Unexpected Crystallographic Phase Transformation in

May 1, 2018 - Australian Nuclear Science and Technology Organisation, Lucas Heights , NSW 2234 , Australia. § Australian Synchrotron, 800 Blackburn ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4−x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature Gabriel L. Murphy,†,‡ Chun-Hai Wang,† George Beridze,∥,⊥ Zhaoming Zhang,‡ Justin A. Kimpton,§ Maxim Avdeev,‡ Piotr M. Kowalski,∥,⊥ and Brendan J. Kennedy*,† †

School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia § Australian Synchrotron, 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 SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4−x, in space group R3̅m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4−x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4−x. Cooling δ-SrUO4−x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4−x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4−x to enable the transformation to δSrUO4−x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δSrUO4−x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4−x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4−x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

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INTRODUCTION

preserve the desired phase of these materials with successes particularly seen in the development of ceramic superconductors.5,6 Recent studies of the intricate order−disorder processes in low dimensional materials7,8 have inspired new avenues of investigation in bulk material design, for instance, reviving the notion of order emerging through disorder.9 Ordering phenomena related to the formation of anion and cation defects in crystalline metal oxides are well-known, for instance, in the transition between the (A0.5B0.5)O1.75 defect-

The design and development of societally important magnetic and superconducting materials among others, which can retain their desirable long-range ordering properties over a wide range of temperatures, have long been the ultima Thule of materials science.1−3 Often intrinsic to many of these materials is the inevitable reversible loss of electronic or structural order and consequent gain of disorder through thermally induced phase transitions. Such phase transitions are often associated with an increase in crystallographic symmetry and structural entropy.4 Considerable efforts have been made to address this, often employing chemical substitution as a means to control and © XXXX American Chemical Society

Received: February 21, 2018

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DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry fluorite and A2B2O7 pyrochlore structures.10−13 The thermally or chemically induced fluorite-pyrochlore transformation involves antisite defects, where the randomly distributed A and B type cations of the fluorite structure begin ordering in a manner that is correlated to the ordering of anionic vacancies, ultimately leading to the formation of the ordered pyrochlore structure.14−17 Ionic defects have been investigated as a means of controlling the dimensionality of structures such as in the case of the reduction of the 3D structured perovskite SrFeO3 to the layered 2D oxide SrFeO2.18−21 The gradual removal of lattice oxygen results in a succession of phase transformations, including the formation of the Sr2Fe2O5 brownmillerite structure as the intermediate, which is coupled with the reduction of the Fe cation from tetravalent to trivalent. Examples of the influence of anion defects on ordering and structure transformation have also been found in actinide systems. For example the disordered fluorite structure of the mineral vorlanite, (CaU)O4, can be transformed to the rhombohedral ordered protovorlanite structure CaUO4 by heating.22−24 The ternary uranium oxide, SrUO4, potentially relevant to spent nuclear fuel partitioning and reprocessing25 is known to exist in two polymorphs, a rhombohedral structure in R3̅m, denoted as α, and an orthorhombic structure in Pbcm, denoted as β.26,27 α-SrUO4 consists of edge sharing UO8 polyhedra forming a layered structure with Sr2+ cations in-between the layers. Similarly, β-SrUO4 consists of edge sharing UO6 polyhedra forming a layered structure also with Sr2+ cations in-between the layers.27,28 The uranium polyhedra can be considered hexagonal and square bipyramids respectively where collinear oxo uranium−oxygen bonds are present consistent with the uranyl moiety, [OUO].2,26,27,29 These uranyl groups are directed at the in-between layers containing the Sr2+ cations, whereas the six or four equatorial uranium−oxygen groups (α and β phases respectively) link together forming the 2D layered structure. The irreversible structural phase transformation from the α to the β polymorph of SrUO4 involves the formation of oxygen vacancies in the α UO8 polyhedra, associated with the reduction of uranium. These apparently reduce the activation energy barrier prior to the transformation to the β variant under oxidizing conditions.30 The transformation can only proceed if a source of oxygen is present and in situ neutron diffraction studies have shown that, without oxygen, the substoichiometric α variant persists at high temperatures with the oxygen vacancies disordered in the plane containing the uranium cations. Ab initio calculations show that the oxygen defects in the orthorhombic form, and by extension in the UO6 polyhedra, are not thermodynamically favored, whereas they are in the UO8 polyhedra of the rhombohedral, α polymorph.30 Recent investigations have shown the significance of local anionic order in the simple binary oxides of uranium UO2 and U4O9.31,32 Markedly different properties may be derived from these materials depending upon the specific details of the shortrange arrangements of the oxygen anions and associated vacant sites. Consequently, it is postulated that the continued reduction of α-SrUO4−x with an associated increase in the number of oxygen defects and electron occupancy in the uranium 5f orbitals may result in the formation of a metastable structure. This is apparently reflected in the literature that contains a couple of tentative reports on the existence of SrUO4−x (with x = 0.5−1.0) structural variants.33,34 Such studies have focused on thermodynamic methods and there is a

paucity of detailed structural analysis, such as in situ neutron or synchrotron X-ray diffraction studies. Understanding the behavior of uranium oxides at high temperatures is pertinent both from the fundamental perspective of understanding the complex chemistry of actinides and the applied approach of reprocessing spent UO2 nuclear fuel and may shed light on the micro to macroscopic processes surrounding the exposure of uranium oxides to extreme scenarios, for example, those in uncontrolled meltdown events.35,36 As a part of our ongoing studies of the solid state chemistry of actinide and nuclear waste form relevant materials,26,30,37−39 we have used in situ synchrotron powder X-ray diffraction to study the high temperature reduction of rhombohedral αSrUO4 by hydrogen gas at temperatures of up to 1000 °C. Herein we show that, under pure H2 gas flow, α-SrUO4−x undergoes a symmetry lowering phase transformation to a novel triclinic phase, denoted as δ, at high temperatures. Remarkably, the δ phase forms through the ordering of previously disordered oxygen vacancies in the α phase. This peculiar oxygen defect ordering and symmetry lowering, observed with increasing temperature, was found to be reversible with subsequent cooling of the sample resulting in the reformation of the substoichiometric disordered α phase. This indicates that the transformation is a consequence of thermodynamic response alone and not reduction of uranium or loss of oxygen. The Rietveld refinement method with symmetry representation analysis was used to establish the structure of the newly discovered δ phase and ab initio calculations using density functional theory were employed to try to understand the observed unusual order−disorder behavior.

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EXPERIMENTAL SECTIONS

Synthesis and Characterization. A polycrystalline sample of αSrUO4 was prepared using conventional solid state synthetic methods, as described previously.26 This was handled to minimize exposure to personnel and contamination of the environment. Room temperature structural characterization was performed on the powder diffractometer (S-XRD) at the Australian Synchrotron40 using X-rays of wavelength 0.82689 Å.26 Data collected from the measurement were analyzed using the Rietveld method against a rhombohedral structure in R3̅m, consistent with previous investigations,26,30 and the sample was found to be single phase (see Supporting Information, Figure 1). The structures were refined by the Rietveld method, as implemented in the program GSAS.41,42 The peak shape was modeled using a pseudo-Voigt function, and the background was estimated by a 12 term 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 reduction studies, a finely ground sample of α-SrUO4 was carefully mixed with coarse analytical grade quartz glass at a 50:50 ratio by wt %. The mixture was packed into a 0.8 mm diameter quartz capillary where the ends were loaded with coarse analytical grade quartz glass and glass wool to allow gas flow and prevent sample loss. The capillary was mounted on a goniometer equipped with a Norby-flow cell apparatus43 that allowed rocking motion during the S-XRD measurements, to minimize the effects of preferred orientation. The gas inlet line was connected to the cell via a flow meter to ensure a constant flow of high purity hydrogen gas (≥99.99% with oxygen ≤10 ppm and moisture ≤20 ppm). Prior to variable temperature measurements, the sample was flushed with high purity H2 for 30 min at room temperature. Temperature control was achieved using a FMB-Oxford hot air blower. Temperatures were increased at a ramp rate of 5 °C per minute and decreased at a rate of 5 or 25 °C per minute, and the data collection was commenced after a 30 s delay at temperature to allow B

DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry thermal equilibration. Data were collected for 5 min at each of the two positions of the MYTHEN II microstrip detector at temperature steps from room temperature to 1000 °C. The wavelength was 0.77466 Å, calibrated using a NIST LaB6 660b standard. Super Structure Analysis. The structure of δ-SrUO4 was analyzed using TOPAS Academic (TA)44,45 and ISODISTORT46 against S-XRD data collected at 1000 °C. The high temperature structure of ordered δ-SrUO4−x (super structure, lower symmetry) was considered to be a distortion of the low temperature disordered αSrUO4−x phase (higher symmetry). The structural model was developed using the following steps to avoid possible overparameterisation and to obtain a stable refinement, reflecting the significantly lower X-ray scattering form factor of oxygen atoms relative to the heavy uranium atoms. First, an average structural model of α-SrUO4−x in space group R3̅m was refined without considering the supercell reflections. Then, a symmetry representation analysis was applied in order to build a model for δ-SrUO4−x, starting with the refined αSrUO4−x model and taking into account the supercell reflections. A distortion model (triclinic cell in space group P1̅) with commensurate distortion vector k = ⟨1/4 1/4 3/4⟩ was used and distortion modes were refined to account for the positions of the observed supercell reflections. Bond distances restraints of 1.99 ± 0.02 and 2.33 ± 0.02 Å with a weighting of 1 were imposed for the uranyl oxo-like and equatorial uranium−oxygen bonds, respectively. In the final refinement cycles, the structural model was fully relaxed with all free parameters refined. Ab Initio Calculations. The ab initio calculations were performed with density functional theory (DFT) using the plain-wave QuantumESPRESSO simulations package.47 The computational setup was exactly the same as that used in our previous studies of the α and β phases of SrUO4−x.30 The plane-wave energy cutoff was set to 50 Ryd and ultrasoft pseudopotentials were used to mimic the presence of core electrons.48 To be consistent with previous calculations, we applied a PBE exchange-correlation functional. The stoichiometric δ phase was modeled by a 2 × 2 × 2 supercell containing 192 atoms. Consequently, gamma point (k-point grid) calculations were sufficient to achieve a converged solution. The results of these calculations are consistent with the computation of the smaller 1 × 1 × 1 supercell but we used the larger supercell as it gave us better flexibility to consider different O-vacancy arrangements. In order 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.49 The same procedure has been successfully tested on large sets of uranium-bearing molecular and solid complexes50 and applied in the computational studies of the α and β phases of SrUO4−x.30 The derived Hubbard U parameter used in the current study is 2.5 eV, which is the same value as derived for the α and β phases of SrUO4−x.30 This value is in the range expected for U(VI) and U(V) compounds.50

Figure 1. S-XRD data at 2θ ranging from 12.5 to 15°, illustrating the temperature dependence of α-SrUO4−x heated from 50 to 350 °C under high purity hydrogen flow, where the onset of the disordered δSrUO4−x phase and loss of the α-SrUO4−x phase is apparent with increasing temperature. Note the splitting of the peak near 14°, indicating the lowering of symmetry with temperature.

until 350 °C, at which temperature the sample is single phase. This new phase is denoted as disordered δ-SrUO4−x. The diffraction data suggest that the sample remains single phase δ-SrUO4−x from 350 to 450 °C, the gradual shift in the peaks to lower angles being a consequence of thermal expansion (which seems to be strongly anisotropic in this temperature range). Continued heating of the sample to above 450 °C resulted in the appearance of some extremely weak reflections in the 2θ region from 8 to 12.7°, as shown in Figure 2. Further heating of the sample, while maintaining the flow of hydrogen gas, to 1000 °C did not result in any significant change to the diffraction data, other than peak shifts due to thermal expansion. The intensity of the weak superlattice reflections does not change significantly as the sample was heated from 475 to 1000 °C. The sample was then cooled to 50 °C, at a rate of 25 °C/min, while maintaining the hydrogen flow. Examination of the resulting S-XRD profile at 50 °C showed that, under these conditions, the sample was a twophase mixture, with both phases being able to be fitted with rhombohedral models in space group R3̅m. This is consistent with the reformation of α-SrUO4−x, albeit with slightly different lattice parameters, see Figure 3 for refinement profiles. The two cation sites in both phases were completely occupied. It was not possible to refine the anion occupancies of the two phases, but it seems reasonable to propose that the larger cell contains more vacancies and is hence more reduced and that the twophase anomaly is a consequence of variable amounts of defects between the two phases. The sample was next reheated while still under hydrogen flow, with diffraction data measured at 50 °C intervals up to 550 °C and recooled back to 50 °C with a slower cooling rate of of 5 °C/min. It was observed (Figure 4) that the peaks corresponding to the larger rhombohedral cell began to lose intensity as the sample was reheated toward 200 °C, whereas the peaks corresponding to the smaller cell began to gain intensity. This is consistent with oxygen diffusion within the sample that allows the sample to reach an equilibrium state in terms of oxygen stoichiometry. Around 300 °C additional reflections, near 13.0° and 13.7° in Figure 4, were observed consistent with the re-emergence of the disordered δ phase. The sample was observed to contain two phases, α and δ, over the temperature range of 300 to 400 °C, thereafter it was single

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RESULTS AND DISCUSSION In Situ Synchrotron X-ray Diffraction Studies. The temperature dependence of the structure of α-SrUO4−x, heated under high purity hydrogen to 1000 °C, was established by SXRD. The thermodiffraction data could be divided into four regions. From 25 to 75 °C the sample remains single (rhombohedral) phase with unexceptional thermal expansion. It is appropriate to describe the material present at temperatures below 125 °C as α-SrUO4−x. Heating this to around 150 °C resulted in the formation of weak reflections to the left of some strong Bragg reflections, most obvious of these weak reflections being near 2θ = 13.0 and 13.7°, as shown in Figure 1. As the sample was further heated to 200 °C, the intensity of these reflections increased, while the reflections from rhombohedral α-SrUO4−x began to lose intensity, indicating that the material undergoes a discontinuous first order structural transformation. The two-phase region coexists up C

DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Variable temperature S-XRD data taken from δ-SrUO4−x at temperatures from 350 to 1000 °C, shown at 2θ ranging from 5.3 to 15.5°. The inset illustrates the onset of superlattice reflections with heating to T > 450 °C, indicating the transition from the disordered δ-SrUO4−x to the ordered δ-SrUO4−x.

δ phase. Continual heating to above 450 °C resulted in the reappearance of the extremely weak superlattice reflections at 8° < 2θ < 12°. These persisted to 550 °C, at which time the sample was recooled, as illustrated in Figure 4. That the superlattice reflections re-emerge upon heating and are lost on cooling demonstrates that these are the result of a reversible phase transition rather than irreversible sample decomposition. Comparing the diffraction pattern measured at 50 °C (Figure 3c), after the sample had been heated to 1000 and 550 °C as described above, with that measured at 50 °C prior to heating (Figure 3a), showed the diffraction patterns to be essentially identical and both could be fitted with a α-SrUO 4−x rhombohedral model in space group R3̅m. Evidently, the sample is stable to decomposition when heated to 1000 °C under a H2 environment. To summarize, heating α-SrUO4−x under H2 gas flow results in a reversible first order reconstructive transformation between the α and δ phases. Second, the emergence of weak superlattice reflections, most obvious in the 2θ region of 8 to 12°, in patterns measured above 450 °C is suggestive of the formation of a superstructure which most likely involves oxygen-vacancy ordering. Remarkably these measurements show that the intensity of these reflections appears to increase slightly with increasing temperature, suggesting that the δ phase is becoming more ordered as the temperature is increased. When the temperature is reduced to below ∼200 °C the superlattice reflections are lost, corresponding to the loss of the ordered superstructure, and regeneration of the disordered α phase. It is postulated that the δ phase only forms when the concentration of oxygen defects reaches a critical value and that these oxygenvacancy defects are ordered at high temperatures. Remarkably, the oxygen-vacancy ordering is lost upon cooling, even while maintaining a H2 atmosphere and, presumably, a constant number of vacancies. Consequently, this implies that once this critical value of oxygen vacancies is obtained in the α phase, the transformation may occur reversibly through thermodynamic response alone, and is not a result of further reduction of the sample.

Figure 3. Rietveld profiles for α-SrUO4−x taken under high purity H2 gas flow at 50 °C (a) before high temperature treatment, (b) after the first high temperature treatment to 1000 °C and (c) after the second high temperature treatment to 550 °C. The insets highlight the purity and absence of sample decomposition during the heating cycles. λ = 0.77466 Å. Note the two phases apparent in the (b) profile where both phases could be refined against rhombohedral models in space group R3̅m.

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Figure 4. S-XRD heating data taken from SrUO4−x at temperatures ranging from 50 to 550 °C and cooling back to 50 °C under high purity hydrogen gas flow (5.3 ≤ 2θ ≤ 15.5°). The inset illustrates the onset of superlattice reflections with heating and loss thereof from cooling, highlighting the reversibility of the disorder (at lower temperatures) and order (at higher temperatures) transformation.

Structural Analysis and Solution of δ-SrUO4−x Super Structure. The δ-SrUO4−x structure can be considered as a distortion of the α-phase, evident by the observed superlattice reflections in the S-XRD pattern (Figure 2). These reflections follow a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩ related to the parent rhombohedral unit cell (α-SrUO4−x). Structural analysis of δ-SrUO4−x was based on S-XRD data measured under H2 at 1000 °C, since these data showed the best resolution of the superlattice reflections. The structural model of the ordered δ-phase was built and refined with the use of the program ISODISTORT. The pattern at 1000 °C was indexed to a triclinic unit cell in space group P1̅ with a cell volume ∼355 Å3. The undistorted triclinic cell, where the atoms are constrained to the equivalent positions of the parent rhombohedral cell resulted in a Rwp factor of the refinement of 9.20% and this was significantly improved through the introduction of one displacement distortion mode for the U and Sr cations, two anion displacement distortion modes and four anion occupancy distortion modes (Rwp = 3.98%). A further relaxation of the model to allow for refinement of the anion positional parameters resulted in a slight improvement to the refinement (Rwp = 3.21%) with the profile shown in Figure 5. In the final refinement cycles the atomic displacement parameters of each type of ions were constrained to be equal. In Figure 5 the intensity (I) was plotted as √I to highlight the details of the weak superlattice reflections. The structural model, described in Table 1, gives an excellent fit to the experimental data and the positions and relative intensities of the super-reflections are well described. This is apparently the first triclinic structural model for SrUO4−x reported to date.33,34 A representation of the structure of δ-SrUO4−x is given in Figure 6. Similar to other SrUO4−x phases, δ-SrUO4−x adopts a layered structure. The U−O polyhedra form UO4−x layers via edge-sharing connections and the Sr2+ cations sit between the layers. There are two types of oxygen sites with respect to the layers, the oxo-oxygens at sites O(1)−O(4) and the in plane oxygens at sites O(5) − O(8). The structural analysis suggested that the four oxygen sites O(1)−O(4) were fully occupied and

Figure 5. Rietveld refinement profile of the ordered δ-SrUO4−x against the S-XRD data at 1000 °C; Dots, observed; solid line, calculated curve; gray line below, difference curve; vertical tick marks, peak positions. wRp = 3.21% and Rp = 2.42%.

this was assumed in the final refinements. The occupancies of the other four anion sites O(5)−O(8) were varied in the final refinement cycles and, with the exception of O(7), these are all partially occupied. The occupancy of O2− is illustrated by shading of the spheres in Figures 6 and 7 and is tabulated in Table 1. The refined occupancies of the O(5), O(6), O(7), and O(8) sites are 0.90(2), 0.61(3), 1.00(3), and 0.89(3), respectively, corresponding to the compositional formula SrUO3.70(3). The out-of-plane oxo U−O bond length varies from 1.892(17) to 2.083(17) Å and the in plane U−O bond length ranges from 1.85(2) to 2.761(16) Å. δ-SrUO4−x can be considered as a structural distortion of the rhombohedral α-SrUO4−x phase due to the preferential rearrangement of oxygen vacancies from selected in-plane sites. A comparison of the U−O layers in α-SrUO4−x and δSrUO4−x is shown in Figure 7. In α-SrUO4−x the uranium cations have (near) 8-fold coordination with oxygen forming E

DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Parameters of the Ordered δ-SrUO4−x at 1000 °C from Rietveld Refinement a (Å) b (Å) c (Å) V (Å3) space group site U(1) U(2) U(3) Sr(1) Sr(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8)

x 0 0 0.0025(3) 0.4859(8) 0.4825(8) 0.299(3) 0.300(3) −0.326(3) 0.323(3) 0.023(4) −0.048(6) −0.035(3) −0.044(4)

α (deg) β (deg) γ (deg) Dtheo (g cm−3) formula z 0 0.5 0.2517(2) 0.6173(10) 0.1189(10) 0.067(3) 0.589(3) 0.162(4) 0.325(3) 0.240(3) 0.314(4) −0.044(2) 0.510(3)

6.72851(3) 6.97217(6) 8.06596(6) 355.159(4) P-1 (No 2) y 0 0 0.4737(2) 0.7580(6) 0.7479(6) 0.065(3) 0.073(3) 0.444(3) 0.508(3) 0.128(3) 0.812(4) 0.254(3) 0.297(3)

occupancy 1 1 1 1 1 1 1 1 1 0.90(2) 0.61(3) 1.00(3) 0.89(3)

89.8732(6) 107.3712(5) 99.9942(8) 7.20(1) SrUO3.70(3) Beq (Å2) 2.283(9) 2.283(9) 2.283(9) 2.99(3) 2.99(3) 2.71(12) 2.71(12) 2.71(12) 2.71(12) 2.71(12) 2.71(12) 2.71(12) 2.71(12)

Figure 6. Representation of the ordered δ-SrUO4−x. Red: in plane oxygen (with partially shaded red spheres denoting the partial occupancy of the sites); light blue: out of plane oxygen; navy: uranium; green: strontium.

Figure 7. Representation of a U−O layer in α-SrUO4−x and ordered δ-SrUO4−x. Red: in plane oxygen; light blue: out of plane oxygen, navy: uranium. The apparent ordered oxygen defects sites in δ-SrUO4−x are O(6) with occupancy of 0.61(3), as shown by the partially shaded red spheres, which are highlighted by the black circles. The oxygen vacancies in α-SrUO4−x are evenly distributed over the in plane O(2) sites (i.e., disordered), also shown by the partially shaded red spheres. F

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oxygen content results in the energy of the α phase becoming lower than that of the β phase.30 The calculations suggest that as the number of vacancies increases the δ phase becomes slightly lower in energy relative to both the α and β phases. We note that the introduction of vacancies does not result in a dramatic stabilization of the δ phase, which could have possibly explained the formation of δ superstructure. This points toward entropy as a driver for the observed high T ordering transition. To understand the observed stability of these phases, entropy needs to be considered as well. The random distribution of vacancies in α-SrUO4−x should result in an additional configurational (disordering) entropy and the stabilization of the α phase at ambient condition. On the other hand, since the U in SrUO4−x is in a mixed valence (U6+ and U5+) state there is the additional possibility of short-range ordering of the U5+ cations. The local ordering of U5+ cations should decrease as the temperature increases leading to a rise in the configurational entropy with temperature. Our calculations of the U5+ cation disordering energy shows that significant short-range ordering should be present at ambient conditions in all three phases (the obtained disordering energy in the δ phase is ∼4 kJ/mol comparing to RTambient = 2.49 kJ/mol) and that it is much easier to disorder U5+ cations in the vacancy-ordered δ phase than in the α phase. This strongly suggests that at some critical temperature this effect should make the vacancy-ordered structure preferable. The importance of configurational electronic entropy terms in accounting for the stability of solid phases has been noticed before,51 but to the best of our knowledge, our studies would be the first demonstration that the electronic entropy could also influence phase diagrams. We note that in order to correctly describe these high-T ordering effects at the quantitative level, a complex thermodynamic modeling that accounts for shortrange ordering should be applied, which would involve computationally intensive ab initio calculations of several U5+ cation arrangements. Such studies are well beyond the scope of this paper and could be a topic of subsequent and extensive thermodynamic modeling studies. Defect Ordering and Relation to Other Structural Systems. The transition to the anion-vacancy ordered lower symmetry triclinic δ-SrUO4−x from the disordered higher symmetry rhombohedral α-SrUO4−x does not seem to involve a change in cation order, although it may involve charge ordering analogous to that seen in, for example, brownmillerite, where oxygen ordering is coupled with charge ordering of the Fe cations. Since the current S-XRD data are not sensitive enough to detect possible cation ordering, bond valence sum (BVS) calculations were used to further examine if the U5+ and U6+ ions in δ-SrUO4−x were ordered or if they were randomly distributed. As discussed by Burns et al.,52 such calculations in uranium oxides are limited both by the precision of the structural determination (the heavy U cations dominate X-ray diffraction hence reducing precision) and the impact of the uranyl group. With these reservations in mind, the BVS calculations suggest that the valence of the 8-coordiate U(3) site is significantly less than that of the 8-coordinate U(2) and 6-coordinate U(1) sites, suggesting charge ordering could occur (at least at the local level), which is in line with our calculations. This would need to be confirmed with a more precise structural refinement, possibly using neutron diffraction, or total scattering experiments. In α-SrUO4‑x, the formation of a critical amount of oxygen defects, generated by treatment with H2, facilitates a temperature dependent, reversible ordering of anion

UO8 polyhedra where oxygen vacancies are distributed evenly among the six nonuranyl oxygen sites. Each in plane oxygen atom is shared by three UO8 polyhedra and sits at the center of three neighboring U atoms, forming a regular U3 triangle. This symmetry is broken in δ-SrUO4−x at temperatures above 450 °C. In ordered δ-SrUO4−x, the U3 groups no longer retain a regular triangular configuration, due to the preferential loss of oxygen at the O(5), O(6), and O(8) sites. This results in a reduction of the space group symmetry to triclinic. The appreciable loss at O(6) appears to be the main driving force for the symmetry lowering, with O(6) moving away from the center of the U3 triangles. This displacement results in a change in the effective coordination of the U(1) cation to 6 coordinate, while U(2) and U(3) cations remain 8-coordinate. Thus, 1/3 of the U exists in UO6 octahedra and 2/3 in UO8 polyhedra. Note that the U(3)−O(7) distance is relatively long at 2.761(16) Å. The refinement suggests that the anion vacancies are clustered around the U(2) and U(3) UO8 polyhedra and are absent in the U(1)O6 groups. This observation is similar to that made for α-SrUO4−x and β-SrUO4, namely, oxygen vacancies can be found in the UO8 polyhedra of the former but are not observed in the UO6 polyhedra of the latter.30 Ab Initio Calculations. Having solved the new δ superstructure we computed its variations in order to check its stability against the α-SrUO4−x rhombohedral phase, namely, the oxygen defect formation energy. We performed computations for the δ-SrUO4−x structure at x = 0.5, x = 0.25 and x = 0. The three different structures were obtained by adjusting the content of O(6) oxygen atom sites (empty for the case of x = 0.5 and full for x = 0). We note that our computed δ structure does not exactly represent oxygen occupation of the observed more complex superstructure (since it does not contain vacancies on the O(5) or O(8) sites), and therefore, the results of the DFT calculations only allow qualitative interpretation of the experimental results. The obtained energies are given in Figure 8, together with those of the

Figure 8. Defect formation energies, corresponding to the most stable oxygen vacancy arrangement, for α (black circle), β (red square), and δ (green diamond) structural phases as a function of oxygen content in SrUO4−x, obtained using the PBE+U approach.

stable α and β phases of SrUO4−x at specific oxygen contents taken from our previous studies.30 The solid lines represent the trends as a function of oxygen content and are provided for visual guidance only. As indicated in Figure 8, the stoichiometric δ-SrUO4 structure is higher in energy than the corresponding β structure but its energy is very similar to that of the α phase. The structures of the stoichiometric α and δ phases are nearly identical, and this is reflected in their essentially equal energies. On the other hand, stoichiometric βSrUO4 has the lowest energy by 0.22 eV. Decreasing the G

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Figure 9. Computed structure for rhombohedral α-SrUO3.5; note the apparent occurrence of both UO6 and UO8 polyhedra as the inset highlights. Uranium, strontium, and oxygen atoms are represented as navy, green, and red, respectively, in-plane and out-of-plane oxygens are not distinguished here.

SrUO4−x structure and if such local order is a precursor to the formation of δ-SrUO4−x. Examining the relaxed computed structure for the nonstoichiometric α-SrUO3.5 as shown in Figure 9, it is noteworthy that there is an apparent mixture of UO6 and UO8 polyhedra similar to that observed in the δSrUO4−x. Though tentative, it is suggestive that short-range ordering could be occurring in nonstoichiometric α-SrUO4−x, which might influence the peculiar transformation to the δ phase as observed here (see Figures 6 and 7). Further studies utilizing atomic pair-distribution function analysis are desirable to address this possibility. Supporting Information, Table 1, compares the computed structures of SrUO3.5 and SrUO4 with nonstoichiometric refined structural models. The presence of the oxo uranium uranyl-like moieties in the α and δ phases is postulated to play a key role in the transformation. The structure and directionality of the uranyllike moieties in the AUO4 (A = Ca, Sr) oxides make them distinct to the tetragonal ABO4 scheelite structural variants. The scheelite structures do not display comparable ordering to that encountered here for SrUO4. The oxo uranyl moiety, an example of the general actinyl moiety, is unique to the actinides and its use in chemical structural design has been limited, largely due to the difficulties in studying actinides. Although similar functional groups are known, for instance, the structurally similar vanadyl group, its electronic structure and redox chemistry are markedly different from the actinyl groups.54,55 Indeed more recent studies have demonstrated that the vanadyl group and other nonactinide surrogate elements do not replicate the unique chemistries and properties of the actinides.30,32,37,56−63 The transformation identified here is both novel and remarkable, and coupled with previous investigations,26,30,37 a plethora of unique properties and fascinating structural chemistries are progressively unravelling, which are only realized through the utilization of actinide chemistry.

vacancies. While this ordering may be accompanied by charge ordering in the α- to δ-SrUO4−x transition, it is distinct from the perovskite to brownmillerite transformation of SrFeO3−x. Once the ordered high temperature δ phase is formed, it can reversibly transform back to the disordered low temperature α phase without a change in the average oxidation state of the uranium cations or in the concentration of oxygen vacancies. This does not happen in SrFeO3−x, and we are unaware of any other examples of a system that reversibly transforms to a lower symmetry structure upon heating without a change in its stoichiometry. Prodan and Boswell postulated that the basic building elements in the defect structure of reduced CaUO4−x (with x = 0−0.5), which is isostructural with α-SrUO4−x, are a central 6coordinated U cation surrounded by six 7-coordinated Ca cations or a central 6-coordinated Ca cation surrounded by six 6-coordinated U cations.53 Oxidation and reduction of CaUO4 involves a change in the geometry of the uranium cations between UO6 in reduced CaUO3.5 and UO8 in stoichiometric CaUO4 through the formation of microdomains. That we observe two reduced rhombohedral phases of α-SrUO4−x in the first cooling cycle under hydrogen is in keeping with the earlier proposal of Prodan and Boswell.53 The transformation from δ to α that occurs as the sample is cooled occurs at a temperature where oxygen conductivity and migration is inhibited, resulting in the formation of microdomains of two rhombohedral (α) phases with variable oxygen content. This may also be impacted by the rate of reformation of the intermediate disordered δ phase during cooling. It should be noted that the loss of the superlattice reflections with cooling occurs at a lower temperature than their appearance upon heating; this is likely a result of slower reaction kinetics, as the cooling rate (25 °C/ min) is much faster than the heating rate (5 °C/min), coupled with the coarser temperature steps used in the first cooling cycle. The second heating/cooling cycle under hydrogen used a slower cooling rate (5 °C/min) that is believed to have allowed oxygen to better equilibrate across the bulk sample, resulting in a single α phase. From this, oxygen conductivity in both the α and δ phases is expected to occur at modest temperatures, although this is yet to be directly established. That the oxygen defects are seemingly confined to the UO8 polyhedra and absent in the UO6 polyhedra is consistent with the idea propounded by Prodan and Boswell.53 That uranium oxides can adopt complex structures is well illustrated by the recent work of Desgranges et al.32 who showed that short-range order intrinsically exists in UO2, where it is best described in the space group Pa3̅ rather than Fm3̅m due to local distortions of the UO8 polyhedra. It is not known if similar short-range order also exists in the low temperature α-

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CONCLUSIONS In summary, we have shown that under highly reducing conditions oxygen deficient α-SrUO4−x in space group R3m ̅ undergoes a first order phase transformation above 200 °C to a new phase denoted as δ-SrUO4−x: first as a disordered δ structure without long-range oxygen-vacancy ordering followed by an ordered δ structure at temperatures >450 °C. The ordered δ-SrUO4−x structure could be refined against a triclinic model in space group P1̅ and is characterized by ordering of the oxygen vacancies. Remarkably the transformation between αSrUO4−x and δ-SrUO4−x is reversible, cooling the sample results in the reformation of the rhombohedral α-SrUO4−x phase in H

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ratory usage, and continual support for this research. 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 the JARA-HPC initiative.

which the oxygen vacancies are disordered. That the transformation is reversible suggests that the formation of δSrUO4−x is a thermodynamically driven process, and is not caused by a change in the concentration of oxygen vacancies, which apparently remain constant under the hydrogen gas flow. This transformation has not been observed in less reduced samples of α-SrUO4−x, demonstrating that a critical amount of oxygen defects must be present to allow their ordering and thus the formation of the δ phase. Nevertheless, once obtained, ordering and symmetry lowering with increasing temperature can be readily achieved at relatively low temperatures. Such phenomena are intrinsic to SrUO4−x, while there are examples where oxygen vacancy ordering is favored at high temperatures, such processes are generally irreversible.64,65 We are unaware of examples where oxygen disordering occurs upon lowering the temperature. Presumably this involves a delicate balance between the entropy and enthalpy of the system, and further work is required to understand this. Pertinently the transformation identified in the current study presents a novel direction for the design of a new class of ordered materials that might be expected to possess advanced functional properties for applications such as sensors, catalysts, and in high temperature solid oxide fuel cells.66

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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.8b00463. Rietveld refinement profile of unreduced α-SrUO4 and illustration of the structures of the nonstoichiometric α and δ phases obtained from synchrotron X-ray diffraction measurements and the computed structures for α and δ SrUO4−x phases with x = 0 and 0.5 (PDF).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; b.kennedy@chem. usyd.edu.au. ORCID

Maxim Avdeev: 0000-0003-2366-5809 Brendan J. Kennedy: 0000-0002-7187-4579 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 J.A.K. Ab initio calculations were performed by G.B. and P.K. M.A. performed the initial neutron characterization experiment. 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 beamline at the Australian Synchrotron. 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, laboI

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DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00463 Inorg. Chem. XXXX, XXX, XXX−XXX