond length U−O(1) in CaUO4 obtained from Rietveld refinements against in situ neutron powder diffraction patterns obtained under vacuum. The stoichiometry can be estimated from the occupancy of the O(2) site because the O(1) site remains fully occupied (within the precision of the measurements).

transitions in the temperature range studied. NPD patterns (Figure S2 in Supporting Information) obtained under mildly reducing conditions of high vacuum showed a similar phenomenon except that the accelerated thermal expansion of the CaUO4 unit cell volume started around 600 °C instead of 700 °C, as observed in the S-XRD measurements (Figure 3a). The cell volume determined by NPD is greater than that seen in the S-XRD measurements at temperatures above 600 °C, see Figure 3a, consistent with the formation of more oxygen vacancies in vacuum than in air. The refined occupancy of the O(2) oxygen site, shown in Figure 3b, decreased rapidly above 600 °C, reaching 0.826(14) at 1000 °C, while the O(1) site remained fully occupied. The decrease in the site occupancy of O(2) is mirrored by the increase in the atomic displacement parameter also shown in Figure 3b. Over the temperature range of 60−1000 °C, the U−O(2) distance increased by 1.58% from 2.2967(3) to 2.3331(13) Å (Figure 3c). The U−O(1) distance increased in a nonlinear manner by 2.12% from 1.9604(12) to 2.002(5) Å over the same temperature range (Figure 3d). The much faster increases of the U−O(1) bond length at temperatures above 600 °C are consistent with the reduction of U6+ cations. Single phase samples in the series α-SryCa1−yUO4 could be prepared up to y = 0.4, and Rietveld refinements confirmed that these samples have the rhombohedral structure in space group R3̅m. At higher Sr content, phase separation occurred, as shown in Figure 4, with Rietveld analysis showing both phases to be rhombohedral in space group R3̅m. The two phases coexist until y = 0.8, thereafter forming a single phase rhombohedral structure again also in R3̅m. S-XRD measurements of α-Sr0.4Ca0.6UO4 in a sealed quartz capillary containing air demonstrated that this sample underwent approximately linear thermal expansion up to about 600 °C, as shown in Figure 2. Between 600 and 1000 °C, the thermal expansion rate of the unit cell volume was observed to increase relative to

Figure 4. S-XRD patterns at 13.10° ≤ 2θ ≤ 14.40° taken at RT from the α-SryCa1−yUO4 solid solution samples for 0 ≤ y ≤ 1 in steps of 0.1. The two-phase regions are highlighted by the two red boxes.

that at lower temperatures, presumably due to the formation of oxygen vacancies. The change in the slope of the thermal expansion curve for α-Sr0.4Ca0.6UO4 occurs at a temperature slightly lower than that in CaUO4 (600 vs 700 °C). If the observed cell volumes of α-Sr0.4Ca0.6UO4 and CaUO4 at 1000 °C are compared to the values extrapolated from the linear thermal expansion at temperatures below 500 °C, i.e., assuming no oxygen vacancies had formed up to 1000 °C, the actual cell volume of α-Sr0.4Ca0.6UO4−x is approximately 2% larger compared to the approximate 1% increase in CaUO4−x. This indicates that the partial substitution of Sr2+ cations onto the Ca2+ site of CaUO4 increases the amount of oxygen defects formed and, by extension, the amount of reduced uranium. D


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Inorganic Chemistry Defect Energy Formation Calculations. In Table 1, we provide the energy difference between the rhombohedral and Table 1. Calculated Energy Difference (Per Formula Unit) between the Rhombohedral and Orthorhombic Phases of Stoichiometric AUO4 for A = Ca, Sr0.4Ca0.6, and Sr composition

ΔE (rhombohedral − orthorhombic) (eV)

CaUO4 Sr0.4Ca0.6UO4 SrUO4

−0.16 0.01 0.22

orthorhombic phases of stoichiometric AUO4 (A = Ca, Sr0.4Ca0.6, and Sr) by DFT+U calculations at 0 K. These results clearly show that the stable phase of CaUO4 is rhombohedral, while the stable phase of SrUO4 is orthorhombic. This is confirmed by our experimental studies (this work and previously16,21). In the case of Sr0.4Ca0.6UO4, the energy difference between the two phases is negligible, suggesting that mixtures of both may form. This is also consistent with previous experimental results when Sr0.4Ca0.6UO4 was prepared under oxidizing conditions.20 In the current study, when prepared under reducing conditions, Sr0.4Ca0.6UO4−x is found to be single phase rhombohedral. This is thought to be related to the inability of the orthorhombic structure to retain oxygen vacancy defects that inherently exist under reducing conditions.21 Although no phase transformation was observed in CaUO4 or α-Sr0.4Ca0.6UO4 up to 1000 °C in air, it is apparent that the introduction of Sr2+ cations reduces the temperature at which oxygen defects start to form, which implies a lower oxygen vacancy formation energy for α-Sr0.4Ca0.6UO4 compared to CaUO4. To test this, DFT+U methods were used to calculate the oxygen defect formation energy of the rhombohedral (R3̅m) CaUO4−x and Sr0.4Ca0.6UO4−x structures for (4 − x) = 4, 3.75, and 3.5. These are presented together with the previous results of SrUO4−x21 in Figure 5, showing the oxygen defect formation energy increasing with increasing Ca content:

Figure 5. Oxygen defect formation energies for rhombohedral AUO4−x with A = Ca, Sr, and Sr0.4Ca0.6 as a function of the oxygen content: solid symbols denote the calculated values for CaUO4−x (black circles), α-SrUO4−x (black squares), and α-Sr0.4Ca0.6UO4−x (black triangles) using ab initio methods, and red symbols denote the derived values using eq 3 with the oxygen vacancy content (x) experimentally determined using in situ variable temperature NPD measurements for CaUO4 and α-SrUO4−x.21

l x yz ij x yz| o o x ij x yz ij Sconfig = −4R m o 4 lnjj 4 zz + jj1 − 4 zzlnjj1 − 4 zz} o k { k { k {~ n

With

− R{2x ln(2x) + (1 − 2x)ln(1 − 2x)}

assuming ideal disordering (i.e., no local ordering) of oxygen vacancies (first term in eq 4) and U5+ cations (second term in eq 4). The derived O-vacancy formation energies, EOvac(x), are added to Figure 5, which agree well with the ab initio predictions (within 0.1 eV, which is the expected uncertainty limit of DFT). Consequently, the relative oxygen vacancy formation energies among CaUO4, α-Sr0.4Ca0.6UO4, and αSrUO4 can be understood on the basis of variable oxygen vacancies and U5+ cation disordering. Structural Study 2: In Situ H2 Gas Flow Variable Temperature S-XRD. The temperature dependence of the SXRD profiles for CaUO4 and α-Sr0.4Ca0.6UO4 heated under an atmosphere of flowing high purity H2 are presented in Figures 6 and 7, respectively. The reducing environment enhances oxygen defect formation even at ambient temperature; consequently, the oxides will be denoted as substoichiometric, CaUO4−x and α-Sr0.4Ca0.6UO4−x, in the following discussions. As shown in Figures 6 and 7, the temperature dependence of the structure can be divided into three regions of interest. The S-XRD profiles show no changes upon heating CaUO4−x toward 400 °C other than the expected thermal expansion. Similar behavior is observed for α-Sr0.4Ca0.6UO4−x upon heating to 350 °C. Heating CaUO4−x to 450 °C results in a first order phase transformation as evident by the appearance of new reflections in the S-XRD profile and the loss of peaks corresponding to the rhombohedral α phase; a similar transition occurs for α-Sr0.4Ca0.6UO4−x albeit at a slightly lower temperature, approximately 400 °C. These new phases are denoted as disordered δ, and it is noteworthy that the temperature at which the transition for α-Sr0.4Ca0.6UO4−x

E(α‐SrUO4 − x ) < E(α‐Sr0.4Ca 0.6UO4 − x ) < E(α‐CaUO4 − x )

The oxygen defect formation energy for CaUO4−x and αSrUO4−x can also be estimated using thermodynamic calculations from the temperature dependence of the oxygen vacancy content (x) which was experimentally determined from variable temperature NPD studies conducted in vacuum. This is because the free energy of the oxygen loss reaction (eq 1) is zero when in equilibrium. x A UO4 → A UO4 − x + O2 , where A = Ca or Sr (1) 2 The free energy of the reaction, ΔG, can be estimated from eq 2, where the reaction enthalpy is given by the oxygen vacancy formation energy EOvac(x), SO2 is the temperature dependent entropy of the oxygen gas,54 and Sconfig is the configurational entropy resulting from the disordering of Ovacancies and the associated U5+ species. ix y ΔG = EOvac(x) − jjj SO2 + Sconfig zzzT 2 k {

(2)

When in equilibrium, ΔG = 0, hence: ix y EOvac(x) = jjj SO2 + Sconfig zzzT k2 {

(4)

(3) E


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Inorganic Chemistry

Figure 6. S-XRD patterns showing the temperature dependence of CaUO4−x heated under pure H2 gas flow from RT to 1000 °C in steps of 25 °C. The emergence of superlattice reflections corresponding to the formation of the ordered δ phase is illustrated in the inset.

Figure 7. S-XRD patterns showing the temperature dependence of α-Sr0.4Ca0.6UO4−x heated under pure H2 gas flow from 50 to 1000 °C (50 °C steps to 200 °C followed by 25 °C steps). The emergence of superlattice reflections corresponding to the formation of the ordered δ phase is illustrated in the inset.

occurs, ∼400 °C, is intermediate between that for CaUO4−x, ∼450 °C, and α-SrUO4−x, ∼200 °C.16 Continued heating of both oxides to above 500 °C under H2 gas flow results in the emergence of additional low intensity reflections at low 2θ angles in the diffraction patterns, as shown in the insets of Figures 6 and 7. Similar to δ-SrUO4−x, the appearance of these is indicative of the formation of an ordered superstructure, presumably due to the ordering of oxygen vacancy defects. With continued heating to 1000 °C, the superlattice reflections for both ordered δ phases were observed to persist, while the strong Bragg reflections showed little change other than peak shifts due to thermal expansion. It was not feasible to establish the precise hydrogen fugacity essential to establish the critical oxygen vacancy concentration required to achieve oxygen vacancy.

The S-XRD profiles of δ-CaUO4−x and δ-Sr0.4Ca0.6UO4−x, after rapidly cooling to 100 °C while maintaining the H2 gas flow revealed that the samples contained two phases, both of which could be refined as rhombohedral structures in space group R3̅m (see Figure S3 in Supporting Information). The formation of multiple phases was also encountered upon rapid cooling of δ-SrUO4−x.16 It was postulated that rapid cooling inhibits oxygen ion transport across the sample, leading to the formation of microdomains with variable oxygen content, which in turn causes variability in unit cell sizes.16 Using a slower cooling rate, a single phase sample of α-SrUO4−x was formed. Consequently to both confirm the reversibility of the observed transformation and in an attempt to form a single phase sample of CaUO4−x upon cooling, the sample was then heated incrementally to 650 °C and then cooled slowly back to 100 °C while under H2 gas flow. As shown in Figure 8, in this F


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Inorganic Chemistry

Figure 8. S-XRD patterns showing the temperature dependence of CaUO4−x reheated under pure H2 gas flow from 50 to 650 °C (50 °C steps to 600 °C followed by 25 °C steps) and then cooled back to 100 °C (50 °C steps down to 450 °C followed by 25 °C steps). The inset highlights the reversibility of the presence of superlattice reflections corresponding to the reversible formation of the ordered δ phase.

second heating cycle, the transformation to ordered δCaUO4−x occurs at roughly the same temperature as that observed in the initial heating cycle (Figure 6) at approximately 550 °C, although details of the ordering seem to be different. S-XRD patterns measured during cooling reveal that the superlattice reflections indicative of the ordered δ phases are maintained at 275 °C before these are lost upon further cooling. This is similar to the behavior of δ-SrUO4−x,16 where superlattice reflections are lost at a temperature during cooling lower than the temperature at which they first appear during heating. This hysteresis in the transition between the ordered and disordered structures is an indicator of a first order phase transition, and the differences between the heating and cooling measurements are attributed to slow nucleation kinetics. In contrast to the reported behavior of δ-SrUO4−x, inspection of the data set measured at 100 °C shows that the CaUO4−x sample remains a mixture of two phases after the second heating cycle, which suggests that oxygen mobility is higher in SrUO4−x as it returned to single phase. At this point, the H2 gas was replaced with O2 (after purging with N2), and the same CaUO4−x sample that had been exposed to the aforementioned heating/cooling cycles was next heated to 700 °C and then cooled to 100 °C under flowing O2 to investigate the reversibility of the process. At about 250 °C, the diffraction data indicate that the sample converts to single phase which is retained on heating to 700 °C and then cooling to 100 °C, see Figure 9. The 100 °C data set obtained at the end was fitted using a rhombohedral model in space group R3̅m consistent with the stoichiometric CaUO4 structure, and the Rietveld analysis provided no evidence for sample decomposition. The Rietveld profile is presented in Figure S4a (Supporting Information). This demonstrates that the process of reducing stoichiometric CaUO4 to form δ-CaUO4−x is completely reversible. Due to time constraints, the reversibility of the ordering in δSr0.4Ca0.6UO4−x was not examined. But, the sample was also heated to 700 °C under O2 gas flow to establish whether the stoichiometric α-Sr0.4Ca0.6UO4 could be recovered. This process is illustrated in Figure S5 (Supporting Information).

Figure 9. A portion of S-XRD data demonstrating the temperature dependence of CaUO4−x heated under pure oxygen from 100 to 700 °C and followed by cooling back to 100 °C.

During this oxidation heating cycle, the weak peaks due to the minor α-Sr0.4Ca0.6UO4−x phase are lost, and there is no evidence for any structural transformation or for the formation of any superlattice reflections. The profile measured at 700 °C could be fitted using a rhombohedral structure in space group R3̅m. The Rietveld profile for the 700 °C data under O2 gas flow is presented in Figure S4b (Supporting Information). Compared to CaUO4, the peaks in α-Sr0.4Ca0.6UO4 are considerably broader, indicative of strain or reduced crystallinity. Superstructure Analysis. There are noticeable variations with respect to the temperature dependence of the superlattice reflections of δ-CaUO4−x and δ-Sr0.4Ca0.6UO4−x, as illustrated in Figure 10. From 550 to 750 °C, there are broad similarities in the superlattice reflections of these two oxides and the previously reported δ-SrUO4−x.16 While the superlattice reflections from δ-Sr0.4Ca0.6UO4−x and δ-SrUO4−x16 do not G


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Inorganic Chemistry

Figure 10. A portion of S-XRD patterns comparing the superlattice reflections for δ-CaUO4−x (left, λ = 0.7764 Å) and δ-Sr0.4Ca0.6UO4−x (right, λ = 0.7755 Å) at 550, 750, and 1000 °C (intensities normalized to the strongest 100 main Bragg peaks).

Table 2. Structural Parameters for the Ordered δ-CaUO4−x Average Structure at 1000 °C from Rietveld Refinement against SXRD Data a (Å) b (Å) c (Å) V (Å3) space group site

x

U(1) U(2) Ca O(1) O(2) O(3) O(4)

0 0 0.4934(17) 0.3277(4) 0.3277(4) 0.078(3) 0.038(2)

α (deg) β (deg) γ (deg) Dtheo (g cm−3) formula

6.47035(2) 6.88559(2) 3.964331(13) 164.9065(10) P1̅ (no. 2) y 0.5 0 0.2448(16) 0.577(2) 0.035(2) 0.210(2) 0.677(2)

z

occupancy

0 0.5 0.252(3) 0.163(3) 0.712(3) −0.047(5) 0.534(4)

1 1 1 1 1 0.835(5) 0.835(5)

89.8719(3) 107.9814(3) 100.4957(3) 6.78 CaUO3.67(1) Uiso (Å2) 0.01228(8) 0.01228(8) 0.0261(7) 0.0046(15) 0.0046(15) 0.0046(15) 0.0046(15)

6.88559(2) Å, c = 3.964331(13) Å, α = 89.7819(3)°, β = 107.9814(3)°, γ = 100.4957(3)° with a cell volume = 164.9065(10) Å3. The refined structural parameters for this are presented in Table 2. The superlattice reflections were fitted using a modulation vector k = ⟨0.278, 0.159, 0.283⟩ based on the average cell. We found no evidence for higher order reflections in the diffraction data, indicating that, if present, the intensities of these are below the detection limit of the measurements. The refinement profile of this approximated structure is illustrated in Figure 11, and the structural parameters are given in Table 3. Attempts were made to establish the oxygen occupancy modulation in this model, but the refinement would not converge to a stable value, presumably as a consequence of the comparatively weak scattering of the oxygen anions relative to the uranium cations.

change much with further increase in temperature up to 1000 °C, δ-CaUO4−x undergoes further changes in its superlattice reflections above 875 °C, and this is reflected in the inset of Figure 6 and Figure 10. This change involves the splitting of some superlattice reflections and also merging of some others with increasing temperature, indicating a phase transformation with respect to the superstructure. This phase transformation was not apparent in the diffraction measurements of δSr0.4Ca0.6UO4−x (inset of Figure 7 and Figure 10) and was not observed in δ-SrUO4−x16 either. In an attempt to understand the peculiar high temperature ordering in δ-CaUO4−x, particularly in comparison to δ-SrUO4−x, symmetry representation analysis was carried out. The average structural model of δ-CaUO4−x at 1000 °C is described in space group P1̅ with a = 6.47035(2) Å, b = H


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Inorganic Chemistry

O polyhedra layers. There are two types of out-of-plane oxo oxygen atoms, O(1) and O(2), which respectively bond to U(1) and U(2) atoms with lengths of 1.985(3) and 1.994(3) Å. The ordered defect containing in-plane oxygen atoms (O(3) and O(4)) have bond lengths ranging from 2.169(13) to 2.368(14) Å bonded to U(1) and 2.181(18) to 2.728(15) Å bonded to U(2). The inability to assign a commensurate distortion vector to δ-CaUO4−x and precisely determine its structure highlights the markedly different superstructure and topological and particularly positional ordering of oxygen vacancies between δ-CaUO4−x and δ-SrUO4−x at 1000 °C. The displacement modulation of the atoms in δ-CaUO4−x at 1000 °C is shown in Figure S6 and the modulation structure in Figure S7 of Supporting Information. The most significant modulation in the structure occurs through the oxygen anions due to the ordering of the partially occupied anion sites. Generally, the structural modulation of δ-CaUO4−x is similar to that of δ-SrUO4−x, although the detail is somewhat more complex. The in-plane oxygen atoms O(3) and O(4) of the UO8 groups have an average occupancy of 0.83 that gives the most significant displacement modulation both in-plane (bc plane) and out-of-plane, which is correlated with the occupancy modulation. The displacement modulation of O(1) and O(2) is mainly in-plane (bc plane), approximately out-of-phase to that of O(3) and O(4). The U atoms also show a modulation in their displacement mostly along the caxis that is phase-related with the O(3) and O(4) modulation. Thus, the structural modulation of δ-CaUO4−x is expected to be caused by the O(3)−O(4) ordering. Figure 13 compares

Figure 11. Rietveld refinement profile of ordered δ-CaUO4−x at 1000 °C. The black line represents experimental data; red line is the refined model, and gray line is the difference plot. The blue and purple vertical markers are the allowed space group reflections of the triclinic main structure and the satellite superstructure, respectively. Note that the square root of the intensity is plotted to make the satellite reflections clearer.

Table 3. Structural Parameters for the Ordered CaUO4−x Modulation Structure at 1000 °C from Rietveld Refinement a (Å) b (Å) c (Å) V (Å3) space group modulation vector

6.47035(2) α (deg) 89.8719(3) 6.88559(2) β (deg) 107.9814(3) 3.964331(13) γ (deg) 100.4957(3) 164.9065(10) Dtheo (g cm−3) 6.78 P1̅(αβγ) formula CaUO4−x 0.278(1)a* + 0.159(1)b* + 0.283(1)c*

The structure of δ-CaUO4−x at 1000 °C is illustrated in Figure 12. Similar to δ-SrUO4−x16 and the lower temperature disordered rhombohedral CaUO4−x, δ-CaUO4−x forms a 2D structure consisting of alternating U−O and Ca−O polyhedra. The in-plane U−O polyhedra layers contain the ordered anion defects through an edge sharing motif (inset of Figure 12). Out-of-plane oxo U−O bonds are directed in a canted manner toward the Ca2+ cations which are contained between the U−

Figure 13. Comparison of a U−O layer in disordered rhombohedral CaUO4−x (left) with ordered δ-CaUO4−x (right). Navy: uranium; red: in-plane oxygen (partial shading indicates partial occupancy); aqua: out-of-plane oxygen. U(1)O8−x and U(2)O8−x polyhedra in δCaUO4−x are represented in purple and yellow, respectively.

the UO8 polyhedra layers of the higher symmetry CaUO4−x and the lower symmetry δ-CaUO4−x where the modulation of ordered δ structure is inferred through the alternating U(1)O8 and U(2)O8 polyhedra parallel to the bc plane. It is noteworthy that the transformation temperature from the stoichiometric rhombohedral α phase into the nonstoichiometric disordered δ phase is lowered as the Sr content increases for the three oxides CaUO 4−x (∼450 °C), Sr0.4Ca0.6UO4−x (∼400 °C), and SrUO4−x (∼200 °C). It is also observed that the transformation to the disordered δ phase in CaUO4−x occurs at a slightly lower temperature upon reheating, 400 vs 450 °C. Therefore, it is reasonable to postulate that the thermally induced transformation to the δ phase requires both a critical amount of oxygen vacancies and sufficient thermal energy and that this combination is not initially achieved until the sample is heated to ∼450 °C under H2. The necessary stoichiometry is maintained by cooling

Figure 12. Average triclinic structure for δ-CaUO4−x at 1000 °C in P1̅. The U and Ca cations are represented by navy and dark green spheres, respectively, whereas the in-plane and out-of-plane oxo site oxygen anions are represented by small aqua and red spheres, respectively. U(1)O8−x and U(2)O8−x polyhedra are represented in purple and yellow, respectively. I


Article

Inorganic Chemistry under H2 such that the regeneration of the δ phase is simply controlled by temperature. Evidently defect−defect interactions and their contribution to free energy play a role here.55−58 Because rhombohedral α-SrUO4 has the largest unit cell, followed by α-Sr0.4Ca0.6UO4 and then CaUO4, it is likely that defect−defect interactions would be more significant in the latter structures, as the defects would be in closer proximity and it could be energetically more costly for successive defects to form upon further reduction. This is consistent with the observation in Figure 2 for α-Sr0.4Ca0.6UO4 and CaUO4 in sealed capillaries and the ab initio calculations and derived oxygen vacancy formation energies based on thermodynamic considerations. Consequently, more energy is required to overcome stronger defect−defect interactions in CaUO4−x to obtain the critical defect amount necessary for the formation of the δ phase compared to α-SrUO4 or αSr0.4Ca0.6UO4. Interestingly, once the critical amount of defects has been obtained in CaUO4−x, it can transform to the δ phase at lower temperature upon reheating. Similar behavior has been observed for α-SrUO4−x.21 This suggests that the α to δ phase transformation is energetically less costly than the initial reduction. As we have discussed previously,16 there are several prominent examples of systems that undergo symmetry lowering transformations or exhibit ordering upon heating; however, they are invariably irreversible processes or require a change in composition. The reversible symmetry lowering through the ordering of oxygen vacancies upon heating exhibited by δ-CaUO4−x, δ-Sr0.4Ca0.6UO4−x, and δ-SrUO4−x is distinct, occurring through thermodynamic response alone. It is not immediately clear with respect to free energy what is the thermodynamic driver behind the ordering phenomena. Ab initio calculations show that the difference in enthalpy between the α and δ phases is negligible in SrUO4−x,16 thus pointing toward an entropic factor. Systems that contain disordered defects are known to often exhibit ordering at the local scale.59,60 The R3̅m rhombohedral model used to describe the average structure of the α phases does not account for this, and the possibility of local ordering in these materials cannot be precluded. The thermally induced rearrangement of anions and cations in the δ phase would be expected to contribute to a substantial change in electronic or magnetic entropy. Investigations utilizing local structure and thermodynamic analysis methods are thus necessary to understand this (apparently hidden) entropic component which might counter balance the decrease in entropy caused by the long-range oxygen-vacancy ordering. Regardless, these oxides present a novel direction for the design of materials that can reversibly exhibit ordering at high temperatures and disordering at low temperatures. Examples where this may be desirable include topological insulators and superconductors, particularly if this phenomenon can be replicated in nonactinide materials.

transformations involving the ordering of oxygen vacancies and the lowering of crystallographic symmetry, forming layered triclinic structures denoted as δ in space group P1̅. That the order−disorder transformations are reversible demonstrates that they are not related to any chemical changes such as decomposition or reduction of the sample but are thermodynamic in origin. The transformation temperature from the stoichiometric oxide to the nonstoichiometric δ phase is apparently influenced by the size of the alkaline metal cation (or unit cell size). An increase in the A-site cation size results in the lowering of the transition temperature, and it is postulated that this is related to the reduction in energetically unfavorable defect−defect interactions which inhibit the ability for further defects to form and by extension the critical amount necessary for the transformation to the δ phase. Both ab initio calculations and estimated oxygen vacancy formation energies (based on thermodynamic considerations utilizing experimental data) support this postulate where the relative defect formation energies between CaUO4−x, α-Sr0.4Ca0.6UO4−x, and α-SrUO4 can be understood on the basis of oxygen vacancy and U5+ disordering. It was found that single phase rhombohedrally structured solid solution α-SryCa1−yUO4 is not continuous for all y but only for 0 ≤ y ≤ 0.4 and 0.9 ≤ y ≤ 1.0. The intermediate compositions separate into two rhombohedral phases. Despite being isostructural, when CaUO4−x and α-Sr0.4Ca0.6UO4−x are heated under highly reducing conditions, they form δ phases with subtly different defect ordering arrangements, as indicated by the appearance of superlattice reflections. The average layered structures of both δ-CaUO4−x and δ-Sr0.4Ca0.6UO4−x are similar to the average structure of δ-SrUO4−x, such that they can be considered distortions with respect to the ordering of oxygen defects. Close examination of the diffraction patterns of δCaUO4−x shows that it contains markedly different superlattice reflections to those of δ-Sr0.4Ca0.6UO4−x and δ-SrUO4−x at temperatures above 850 °C, and changes in the superlattice reflections of δ-CaUO4−x with temperature suggests that it may undergo further phase transformations involving the modulated superlattice. The complexity and difference of the superstructures is highlighted in the structural solution of δCaUO4−x using symmetry representation analysis, where the ordering apparently follows an incommensurate modulation as opposed to the commensurate ordering observed in δSrUO4−x. This structural variation between the δ phases is attributed to the differences in their ordered anionic sublattices, which originates from subtle differences in their lower temperature disordered rhombohedral phases. Through variation of the A site composition, anion defect−defect interactions and cation local disorder are altered, and this allows for the ordering motif and superlattice morphology to be modified. The stoichiometric structures can be readily recovered by heating under pure oxygen, demonstrating the total reversibility from ambient temperature stoichiometric AUO4 to high temperature oxygen-vacancy ordered δ-AUO4−x. To our knowledge, these are the only examples of materials which can undergo reversible symmetry lowering and disorderto-order phase transformations with increasing temperature. The implications of this are profound in terms of advanced functional material design and acquisition, as it promotes the idea that materials that disorder at room temperature may be induced to reversibly order at high temperatures.

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CONCLUSION We demonstrated that the ability for rhombohedral CaUO4 to host oxygen defects at high temperatures under oxidizing conditions can be controlled and increased by partially substituting Ca2+ ions with Sr2+ ions up to 40%. Sr-doping leads to an increase in the unit cell volume, which apparently decreases defect−defect interactions, allowing more defects and, by extension, reduced uranium cations, to form upon heating. When CaUO4 and α-Sr0.4Ca0.6UO4 are heated under highly reducing conditions, they undergo reversible phase J


Article

Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00406.

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Neutron diffraction profiles; Rietveld fits for S-XRD data of CaUO4−x and Sr0.4Ca0.6UO4−x post reduction under hydrogen atmosphere and reoxidized with oxygen; modulation function for CaUO4−x and representation of the modulated structure (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhaoming Zhang: 0000-0003-3273-8889 Maxim Avdeev: 0000-0003-2366-5809 Brendan J. Kennedy: 0000-0002-7187-4579 Present Address

# G.L.M.: Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany.

Author Contributions

G.L.M. fabricated the samples. The synchrotron X-ray diffraction experiment was designed and performed by G.L.M., B.J.K., and Z.Z. with the assistance of O.M., H.B., and Q.G. Ab initio calculations were performed by G.B. and P.K. M.A. performed the neutron diffraction experiments. C.H.W. performed the structural solution analysis. G.L.M., B.J.K., and Z.Z. prepared the manuscript. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was, in part, performed at the powder diffraction beamlines at the Australian Synchrotron and the Australian Centre for Neutron Scattering. B.J.K. acknowledges the support of the Australian Research Council. G.L.M. acknowledges and greatly appreciates the support and encouragement from Dr. Chris Griffith, Dr. Robert Gee, and ANSTO Minerals with radioactive materials handling, laboratory usage, and continual support for this research. We thank Prof Siggi Schmid for helpful discussions of the modulated structure. G.L.M. also acknowledges and greatly appreciates the research funding support from the Australian Institute of Nuclear Science and Engineering (AINSE). The supercomputing resources were provided by JARA-HPC initiative.

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