High-Pressure Synthesis, Structural, and Spectroscopic Studies of the

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

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High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni−U−O System Gabriel L. Murphy,†,‡ Philip Kegler,§ Yingjie Zhang,‡ Zhaoming Zhang,*,‡ Evgeny V. Alekseev,§ Martin D. de Jonge,∥ 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 § Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany ∥ Australian Synchrotron, Australian Nuclear Science and Technology Organisation, 800 Blackburn Road, Clayton, Victoria 3168, Australia

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The first comprehensive structural study of the Ni−U−O system is reported. Single crystals of α-NiUO4, β-NiUO4, and NiU3O10 were synthesized, and their structures were refinedusing synchrotron singlecrystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4 adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO4 containing corrugated two-dimensional (2D) layers of corner-sharing UO6 polyhedra and edge-sharing onedimensional (1D) zigzag α-PbO2 rutile-like chains of NiO6 polyhedra in the [001] direction. β-NiUO4 is isostructural to MgUO4 and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6 and NiO6 polyhedra in the [001] direction as in regular TiO2 rutile. NiU3O10 forms a triclinic structure in space group P1̅ and is isostructural with CuU3O10, where it forms a three-dimensional (3D) framework structure built through a mixture of UO6 and UO7 polyhedra in which the NiO6 polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO4 were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni2+ and U6+ cations, where the Ni2+ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6 polyhedra sit isolated within the framework of 3D connected UO6/UO7 polyhedra. Synthesis conditions indicate that βNiUO4 is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni−U−O system, the larger AUO4 family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

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chemistry that can be found within the actinides,6−16 which has the potential to further guide advances in general inorganic chemistry and materials science. Advanced characterization methods can provide the critical understanding necessary to efficiently pursue innovative waste-form development.3 A salient example is our recent investigation of the reversible phase transformation between α-SrUO4−x and δ-SrUO4−x.17 This reversible transformation involving the lowering of symmetry and ordering of oxygen defects upon heating is, to the best of our knowledge, the first of its kind and is apparently associated with the presence of the actinyl moiety.17,18 αSrUO4−x and δ-SrUO4−x are members of the AUO4 class of

INTRODUCTION Nickel-based materials have applications in molten salt nuclear reactors and are being considered as part of future generation IV nuclear reactor designs.1,2 Considering this, it is pertinent to examine the solid-state chemistry of the ternary Ni−U−O system, particularly in relation to the understanding of phases that may form from potential fuel-material interactions, waste from processing, or during decommissioning.3 Traditionally, studies of uranium oxides have focused on tetravalent uranium due to its relevance to nuclear fuels.4,5 More recently, emphasis has shifted toward examining higher-valence uranium and other actinide materials in relation to both waste-form research and improving fundamental chemical insight. This has resulted in what could be described as the renaissance of actinide chemistry, and several recent studies have highlighted the rich © XXXX American Chemical Society

Received: August 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Since α-NiUO4 is apparently isostructural to CrUO4, which possesses trivalent Cr and pentavalent U cations,14 and βNiUO4 is isostructural to MgUO4 in which the Mg is divalent and U hexavalent,14 it is possible that the relative stability of the two structures is related to electron transfer between the Ni and U cations. Generally Ni3+ is far less commonly encountered than Ni2+ owing to the stability of the 3d8 electron configuration of the latter. This presents an interesting hypothesis to be tested here: does an orthorhombic structure of NiUO4 in space group Pbcn exist, and if so, does it require the presence of trivalent Ni and pentavalent U cations (isoelectric with CrUO4 and FeUO4), or can the structure be achieved with divalent Ni with corresponding hexavalent uranium cations? In the present investigation, flux growth and HT/HP synthetic methods were used to first understand the conditions under which the oxides NiU3O10 and the two polymorphs of NiUO4 form. Single-crystal synchrotron X-ray diffraction measurements were used to determine the precise structures of these oxides. To determine the U oxidation state, X-ray absorption near edge structure (XANES) measurements were performed at the U L3-edge from single crystals using XANES mapping. The results are discussed with respect to the structural chemistry of the Ni−U−O system, other members of the AUO4 family of oxides, and the relationship to the rutile variants in the AUO4 family of oxides.

oxides that are potentially relevant to spent nuclear fuel reprocessing4,19 and uranium minerals science.20−23 These oxides can support a variety of different metal cations with variable oxidation states on the A site, and a plethora of structures are known.24−31 Despite the variation in the precision of the reported structures, the structural-property relationships that exist between them, including the understanding of the chemical requirements for structural formation, are becoming unravelled.14,17,18,25,26 However, contention still exists in the literature regarding the ability for these oxides to incorporate certain cations, such as nickel.26 Several examples of structural transformations in AUO4 oxides are known. For instance, the transformation between the alkaline earth metal rhombohedral and orthorhombic variants is well-described, in particular, the irreversible transition between rhombohedral α-SrUO4 and orthorhombic β-SrUO4.18 The transformations that occur between U6+ oxides with rutile-related structures have only been described for CdUO4, where the rhombohedral α-CdUO4 to orthorhombic β-CdUO4 transformation is reported to be irreversible.28,32 There appears to be no reports of phase transformations involving any of the orthorhombic AUO4 oxides in space group Ibmm seen in MnUO4,26 CoUO4,30 and MgUO429 or in space group Pbcn seen in CrUO431 and FeUO4.26,33 Indeed, with the exception of NiUO4 none of the AUO4 oxides containing a transition metal are known to be polymorphic, and as such NiUO4 may represent a “missing link” between the Ibmm and Pbcn structural types. The details of these two orthorhombic structures are described below. There are two reports from the 1960s on the structures of αNiUO4 and β-NiUO4.34,35 The formation of these compounds was however questioned in a recent paper.26 In 1967 Hoekstra and Marshal34 examined several ternary uranium oxides describing a NiU3O10 structure and two polymorphs of NiUO4. On the basis of their single-crystal X-ray diffraction analysis, it was concluded that NiU3O10 is isostructural to CuU 3 O 10 . They also reported the formation of two polymorphs of NiUO4 using high-temperature and highpressure (HT/HP) synthesis techniques. With very similar synthetic conditions (∼4 GPa at 1000 °C for 30 min), they identified a polymorph isostructural with orthorhombic CrUO 431 (denoted α) and another isostructural with orthorhombic MgUO429 (denoted β). They failed to identify optimal synthetic HT/HP conditions for either polymorph, suggesting their experimental conditions were near the pressure-temperature (PT) boundary of the two phases. Furthermore, they could not unequivocally discern, from their synthesis, which orthorhombic polymorph was the higher pressure structure and whether there was a direct phase transformation between the two polymorphs. Young 35 independently described, in 1966, the preparation of βNiUO4 at higher pressures and temperatures (6 GPa at 1100 °C), suggesting that the β-form might be preferred with increasing pressure/temperature (although he did not attempt to make α-NiUO4). Other than these reported structures, the only other structure well-described in the Ni−U−O system is NiU2O6. This has been studied by single-crystal X-ray diffraction,26 neutron power diffraction,36 and magnetic measurements.37 NiU2O6 can be generated either from flux26 or by heating its constituent oxides in an evacuated quartz tube36,37 or in air;38 it forms a trigonal structure in space group P321 containing divalent nickel and pentavalent uranium.

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EXPERIMENTAL SECTIONS

High Temperature/Pressure Synthesis of NiUO4. Stoichiometric quantities of NiO (obtained from sintering nickel nitrate, AlfaAesar, 99.9%) and U3O8 (obtained from sintering uranyl nitrate, International Bioanalytical Industries, Inc. 99%) were thoroughly mixed together, loaded into platinum crucibles, and sealed. To understand the conditions at which the NiUO4 polymorphs form, several synthesis experiments were undertaken using either piston cylinder or multi anvil apparatus (Voggenreiter LP 1000-540/50) located at IEK-6, Forschungszentrum Jü lich, Germany. The conditions of these are presented in Table 1, and the results of

Table 1. High-Temperature and High-Pressure Conditions Used to Synthesize Polymorphs of NiUO4a

a

temperature (°C)

pressure (GPa)

phases present

900 700 900 950 1000 1200

2 4 4 4 4 10

NiU3O10 α-NiUO4 + NiU3O10 α-NiUO4 + NiU3O10 α-NiUO4 + NiU3O10 β-NiUO4 + NiU3O10 β-NiUO4 + NiU3O10

Phases were determined using SC-XRD and powder XRD.

laboratory powder X-ray diffraction (XRD) and single-crystal X-ray diffraction (SC-XRD) are also included in Table 1. Samples obtained from two synthesis experiments, one subjected to a pressure of 4 GPa and a temperature of 900 °C and another to a pressure of 10 GPa and temperature of 1200 °C, were selected for further analysis. The conditions for these and the corresponding NiUO4 polymorph produced are described here: • α-NiUO4 (4 GPa and 900 °C): The sample was initially placed under a pressure of 4 GPa with a compression time of 30 min using a piston cylinder apparatus. When the desired pressure was reached, the sample was then rapidly heated to 900 °C in 20 min. These conditions were maintained for 12 h before cooling to 600 °C over 48 h and subsequently to room B

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

beamline offers less opportunity for applying multiscan absorption correction as is possible with the data collections on a four circle goniometer. Consequently, ripples around U atoms are often present due to ineffective absorption correction. In addition, the data completeness is relatively low for crystals in low-symmetry space groups. X-ray Absorption Near-Edge Structure (XANES) Measurements. XANES spectra were collected at the uranium L3-edge for single crystals of α-NiUO4, β-NiUO4, and NiU3O10 as well as for powder samples of CrU5+O4 and CoU6+O4 (as U standards), on the X-ray Fluorescence Microscopy (XFM) beamline46 at the Australian Synchrotron. The structures of each of the crystals used for these measurements were checked using the MX2 beamline described above. The measurements were performed at room temperature in fluorescence mapping mode using the Maia detector system.47,48 Despite the excellent performance of the XFM monochromator, all specimens were measured in a single energy series to preclude errors resulting from any non-reproducibility of the X-ray energy; thus, while the individual energies may suffer some small uncertainty, all features are perfectly aligned. We estimate the potential impact of selfabsorption effects, which suppress peak amplitudes in the XANES spectrum,49 to be less than 5%, and this level of self-absorption is not detrimental to this study. Fluorescence data were analyzed by fitting fluorescence spectra at each energy using a full forward model and with a “moving edge” formulation50 within GeoPIXE.51 The series of images were aligned using one of the lower-energy spectator elements, and the fluorescence intensities for the three unknown and two reference specimens were extracted from the entire specimen on the basis of their spatial location within the scan area. Further analysis on the fluorescence intensities was performed using the software package ATHENA.52

temperature rapidly. Then the pressure was gradually reduced to ambient over a period of 30 min. • β-NiUO4 (10 GPa and 1200 °C): The sample was initially placed under a pressure of 10 GPa with a compression time of 7.5 h using a multi anvil apparatus. When the desired pressure was reached, the sample was then rapidly heated to 1200 °C using a ramp rate of 100 °C/min. These conditions were maintained for 1 h before cooling to 600 °C over 3 h and subsequently to room temperature rapidly. Then the pressure was gradually removed over a period of 24 h. For these synthesis experiments, the crucibles were dismantled postsynthesis and found to contain either a mixture of α-NiUO4 and NiU3O10 (4 GPa and 900 °C) or a mixture of β-NiUO4 and NiU3O10 (10 GPa and 1200 °C) as determined by XRD and SC-XRD. Fine single crystals of α-NiUO4 (black), β-NiUO4 (dark red), and NiU3O10 (also black but much smaller in size) were mechanically separated for further analysis. Synthesis of NiU3O10 (Ambient Pressure Conditions). NiU3O10 single crystals were recovered from the HT/HP experiments, but to confirm there was no structural modification associated with the use of high pressures, single crystals were also grown under ambient pressure conditions using a combined solid state and molten salt regrowth method. Stoichiometric quantities of NiO and U3O8 were intimately mixed and calcined for 72 h using a platinum crucible in a box furnace at 1000 °C with intermittent mixing. The final product was mixed with excess NiCl2·6H2O (Alfa Aesar 99.99%) and heated to 1100 °C at a ramp rate of 100 °C/hour, and the molten mixture was held at this temperature for 24 h before being slowly cooled to 600 °C at 1 °C/min and then to room temperature at 10 °C/min. The final product was washed extensively with deionized water, to remove excess NiCl2, before single crystals of NiU3O10 were mechanically separated for further analysis. Laboratory Characterization. Powder XRD measurements were made at room temperature using a Bruker AXS D8 Endeavor diffractometer (40 kV/40 mA) in Bragg−Brentano geometry. The diffractometer has a copper X-ray tube and a primary nickel filter, producing Cu Kα1,2 radiation (λ = 1.541 87 Å). A linear silicon strip LynxEye detector (Bruker-AXS) was used. Data were collected in the range of 2θ = 5−100° with 10 s/step and a step width of 0.02°. The aperture of the fixed divergence slit was set to 0.2 mm, and the receiving slit was set to 8 mm. The discriminator of the detector was set to an interval of 0.16−0.25 V. Samples were contained in domed sample holders due to their radioactive nature. Data were analyzed by pattern matching using the program FullProf, where space group and lattice parameter information were obtained from single-crystal X-ray measurements, as described below. SC-XRD data were collected at room temperature to confirm crystal quality. SC-XRD measurements were undertaken at room temperature using an Agilent Technologies SuperNova diffractometer with Mo Kα radiation (λ = 0.710 73 Å) using an EOS CCD detector. All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multiscan method. The morphology and elemental composition of the crystals were determined using a FEI Quanta 200F Environment Scanning Electron Microscope (SEM) fitted with an energy-dispersive X-ray spectrometer (EDS). This verified the sample compositions based on the SCXRD structure solutions. Synchrotron Single-Crystal X-ray Diffraction (SSC-XRD). SSC-XRD data were collected at 100(2) K on the MX2 beamline39 at the Australian Synchrotron with a Si double-crystal monochromator (λ = 0.710 85 Å). Data were collected on small fragments (a few to tens of micrometers) to reduce the absorption effects; the sizes of these are available in the CIF files. All data were collected using BluIce software.40 Cell refinements and data reductions were undertaken using XDS software.41 An empirical absorption correction was applied to the data using SADABS.42 The structures were solved by direct methods using SHELXT,43 and the full-matrix least-squares refinements were performed using SHELXL44 via the Olex2 interface.45 The one circle goniometer geometry of data collections at the MX2

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RESULTS AND DISCUSSION Syntheses. Initially it was attempted to prepare NiUO4 using conventional solid-state methods at ambient pressure, which was previously found to be effective for other metal ternary actinide oxides,9,25 but as reported by earlier workers34 this was unsuccessful. HT/HP methods were then used. Since the report by Hoekstra and Marshall34 does not provide sufficient information to establish the optimal conditions to form either polymorph, a variety of temperature and pressure combinations was explored to understand which of the phases are preferred under given conditions. These are summarized in Table 1, where the phases were identified using both singlecrystal and powder XRD analyses. It is apparent that high pressure is necessary to obtain either α-NiUO4 or β-NiUO4, but temperature also plays a significant role. Two gigapascals is insufficient to stabilize either NiUO4 polymorph at 900 °C, whereas 700 °C was found to be sufficient to form α-NiUO4 when P was increased to 4 GPa. β-NiUO4 is apparently favored at both higher temperature and higher pressure conditions relative to α-NiUO4. Under 4 GPa pressure α-NiUO4 is expected to undergo a phase transformation to β-NiUO4 at a temperature between 950 and 1000 °C. Hoestra and Marshal34 reported that both NiUO4 polymorphs undergo decomposition above 920 °C under ambient pressure conditions to NiU3O10 + NiO. From this it is deduced that high pressure is necessary for the phase transformation between the two NiUO4 polymorphs to proceed. Small crystals of NiU3O10 were readily identified and recovered from all the α-NiUO4 or β-NiUO4 HT/HP synthesis experiments. Analysis showed these to be structurally identical to the NiU3O10 crystals obtained from the ambient pressure solid-state and molten salt regrowth synthesis described above. Powder XRD patterns were analyzed using profile matching, and examples for synthesis experiments at T = 900 °C and P = C

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 4 GPa (α-NiUO4) and T = 1000 °C and P = 4 GPa (βNiUO4) are presented in Figure S1, Supporting Information. The presence of NiU3O8 may reflect non-isostatic conditions during synthesis. In general α-NiUO4 or β-NiUO4 was found to form larger single crystals than NiU3O10. Structural Studies (a) α-NiUO4. The structure of αNiUO4 was solved in the orthorhombic space group Pbcn using SSC-XRD measurements with a unit-cell volume of 277.58(9) Å3. Crystal data and refinement details are presented in Table 2, and structural parameters can be found in Table S1,

Hoekstra and Marshall34 who concluded, based on the comparison of their powder XRD patterns, that α-NiUO4 is isostructural with CrUO4. The structure consists of cornersharing UO6 polyhedra forming corrugated two-dimensional (2D)-like layers parallel to the (010) plane. Between these layers NiO6 polyhedra edge share forming zigzag onedimensional (1D) chains in the [001] direction similar to the PbO6 chains in the zigzag rutile structure of α-PbO2.53 None of the six U−O bonds are collinear, all forming O−U−O angles less than 170°. Furthermore, the linkage between adjacent UO6 polyhedra occurs through four of the six oxygens; the two remaining oxo U−O bonds are cis and not in the trans collinear configuration associated with the uranyl moiety.54 The U−O bond lengths range from 1.949(10) to 2.227(10) Å, and the Ni−O bond lengths range from 2.004(10) to 2.064(10) Å; details of the bond distances can be found in Table 3. Considering the arrangement of the U−O bonds in the UO6 polyhedra, the typical uranyl oxo moiety encountered in other ternary uranium oxides25 is not present in α-NiUO4. This is consistent with the remarks by Read et al.26 in their study of FeUO4. Bond valence sum (BVS) calculations for the Ni and U sites in α-NiUO4 produced values of 2.19 and 5.97, respectively, indicating divalent Ni and hexavalent U. In these calculations we used the BVS parameters (2.051/0.519) for six-coordinate U reported by Burns and co-workers.55 It is interesting to note that, although α-NiUO4 is isostructural to Cr3+U5+O4 and Fe3+U5+O4, BVS calculations indicate that the oxidation states of Ni and U in αNiUO4 are 2+ and 6+, respectively. This difference in valence states is consistent with the longer U−O bond distances reported for CrUO4 and FeUO4 compared to those for αNiUO4.14 (b) β-NiUO4. The structure of β-NiUO4 is described by the orthorhombic space group Ibmm based on our SSC-XRD data and has a unit-cell volume of 275.10(10) Å3. Crystal data and refinement details are presented in Table 2, and structural parameters can be found in Table S2 (Supporting Information). The structure is isostructural to that of MgUO4 (described in the alternative space group of Imma),29 and it is presented in Figure 2. This is consistent with both Young’s and Hoekstra and Marshall’s original investigations of β-NiUO4.34,35 The structure consists of UO6 polyhedra edge-sharing to form infinite 1D chains in the [001] direction similar to that in the TiO2 rutile structure. The UO6 polyhedra form near tetragonal bipyramids, where the shorter oxo-uranyl like collinear trans U−O(1) bonds (=1.921(13) Å) point toward Ni cations along the [010] direction. This uranyl length is considered long for hexavalent uranium.54 There are two longer U−O(2) bonds, labeled U−O(2)i and U−O(2)ii, of length 2.113(12) and 2.120(12) Å, that link adjacent uranium polyhedra to form the 1D chains. The NiO6 polyhedra form tetragonal bipyramids that edge share also forming infinite 1D chains along the [001] direction. Four long Ni−O(1) bonds are observed as a consequence of the coordination with the oxo-uranyl like O(1) oxygens at a length of 2.124(8) Å and provide the edge-sharing link of the NiO6 chain. The two short Ni−O(2) bonds at 1.965(11) Å are collinear to each other and lie near parallel to the [100] direction. The UO6 polyhedra are considerably distorted as highlighted in Figure 2c. With the method of Robinson et al.56 an approximate bond angle variance of 162.82 (σ2) is calculated for the UO6 polyhedra compared to the NiO6 polyhedra, which are relatively free of distortion with an

Table 2. Crystal Data and Structure Refinement Details for α-NiUO4, β-NiUO4, and NiU3O10 compound

α-NiUO4

β-NiUO4

NiU3O10

formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z/μ (mm−1) min/max θ [deg] dcalcd (g cm−3) GOF final R1a [I > 2σ(I)] final wR2b [I > 2σ(I)]

NiUO4 360.74 orthorhombic Pbcn 4.7500(9) 11.436(2) 5.1100(10) 90 90 90 277.58(9) 4/64.846 3.564/27.442 8.632 1.108 0.0353 0.0734

NiUO4 360.74 orthorhombic Ibmm 6.3840(13) 6.3709(13) 6.7639(14) 90 90 90 275.10(10) 4/65.431 4.391/24.860 8.710 1.195 0.0257 0.0699

NiU3O10 932.80 triclinic P1̅ 5.5010(11) 5.5400(11) 7.5450(15) 70.93(3) 70.48(3) 72.94(3) 200.40(9) 1/62.771 2.962/24.997 7.729 1.196 0.0501 0.1287

R1 = ∑||Fo| − |Fc||/|Fo|. bwR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

a

Supporting Information. α-NiUO4 is isostructural to CrUO431 and FeUO4,26,33 and the structure is presented in Figure 1. This structural model is consistent with early investigation by

Figure 1. (a) Structural representation of α-NiUO4 in space group Pbcn (UO6 and NiO6 are represented as dark blue and silver gray polyhedra, where oxygen atoms are represented as small red spheres). (b) Highlights corrugated 2D layers of corner-sharing UO6 polyhedra parallel to the (010) plane, and (c) highlights edge-sharing 1D zigzag rutile chains of NiO6 polyhedra in the [001] direction. D

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Selected Bond Distances (Å) for α-NiUO4, β-NiUO4, and NiU3O10 α-NiUO4 U

Ni

U U U U U U Ni Ni Ni Ni Ni Ni

O1 O1 O2i O2i O2ii O2ii O1 O1 O1 O1 O2 O2

β-NiUO4 1.949(10) 1.949(10) 2.022(9) 2.022(9) 2.227(10) 2.227(10) 2.004(10) 2.004(10) 2.064(10) 2.064(10) 2.015(10) 2.015(10)

U

Ni

U U U U U U Ni Ni Ni Ni Ni Ni

O1 O1 O2i O2i O2ii O2ii O2 O2 O1 O1 O1 O1

NiU3O10 1.921(13) 1.921(13) 2.113(12) 2.113(12) 2.120(12) 2.120(12) 1.965(11) 1.965(11) 2.124(8) 2.124(8) 2.124(8) 2.124(8)

U1

U2

Ni

U1 U1 U1 U1 U1 U1 U2 U2 U2 U2 U2 U2 U2 Ni Ni Ni Ni Ni Ni

O2 O2 O3 O3 O5 O5 O1 O1 O2 O3 O3 O4 O5 O1 O1 O2 O2 O4 O4

1.936(19) 1.936(19) 2.020(17) 2.020(17) 2.287(18) 2.287(18) 2.122(18) 2.27(2) 2.450(17) 2.525(19) 2.34(2) 1.835(17) 1.894(17) 2.038(17) 2.038(17) 2.00(2) 2.00(2) 2.126(17) 2.126(17)

the Glazer notation.58 The geometry of the edge-sharing NiO6 chains allows tilting to occur only along the edge. This acts as a fulcrum for the tilt to occur along the chain linkage, causing the NiO6 polyhedra to successively reverse tilt relative to the [010] axis, that is, out-of-phase tilting. The U−O(2) bonds exhibit a consequential lengthening and shortening of the bonds, which apparently induces the observed distortion in the UO6 polyhedra. BVS calculations were also used to provide insight into the valence state of the metal cations in β-NiUO4; values of 1.99 and 6.10 were obtained for Ni and U, respectively, indicating divalent Ni and hexavalent U. Comparing the unit-cell volumes between α-NiUO4 and β-NiUO4, the latter has a slightly smaller volume of 275.10(10) Å3 compared to 277.58(9) Å3. Typically smaller unit-cell volumes are favored by higher pressures; thus, it is unsurprising that β-NiUO 4 was synthesized under higher-pressure conditions, and α-NiUO4 is expected to undergo a phase transformation to the βpolymorph with increasing pressure. (c) NiU3O10. SSC-XRD measurements showed the structure of NiU3O10 to be triclinic in space group P1̅ with a unit-cell volume of 200.40(9) Å3. Crystal data and refinement details are presented in Table 2, and structural parameters can be found in Table S3 in Supporting Information. It is isostructural with CuU3O10,59 and the structure is related to that of USbO5.60 The structure is represented in Figure 3 and consists of two distinct uranium sites, namely, U(1) and U(2), forming U(1)O6 and U(2)O7 polyhedra, respectively, which link together to form a three-dimensional (3D) framework-like structure. Contained within the structure are NiO6 polyhedra isolated from other NiO6 groups by the UO6/UO7 framework. As presented in Figure 3b, the U(2)O7 polyhedra edge share with other U(2)O7 polyhedra forming 1D zigzag chains along the [001] direction. The U(1)O6 polyhedra bridge these chains via an in-plane edge and out-of-plane corner-sharing motif. This edge sharing between the U(1)O6 with the U(2)O7 polyhedra involves U(1)−O(2) and U(1)−O(3) bonds, where the former bonds corner share with the NiO6 polyhedra. The U(1)O6 polyhedra also corner share with U(2)O7 polyhedra, linking the layers via the U(1)−O(5) bonds. The U(1)−O(2)

Figure 2. (a) Structural representation of orthorhombic β-NiUO4 in space group Ibmm (UO6 and NiO6 are represented as dark blue and silver gray polyhedra, where oxo and nonoxo oxygen atoms are represented as small aqua and red spheres, respectively). (b) Highlights alternating 1D chains of UO6 and NiO6 polyhedra in the [001] direction similar to regular TiO2 rutile, and (c) highlights the distortion of UO6 polyhedra and the tilt of NiO6 polyhedra.

approximate bond angle variance of 108.08 (σ2). Refined bond lengths for β-NiUO4 are listed in Table 3. Examining the NiO6 polyhedra in β-NiUO4 reveals a considerable off-axis tilt around the [010] axis as seen in Figure 2c. This is reminiscent of that observed in ABX3 perovskite structures.57 The ideal perovskite structure is cubic in space group Pm3̅m, consisting of corner-sharing BX6 octahedra and AX12 cuboctahedra. The structure can readily depart from the archetype cubic symmetry through a variety of distortion mechanisms including BX6 tilting. This occurs when the A site cation is too small for the BX6 corner-sharing connectivity. The BX6 corner-sharing geometry is such that the tilting can either be in-phase or out-of-phase, described well by E

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

samples in the HT/HP synthesis experiments precluded the option to prepare a pure-phase powder sample suitable for standard XANES measurements. Mechanically isolating crystals of a sufficient quantity to generate a pure-phase powder sample was found to be unreliable especially due to the similar morphologies and colors of crystals. To overcome this problem, XANES measurements at the U L3-edge were undertaken by XANES mapping at the XFM beamline of the Australian Synchrotron using single crystals of α-NiUO4, βNiUO 4 , NiU 3 O 10 and powder standards of Cr 3+ U 5+ O 4 (isostructural to α-NiUO4) and Co2+U6+O4 (isostructural to β-NiUO4). The U L3 map of the crystals used in these measurements is presented in Figure 4, and the normalized U

Figure 3. (a) Structural representation of NiU3O10 in space group P1̅ (UO6, UO7, and NiO6 are represented as dark blue, purple, and silver gray polyhedra, where uranyl-like oxygen and nonuranyl oxygen atoms are represented as small aqua and red spheres). (b) Highlights the UO6−UO7 edge-sharing configuration and the 1D zigzag chains of U(2)O7 polyhedra along the [001] direction, and (c) highlights the UO6 and UO7 corner-sharing configuration, which forms the 3D UO6−UO7 framework. Note that NiO6 polyhedra were removed for clarity in (b, c).

Figure 4. Uranium L3 map of β-NiUO4 (left), α-NiUO4 (center), and NiU3O10 (right) crystals obtained using XFM. The intensity reflects both the U concentration as well as thickness of the crystals within each pixel area (1 μm × 1 μm).

L3-edge XANES spectra obtained from the three single-crystal samples and two powder standards are shown in Figure 5.

bond is shorter, relative to the U(1)−O(3) and U(1)−O(5) bonds, with a refined length of 1.936(19) Å, and it has a transcollinear configuration with another U(1)−O(2) bond of the same length. This arrangement is suggestive of the presence of the uranyl moiety, despite it not being a terminal oxo as a consequence of its direct coordination to the U(2)O7 polyhedra. The U(1)−O bond lengths in the U(1)O 6 polyhedra range from 1.936(19) to 2.287(18) Å forming relatively regular polyhedra, whereas the bond lengths in the U(2)O7 polyhedra range from 1.835(17) to 2.525(19) Å. The Ni−O distances in the NiO6 polyhedra range from 2.00(2) to 2.126(17) Å. Details of the refined bond lengths can be found in Table 3. BVS calculations for the U(1), U(2), and Ni sites produce respective values of 5.88, 5.76, and 2.05. In these calculations we used the BVS parameters (2.051/0.519) for six-coordinate and (2.045/0.510) for seven-coordinate U reported by Burns and co-workers.55 These calculations suggest that NiU3O10 contains divalent nickel but may contain partially reduced uranium, or it may reflect the limitations of BVS calculations in uranium oxides.55 The XANES results described below support the latter conclusion. X-ray Absorption Near-Edge Structure (XANES) Studies. That α-NiUO 4 contains divalent nickel and hexavalent uranium cations, as suggested by BVS calculations, is in apparent conflict with the recent work of Guo et al.14 who showed, using X-ray absorption spectroscopic measurements, that the isostructural CrUO4 and FeUO4 oxides contain trivalent chromium/iron and pentavalent uranium. As discussed by Burns et al.55 BVS calculations involving uranium oxides are often compromised by the lack of precision in the structural determination (the heavy U cations dominate X-ray diffraction, hence, reducing precision in the anion distances) and the impact of the uranyl group. Consequently we conducted XANES measurements to determine the oxidation states of the uranium cations in both NiUO4 polymorphs as well as in NiU3O10. The persistent formation of multiphase

Figure 5. Normalized U L3-edge XANES spectra from α-NiUO4 (solid red curve), β-NiUO4 (solid blue curve), NiU3O10 (solid black curve), the U6+ standard CoUO4 (dashed black curve), and the U5+ standard CrUO4 (dashed red curve).

XANES spectra are widely used to “fingerprint” oxidation states of the probed atom. The U L3-edge XANES spectra primarily originate from dipole transitions from the U 2p3/2 core electrons into the unoccupied 6d states in the conduction band, and the U valence state can be obtained from the edge position of the spectra.25,61 As shown in Figure 5, the XANES spectra for the three nickel uranium oxide crystals are similar to that of CoU6+O4, supporting the assignment of hexavalent uranium in all of them. Structural Relationships within the Ni−U−O System. With the measured data presented thus far, the structural relationships that exist between the Ni−U−O oxides can now be better understood. The structure of NiU2O6 has previously F

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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UO7 (NiU3O10) connectivity, where the NiO6 polyhedra are isolated from each other. Attempts were made to synthesize NiUO4 with stoichiometric quantities of NiO and U3O8 using solid-state methods under ambient pressure, but no ternary phases were identified. Evidently high pressures are essential for the formation of the 1D rutile-like chain in the Ni−U−O system, just as high pressure is needed to prepare the rutile form of SiO2.63 It is postulated that pressure enables the successive bonding of NiO6 polyhedra in the two NiUO4 polymorphs, which is not necessary in NiU2O6 and NiU3O10, as they do not require interconnected NiO6 polyhedra for the structural formation, instead forming framework structures based on interconnected UO6 or UO6 and UO7 polyhedra, respectively. Crystal Chemistry of the NiUO4 Polymorphs and Relation to Other AUO4 Oxides. That the phase transformation from α-NiUO4 to β-NiUO4, which occurs through either temperature or pressure, is irreversible suggests that βNiUO4 is more stable. Although it is difficult to properly describe the α-NiUO4 to β-NiUO4 phase transformation without in situ measurements, it is postulated that it would be first-order and reconstructive, transitioning from a partially 2D layered and 1D chain α-NiUO4 structure to a more regular and completely 1D chain β-NiUO4 structure. The change in volume between the structures is small; the ∼0.9% reduction in β-NiUO4 supports it being the preferred high pressure structure, which suggests difficulty in experimentally controlling the transition. This seems to be evident when comparing the synthesis conditions used in the present study to that of Hoekstra and Marshal34 and also Young35 in terms of phases recovered at specific pressures and temperatures. Pertinently the α- to β-NiUO4 transition involves the emergence of the uranyl group that apparently plays the role of both a structurally anchoring and stabilizing group in βNiUO4, consistent with previous studies of similar uranium systems.64,65 Its absence in α-NiUO4 is thought to drive the increased thermodynamic stability of β-NiUO4. The enthalpy of formation of some other metal AUO4 oxides in similar uranyl absent Pbcn (FeUO4, CrUO4) and uranyl present Ibmm (MgUO4, CoUO4) structures has been examined previously, where it has been typically found that the Pbcn variants have a more positive enthalpy of formation.14 α-NiUO4 is distinct to the other known AUO4 variants in space group Pbcn, namely, CrUO4 and FeUO4,14 as it does not contain pentavalent uranium or a trivalent A-site metal cation. Guo et al.14 postulated that the preferential oxidation of Cr and Fe to their trivalent oxidation states leads to the reduction of uranium from hexavalent to pentavalent in CrUO4 and FeUO4. Despite having the less-stable 5f1 electron configuration, Guo et al.14 argued that the presence of U5+ does not contribute significantly to the difference in enthalpy and stability. Generally, Ni3+ is encountered less frequently than Ni2+, although it does occur within lanthanoid nickel perovskites of the form Ln3+Ni3+O3, reflecting the preference for the lanthanoid cation to retain its trivalent state, which coerces nickel to be trivalent.66,67 Subsequently, it is suggested here that the presence of Ni2+ and U6+ in α-NiUO4 is a consequence of their preferences to retain the relatively stable 3d8 and 5f0 electron configurations, respectively, although the relative size between the two cations also plays a significant role as previously discussed. Read et al.26 recently argued that the ratio of the ionic radii of the A-site and uranium cations (rA/rU) determines the

been studied using single-crystal X-ray and neutron powder diffraction methods.26,36−38 NiU2O6 crystallizes in the trigonal space group P321 forming a 3D framework structure based on corner-sharing UO6 polyhedra, where isolated NiO6 polyhedra sit within the framework of the structure. The structure contains divalent nickel and pentavalent uranium. The structure is represented in Figure 6. Both NiU2O6 and

Figure 6. (a) Structural representation of NiU2O6 in space group P321.36 Dark blue and silver gray polyhedra represent UO6 and NiO6, and small red spheres represent oxygen atoms. (b) Highlights the UO6 corner-sharing configuration (nickel polyhedra were removed for clarity), and (c) highlights the edge and corner-sharing motif of the UO6 and NiO6 polyhedra.

NiU3O10 contain a 3D framework connectivity of UO6 (NiU2O6) or UO6 and UO7 (NiU3O10) polyhedra with isolated NiO 6 polyhedra, but unlike the two NiUO 4 polymorphs, they do not contain interconnected NiO6 polyhedra. Of the two NiUO4 polymorphs, β-NiUO4 has the more regular rutile-like structure, with its UO6 and NiO6 polyhedra edge sharing forming alternate chains in the [001] direction (see Figure 2c), similar to that seen in TiO2 (rutile) itself. Previously Baur et al.62 derived a structural hierarchy relating rutile and MgUO4, and hence β-NiUO4, through a maximal nonisomorphic subgroup of order four. These authors argued that MgUO4 in space group Ibmm could have been assigned to the more regular CoReO4 structure in space group Cmmm if not for the presence of the uranyl moiety, which leads to a distortion of the UO6 polyhedra that requires the larger cell provided by Ibmm. The present study suggests that the impact of the uranyl group is even more profound. It is apparently able to assist in the formation of the β-NiUO4 structure through the lengthening of the U−O(1) uranyl bond, providing the critical Ni−O(1) bond, which allows successive NiO6 polyhedra to link to form the described 1D chain. α-NiUO4 has a similar structure to the zigzag distorted rutile of α-PbO2. The structure of α-PbO2 consists of PbO6 polyhedra edge sharing, forming 1D zigzag chains along the [001] direction, and also corner-sharing, resulting in a corrugated 2D layered arrangement parallel to the (010) plane.53 This is similar to the respective connectivity of the NiO6 and UO6 polyhedra in α-NiUO4. In contrast to the 1D connectivity of the NiO6 groups in the two NiUO4 polymorphs, NiU2O6 and NiU3O10 form 3D framework structures, based on UO6 (NiU2O6) or UO6 and G

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Inorganic Chemistry Table 4. Comparison of AUO4 Structure Types, Ionic Radiia Taken from the Shannon Ionic Radii Tables73 compound

structural type

space group

BaUO4 PbUO4 β-SrUO4 α-SrUO4 CaUO4 α-CdUO4 β-CdUO4 MnUO4 CoUO4 CuUO4

orthorhombic orthorhombic orthorhombic rhombohedral rhombohedral rhombohedral orthorhombic orthorhombic orthorhombic monoclinic

Pbcm Pbcm Pbcm R3̅m R3̅m R3̅m Cmmm Ibmm Ibmm P21/n

MgUO4 β-NiUO4

orthorhombic orthorhombic

Ibmm Ibmm

α-NiUO4

orthorhombic

Pbcn

FeUO4 CrUO4

orthorhombic orthorhombic

Pbcn Pbcn

reference Murphy et al.25 Cremers et al.24 Murphy et al.25 Murphy et al.25 Murphy et al.25 Yamashita et al.28 Yamashita et al.28 Read et al.26 Bertaut et al.30 Siegel and Hoekstra27 Zachariasen29 present investigation present investigation Read et al.26 Bacmann et al.31

uranium cation valence

A site cation valence

uranium ionic radii (Å)

A site ionic radii (Å)

cation ratio rA/rU

6 6 6 6 6 6 6 6 6 6

2 2 2 2 2 2 2 2 2 2

0.73 0.73 0.73 0.86 0.86 0.86 0.73 0.73 0.73 0.73

1.35 1.19 1.18 1.26 1.12 1.1 0.95 0.83 0.745 0.73

1.85 1.63 1.61 1.47 1.30 1.28 1.30 1.14 1.02 1

6 6

2 2

0.73 0.73

0.72 0.69

0.99 0.95

6

2

0.73

0.69

0.95

5 5

3 3

0.76 0.76

0.645 0.615

0.85 0.81

Note that the different Sr2+ and U6+ ionic radii for α-SrUO4 and β-SrUO4 reflect the different coordination number around Sr and U in the two structures. a

when compared to the other A-site cations of the Ibmm AUO4 series. Conversely Ni2+ is larger than Cr3+ and Fe3+ in the Pbcn AUO4 series. It appears that pressure is necessary to mitigate this size “problem” and allow the respective β-NiUO4 and αNiUO4 to form. The structures of CuUO4 and β-CdUO4 are the anomalies in Table 4 (and hence excluded from Figure 7). On the basis of the relative size of the Cu2+ and U6+ cations, an orthorhombic structure in space group Ibmm would be expected for CuUO4; instead, it forms a monoclinic structure in space group P21/n.27 This deviation from the structural trend is related to the presence of the Jahn−Teller active (3d9) Cu2+ cation, which lifts the degeneracy of the eg orbitals leading to a distortion of the CuO6 polyhedra.27 This leads to two of the Cu−O bonds significantly lengthening, whereas the other four shorten. This is in contrast to the pattern of four long and two short A−O bonds observed in the Ibmm and Pbcn AUO4 variants. Interestingly the tetragonal elongation of the CuO6 polyhedra seen in CuUO4 is also observed in the NiO6 polyhedra of NiU3O10. CuU3O10,34 which also contains the Jahn−Teller active Cu2+ cation, is isostructural to NiU3O10, and comparing these two there is no significant variation between the NiO6 and CuO6 polyhedra. Consequentially it can be concluded that the atomic arrangement in the AU3O10 structure can better support the presence of the Jahn−Teller active Cu2+ cation, whereas the orthorhombic AUO4 structures cannot. The structure of β-CdUO4 has been reported to be orthorhombic in space group Cmmm,28 unlike MnUO4, CoUO4, MgUO4, and β-NiUO4, which are all in space group Ibmm. The assignment of space group Cmmm rather than Ibmm, which requires a small difference in the anion sublattice, was undertaken using laboratory X-ray powder diffraction. We are currently revisiting this to confirm whether the structure should be in Ibmm, in light of the discussions by Baur et al.62 and the subtle differences between the structural models in Cmmm and Ibmm. The identification of the two NiUO4 polymorphs adds significant understanding to the solid-state chemistry of the AUO4 oxides. Furthermore, by utilizing the ratio of the ionic

specific structure adopted by the AUO4 series of oxides. We expanded the list of the AUO4 family of oxides by including αNiUO4 and β-NiUO4 as well as the orthorhombic structures in Pbcm. Table 4 summarizes the present knowledge of the structures of the AUO4 oxides, and Figure 7 displays a clear

Figure 7. Trend between the ratio of the ionic radii of the A-site and uranium cations (rA/rU) and the structural types formed within the AUO4 series of oxides.

trend between the structure type and rA/rU (excluding the two anomalies discussed in the next paragraph). As shown in Figure 7, with decreasing ratio of the ionic radii (rA/rU), AUO4 oxides first adopt the orthorhombic structure in space group Pbcm, followed by the rhombohedral structure in R3̅m, the orthorhombic structure in Ibmm, and finally the orthorhombic structure in Pbcn. A related trend between the enthalpy of formation and rA was also identified by Guo et al.14 As displayed in Figure 7, the two NiUO4 polymorphs exist on the periphery of stability with respect to the Pbcn and Ibmm structural types. It was shown above that pressure is necessary to achieve a rutile-like 1D chain configuration of the NiO6 polyhedra in either α-NiUO4 or β-NiUO4. This is reflected in the ionic radius of the divalent nickel cation being the smallest H

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Inorganic Chemistry radii, rA/rU, the structural type of the AUO4 oxides can be readily predicted as shown in Figure 7. However, the story of these structures is far from concluded, as demonstrated by our recent studies of α-SrUO418 and δ-SrUO4−x,17 and the AUO4 oxides still possess a remarkable ability to produce significant novel chemical properties and phenomena. Key to these studies has been the utilization of in situ structural techniques. Subsequently it is desirable to examine the phase transformation between the NiUO4 polymorphs in situ. This would further add to the knowledge of the AUO4 oxides while enabling the poorly known but exotic behavior of actinide oxides under pressure to be examined, potentially unravelling curious chemical and structural phenomena, as theoretical studies68,69 have highlighted and hinted toward. Finally it is noteworthy to consider the implications of αNiUO4 containing different metal and uranium oxidation states to those encountered in the isostructural CrUO4 and FeUO4. It is unclear whether a continuous solid solution would exist between these compounds, and if so, how the oxidation states in such a system would vary. Simplistically it is postulated for a solid solution of the form, α-NixCr1−xUO4 or α-NixFe1−xUO4, the amount of nickel present would linearly increase the amount of U6+ that would form. However, considering the different affinities for the nickel, chromium, or iron cations to oxidize from divalent to trivalent this linear trend might not be realized. Consequentially such a system would potentially provide a means to measure the relative affinity for these transition-metal cations to undergo oxidation in the presence of uranium. The knowledge is potentially relevant to the understanding of behavior within spent nuclear fuel systems. Furthermore, it would provide a means to modulate the apparent magnetism previously examined in these oxides.31,33,70−72

MgUO4, CoUO4, and MnUO4) were reported prior to this investigation. The identification of α-NiUO4 and β-NiUO4 and their position within the greater hierarchy of AUO4 oxides first provides the “missing link” between the space group Pbcn and Ibmm variants and further allows the structural chemistry of the AUO4 oxides to be more comprehensively understood. In these oxides the structural preference is related to the ratio of the ionic radii of the A-site and uranium cations (rA/rU). It is pertinent and somewhat surprising that α-NiUO4 contains Ni2+ and U6+ as opposed to the isostructural CrUO4 and FeUO4, which both contain trivalent A-site cation and pentavalent uranium.

CONCLUSIONS In summary, we have synthesized the ternary nickel uranium oxides, α-NiUO4, β-NiUO4, and NiU3O10 and refined the structures of these for the first time using SSC-XRD data supported by XANES measurements. The three oxides were shown to only contain hexavalent uranium from U L3-edge XANES mapping of selected single crystals. The 1D NiO6 chain containing oxides α-NiUO4 and β-NiUO4 were found to only form under HT/HP conditions, whereas β-NiUO4 is favored under higher-temperature and higher-pressure conditions relative to α-NiUO4. From ex situ synthesis experiments it is concluded that α-NiUO4 is expected to undergo a phase transformation to β-NiUO4 irreversibly between T = 950 and 1000 °C when P = 4 GPa. That HT/HP conditions are necessary for the structural formation of α-NiUO4 and βNiUO4 is argued to be a consequence of the size difference between the Ni2+ and U6+ cations. When compared to their isostructural orthorhombic counterparts in space group Pbcn (Cr3+U5+O4, etc.) and Ibmm (Mg2+U6+O4, etc.), the Ni2+ cation is essentially too large or too small relative to the U6+ cation, respectively, to form these structural types under ambient pressure conditions. NiU2O6 and NiU3O10 apparently mitigate this cation size problem and the requirement for high pressure synthesis conditions, as they both form framework structures through connectivity among respective UO6 or UO6 and UO7 polyhedra, while NiO6 polyhedra sit unconnected in void positions in both structures. Within the family of AUO4 oxides, no phase transformations between the orthorhombic space group Pbcn (e.g., CrUO4 and FeUO4) and Ibmm (e.g.,

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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.8b02355. Powder diffraction profiles for α-NiUO4 and β-NiUO4 and tables of refined structural parameters for α-NiUO4, β-NiUO4, and NiU3O10 (DOCX) Accession Codes

CCDC 1863994−1863996 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Z.-M.Z.) *E-mail: [email protected]. (B.J.K.)

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Yingjie Zhang: 0000-0001-6321-4696 Zhaoming Zhang: 0000-0003-3273-8889 Evgeny V. Alekseev: 0000-0002-4919-5211 Brendan J. Kennedy: 0000-0002-7187-4579 Author Contributions

The synthesis experiments were conceived and designed by G.L.M., P.K., and E.A.; preliminary X-ray diffraction measurements and electron microscopy measurements were performed by G.L.M., P.K., and E.A.; synchrotron single-crystal X-ray diffraction measurements were performed by Y.Z.; X-ray fluorescence microscopy measurements were performed by G.L.M., Z.Z., B.J.K., and M.D.J.; the manuscript was prepared by G.L.M., Z.Z., and B.J.K. with input from all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SSC-XRD and XANES data were collected on the MX2 and XFM beamlines at the Australian Synchrotron, a part of ANSTO. The Australian Cancer Research Foundation (ACRF) is acknowledged for funding the new detector on the MX2 beamline. B.J.K. acknowledges the support of the Australian Research Council. G.L.M. acknowledges and greatly appreciates the support and encouragement from ANSTO Minerals and Dr. C. Griffiths and Dr. R. Gee for radioactive materials handling and laboratory usage. G.L.M. also acknowledges the funding and support from AINSE, the United I

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(17) Murphy, G. L.; Wang, C.-H.; Beridze, G.; Zhang, Z.; Kimpton, J. A.; Avdeev, M.; Kowalski, P. M.; Kennedy, B. J. Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4−x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature. Inorg. Chem. 2018, 57 (10), 5948−5958. (18) Murphy, G. L.; Kennedy, B. J.; Kimpton, J. A.; Gu, Q. F.; Johannessen, B.; Beridze, G.; Kowalski, P. M.; Bosbach, D.; Avdeev, M.; Zhang, Z. M. Nonstoichiometry in Strontium Uranium Oxide: Understanding the Rhombohedral-Orthorhombic Transition in SrUO4. Inorg. Chem. 2016, 55 (18), 9329−9334. (19) Glasser, F. P. Mineralogical aspects of cement in radioactive waste disposal. Mineral. Mag. 2001, 65 (5), 621−633. (20) Othmane, G.; Allard, T.; Menguy, N.; Morin, G.; Esteve, I.; Fayek, M.; Calas, G. Evidence for nanocrystals of vorlanite, a rare uranate mineral, in the Nopal I low-temperature uranium deposit (Sierra Pena Blanca, Mexico). Am. Mineral. 2013, 98 (2−3), 518− 521. (21) Galuskin, E. V.; Armbruster, T.; Galuskina, I. O.; Lazic, B.; Winiarski, A.; Gazeev, V. M.; Dzierzanowski, P.; Zadov, A. E.; Pertsev, N. N.; Wrzalik, R.; Gurbanov, A. G.; Janeczek, J. Vorlanite (CaU6+)O4-A new mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia. Am. Mineral. 2011, 96 (1), 188−196. (22) Galuskin, E. V.; Galuskina, I. O.; Dubrovinsky, L. S.; Janeczek, J. Letter: Actinides in geology, energy, and the environment. Thermally induced transformation of vorlanite to ″protovorlanite″: Restoration of cation ordering in self-irradiated CaUO4. Am. Mineral. 2012, 97 (5−6), 1002−1004. (23) Galuskin, E. V.; Kusz, J.; Armbruster, T.; Galuskina, I. O.; Marzec, K.; Vapnik, Y.; Murashko, M. Vorlanite, (CaU6+)O4, from Jabel Harmun, Palestinian Autonomy, Israel. Am. Mineral. 2013, 98 (11−12), 1938−1942. (24) Cremers, T. L.; Eller, P. G.; Larson, E. M.; Rosenzweig, A. Single-crystal structure of lead uranate (VI). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 1684−1685. (25) Murphy, G.; Kennedy, B. J.; Johannessen, B.; Kimpton, J. A.; Avdeev, M.; Griffith, C. S.; Thorogood, G. J.; Zhang, Z. M. Structural studies of the rhombohedral and orthorhombic monouranates: CaUO4, α-SrUO4, β-SrUO4 and BaUO4. J. Solid State Chem. 2016, 237, 86−92. (26) Read, C. M.; Smith, M. D.; zur Loye, H. C. Single crystal growth and structural characterization of ternary transition-metal uranium oxides: MnUO4, FeUO4, and NiU2O6. Solid State Sci. 2014, 37, 136−143. (27) Siegel, S.; Hoekstra, H. R. Crystal structure of copper uranium tetroxide. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1968, 24, 967−970. (28) Yamashita, T.; Fujino, T.; Masaki, N.; Tagawa, H. The crystalstructures of alpha-CdUO4 and beta-CdUO4. J. Solid State Chem. 1981, 37 (2), 133−139. (29) Zachariasen, W. H. Crystal chemical studies of the 5f-series of elements. XXI. The crystal structure of magnesium orthouranate. Acta Crystallogr. 1954, 7 (12), 788−791. (30) Bacmann, M.; Bertaut, E. F. A non colinear model of magnetic structure of UCoO4. J. Phys. 1969, 30 (11−12), 949−953. (31) Bacmann, M.; Bertaut, E. F.; Bassi, G. Parametres atomiques et structure magnetique de UCrO4. Bulletin De La Societe Francaise Mineralogie Et De Cristallographie 1965, 88 (2), 214−218. (32) Tagawa, H.; Fujino, T. Anomalous change of the oxidationstate of uranium in the phase-transformation of cadmium monouranate. Inorg. Nucl. Chem. Lett. 1980, 16 (2), 91−96. (33) Bacmann, M.; Bertaut, E. F. Structure du mouveau compose UFeO 4. Bulletin De La Societe Francaise Mineralogie Et De Cristallographie 1967, 90 (2), 257−258. (34) Hoekstra, H. R.; Marshall, R. H. Some uranium-transition element double oxides. Adv. Chem. Ser. 1967, 71 (71), 211−227. (35) Young, A. P. Nickel orthouronate - high-pressure synthesis. Science 1966, 153 (3742), 1380−1381.

Uranium Trust Fund, and The Joan R. Clarke research fund. E.V.A. is grateful for Helmholtz Association for the support within the VH-NG 815 project.

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REFERENCES

(1) Serp, J.; Allibert, M.; Beneš, O.; Delpech, S.; Feynberg, O.; Ghetta, V.; Heuer, D.; Holcomb, D.; Ignatiev, V.; Kloosterman, J. L.; Luzzi, L.; Merle-Lucotte, E.; Uhlíř, J.; Yoshioka, R.; Zhimin, D. The molten salt reactor (MSR) in generation IV: Overview and perspectives. Prog. Nucl. Energy 2014, 77, 308−319. (2) Murty, K. L.; Charit, I. Structural materials for Gen-IV nuclear reactors: Challenges and opportunities. J. Nucl. Mater. 2008, 383 (1), 189−195. (3) zur Loye, H.-C.; Besmann, T.; Amoroso, J.; Brinkman, K.; Grandjean, A.; Henager, C. H.; Hu, S.; Misture, S. T.; Phillpot, S. R.; Shustova, N. B.; Wang, H.; Koch, R. J.; Morrison, G.; Dolgopolova, E. Hierarchical Materials as Tailored Nuclear Waste Forms: A Perspective. Chem. Mater. 2018, 30 (14), 4475−4488. (4) Banerjee, S.; Kutty, T. R. G. Nuclear Fuels. Functional Materials: Preparation, Processing and Applications 2012, 387−466. (5) Zinkle, S. J.; Was, G. S. Materials challenges in nuclear energy. Acta Mater. 2013, 61 (3), 735−758. (6) Alekseev, E. V.; Krivovichev, S. V.; Depmeier, W. A crown ether as template for microporous and nanostructured uranium compounds. Angew. Chem., Int. Ed. 2008, 47 (3), 549−551. (7) Hao, Y.; Murphy, G. L.; Bosbach, D.; Modolo, G.; AlbrechtSchmitt, T. E.; Alekseev, E. V. Porous Uranyl Borophosphates with Unique Three-Dimensional Open-Framework Structures. Inorg. Chem. 2017, 56 (15), 9311−9320. (8) Hao, Y. C.; Klepov, V. V.; Murphy, G. L.; Modolo, G.; Bosbach, D.; Albrecht-Schmitt, T. E.; Kennedy, B. J.; Wang, S.; Alekseev, E. V. Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates. Cryst. Growth Des. 2016, 16 (10), 5923−5931. (9) Murphy, G. L.; Kennedy, B. J.; Zhang, Z. M.; Avdeev, M.; Brand, H. E. A.; Kegler, P.; Alekseev, E. V. Structure and phase transition in BaThO3: A combined neutron and synchrotron X-ray diffraction study. J. Alloys Compd. 2017, 727, 1044−1049. (10) Polinski, M. J.; Garner, E. B.; Maurice, R.; Planas, N.; Stritzinger, J. T.; Parker, T. G.; Cross, J. N.; Green, T. D.; Alekseev, E. V.; Van Cleve, S. M.; Depmeier, W.; Gagliardi, L.; Shatruk, M.; Knappenberger, K. L.; Liu, G.; Skanthakumar, S.; Soderholm, L.; Dixon, D. A.; Albrecht-Schmitt, T. E. Unusual structure, bonding and properties in a californium borate. Nat. Chem. 2014, 6, 387−392. (11) Wang, S.; Alekseev, E. V.; Depmeier, W.; Albrecht-Schmitt, T. E. Surprising Coordination for Plutonium in the First Plutonium(III) Borate. Inorg. Chem. 2011, 50 (6), 2079−2081. (12) Wang, S. A.; Alekseev, E. V.; Depmeier, W.; Albrecht-Schmitt, T. E. New Neptunium(V) Borates That Exhibit the Alexandrite Effect. Inorg. Chem. 2012, 51 (1), 7−9. (13) Wang, S. A.; Alekseev, E. V.; Ling, J.; Skanthakumar, S.; Soderholm, L.; Depmeier, W.; Albrecht-Schmitt, T. E. Neptunium Diverges Sharply from Uranium and Plutonium in Crystalline Borate Matrixes: Insights into the Complex Behavior of the Early Actinides Relevant to Nuclear Waste Storage. Angew. Chem., Int. Ed. 2010, 49 (7), 1263−1266. (14) Guo, X. F.; Tiferet, E.; Qi, L.; Solomon, J. M.; Lanzirotti, A.; Newville, M.; Engelhard, M. H.; Kukkadapu, R. K.; Wu, D.; Ilton, E. S.; Asta, M.; Sutton, S. R.; Xu, H. W.; Navrotsky, A. U(V) in metal uranates: a combined experimental and theoretical study of MgUO4, CrUO4, and FeUO4. Dalton Trans. 2016, 45 (11), 4622−4632. (15) Bugaris, D. E.; zur Loye, H. C. Materials Discovery by Flux Crystal Growth: Quaternary and Higher Order Oxides. Angew. Chem., Int. Ed. 2012, 51 (16), 3780−3811. (16) Read, C. M.; Bugaris, D. E.; zur Loye, H. C. Single crystal growth and structural characterization of four complex uranium oxides: CaUO4, beta-Ca3UO6, K4CaU3O12, and K4SrU3O12. Solid State Sci. 2013, 17, 40−45. J

DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (36) Hinatsu, Y.; Doi, Y.; Nakamura, A. Magnetic studies on CoU2O6 and NiU2O6 by magnetic susceptibility, specific heat and neutron diffraction measurements. J. Nucl. Mater. 2009, 385 (1), 49− 52. (37) Hinatsu, Y. Antiferromagnetic ordering in CoU2O6 and NiU2O6. J. Solid State Chem. 1995, 114 (2), 595−597. (38) Kemmlersack, S. Crystal structure of NiU2O6 and CoU2O6. Z. Anorg. Allg. Chem. 1968, 358 (5−6), 226−232. (39) Aragao, D.; Aishima, J.; Cherukuvada, H.; Clarken, R.; Clift, M.; Cowieson, N. P.; Ericsson, D. J.; Gee, C. L.; Macedo, S.; Mudie, N.; Panjikar, S.; Price, J. R.; Riboldi-Tunnicliffe, A.; Rostan, R.; Williamson, R.; Caradoc-Davies, T. T. MX2: a high-flux undulator microfocus beamline serving both the chemical and macromolecular crystallography communities at the Australian Synchrotron. J. Synchrotron Radiat. 2018, 25, 885−891. (40) McPhillips, T. M.; McPhillips, S. E.; Chiu, H. J.; Cohen, A. E.; Deacon, A. M.; Ellis, P. J.; Garman, E.; Gonzalez, A.; Sauter, N. K.; Phizackerley, R. P.; Soltis, S. M.; Kuhn, P. Blu-Ice and the Distributed Control System: software for data acquisition and instrument control at macromolecular crystallography beamlines. J. Synchrotron Radiat. 2002, 9, 401−406. (41) Kabsch, W. Integration, scaling, space-group assignment and post-refinement. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2010, 66, 133−144. (42) Blessing, R. H. An empirical correction for absorption anisotropy. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 33−38. (43) Sheldrick, G. M. SHELXT - Integrated space-group and crystalstructure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (44) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (45) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339−341. (46) Paterson, D.; de Jonge, M. D.; Howard, D. L.; Lewis, W.; McKinlay, J.; Starritt, A.; Kusel, M.; Ryan, C. G.; Kirkham, R.; Moorhead, G.; Siddons, D. P. The X-ray Fluorescence Microscopy Beamline at the Australian Synchrotron. AIP Conf. Proc. 2010, 1365 (1), 219−222. (47) Kirkham, R.; Dunn, P. A.; Kuczewski, A. J.; Siddons, D. P.; Dodanwela, R.; Moorhead, G. F.; Ryan, C. G.; De Geronimo, G.; Beuttenmuller, R.; Pinelli, D.; Pfeffer, M.; Davey, P.; Jensen, M.; Paterson, D. J.; de Jonge, M. D.; Howard, D. L.; Küsel, M.; McKinlay, J.; et al. The Maia Spectroscopy Detector System: Engineering for Integrated Pulse Capture, Low-Latency Scanning and Real-Time Processing. AIP Conf. Proc. 2009, 1234 (1), 240−243. (48) Ryan, C. G.; Siddons, D. P.; Kirkham, R.; Li, Z. Y.; de Jonge, M. D.; Paterson, D. J.; Kuczewski, A.; Howard, D. L.; Dunn, P. A.; Falkenberg, G.; Boesenberg, U.; De Geronimo, G.; Fisher, L. A.; Halfpenny, A.; Lintern, M. J.; Lombi, E.; Dyl, K. A.; Jensen, M.; Moorhead, G. F.; Cleverley, J. S.; Hough, R. M.; Godel, B.; Barnes, S. J.; James, S. A.; Spiers, K. M.; Alfeld, M.; Wellenreuther, G.; Vukmanovic, Z.; Borg, S. Maia X-ray fluorescence imaging: Capturing detail in complex natural samples. Journal of Physics: Conference Series 2014, 499 (1), 012002. (49) Goulon, J.; Goulonginet, C.; Cortes, R.; Dubois, J. M. On experimental attenuation factors of the amplitude of the exafs oscillations in absorption, reflectivity and luminescence measurements. J. Phys. 1982, 43 (3), 539−548. (50) Etschmann, B. E.; Donner, E.; Brugger, J.; Howard, D. L.; de Jonge, M. D.; Paterson, D.; Naidu, R.; Scheckel, K. G.; Ryan, C. G.; Lombi, E. Speciation mapping of environmental samples using XANES imaging. Environmental Chemistry 2014, 11 (3), 341−350. (51) GeoPIXE software suite. http://nmp.csiro.au/GeoPIXE.html. (52) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12 (4), 537−541.

(53) Filatov, S.; Bendeliani, N.; Albert, B.; Kopf, H.; Dyuzeva, T.; Lityagina, L. High-pressure synthesis of alpha-PbO2 and its crystal structure at 293, 203, and 113 K from single crystal diffraction data. Solid State Sci. 2005, 7 (11), 1363−1368. (54) Burns, P. C. U6+ minerals and inorganic compounds: Insights into an expanded structural hierarchy of crystal structures. Can. Mineral. 2005, 43, 1839−1894. (55) Burns, P. C.; Ewing, R. C.; Hawthorne, F. C. The crystal chemistry of hexavalent uranium: Polyhedron geometries, bondvalence parameters, and polymerization of polyhedra. Can. Mineral. 1997, 35, 1551−1570. (56) Robinson, K.; Gibbs, G. V.; Ribbe, P. H. Quadriatic elongation - quantitative measure of distortion in coordination polyhedra. Science 1971, 172 (3983), 567−570. (57) Zhang, Z.; Howard, C. J.; Knight, K. S.; Lumpkin, G. R. Structures of the cation-deficient perovskite Nd0.7Ti0.9Al0.1O3 from high-resolution neutron powder diffraction in combination with group-theoretical analysis. Acta Crystallogr., Sect. B: Struct. Sci. 2006, 62 (1), 60−67. (58) Glazer, A. M. Classification of tilted octahedra in perovskites. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28 (NOV15), 3384−3392. (59) Dickens, P. G.; Stuttard, G. P.; Patat, S. Structure of CuU3O10. J. Mater. Chem. 1993, 3 (4), 339−341. (60) Dickens, P. G.; Stuttard, G. P. Structure of USbO5 - Powder neutron diffraction study. J. Mater. Chem. 1992, 2 (7), 691−694. (61) Gregg, D. J.; Zhang, Z.; Thorogood, G. J.; Kennedy, B. J.; Kimpton, J. A.; Griffiths, G. J.; Guagliardo, P. R.; Lumpkin, G. R.; Vance, E. R. Cation antisite disorder in uranium-doped gadolinium zirconate pyrochlores. J. Nucl. Mater. 2014, 452 (1), 474−478. (62) Baur, W. H.; Joswig, W.; Pieper, G.; Kassner, D. CoReO4, A new rutile-type derivative with ordering of two cations. J. Solid State Chem. 1992, 99 (1), 207−211. (63) Prakapenka, V. P.; Shen, G.; Dubrovinsky, L. S.; Rivers, M. L.; Sutton, S. R. High pressure induced phase transformation of SiO2 and GeO2: difference and similarity. J. Phys. Chem. Solids 2004, 65 (8), 1537−1545. (64) Denning, R. G. Electronic structure and bonding in actinyl ions and their analogs. J. Phys. Chem. A 2007, 111 (20), 4125−4143. (65) Gibson, J. K.; Haire, R. G.; Santos, M.; Marcalo, J.; de Matos, A. P. Oxidation studies of dipositive actinide ions, An2+ (An = Th, U, Np, Pu, Am) in the gas phase: Synthesis and characterization of the isolated uranyl, neptunyl, and plutonyl ions UO22+(g), NpO22+(g), and PuO22+(g). J. Phys. Chem. A 2005, 109 (12), 2768−2781. (66) Tejuca, L. G.; Fierro, J. L. G. XPS and TPD probe techniques for the study of LaNiO3 perovskite oxide. Thermochim. Acta 1989, 147 (2), 361−375. (67) Sarma, D. D.; Shanthi, N.; Mahadevan, P. Electronic-structure and the metal-insulator transition in LnNiO3 (Ln = La, Pr, Nd, Sm and Ho) - Band structure results. J. Phys.: Condens. Matter 1994, 6 (48), 10467−10474. (68) Matar, S. F.; Demazeau, G. Electronic band structure of CaUO4 from first principles. J. Solid State Chem. 2009, 182 (10), 2678−2684. (69) Osman, H. H.; Pertierra, P.; Salvado, M. A.; Izquierdo-Ruiz, F.; Recio, J. M. Structure and bonding in crystalline cesium uranyl tetrachloride under pressure. Phys. Chem. Chem. Phys. 2016, 18 (27), 18398−18405. (70) Bacmann, M.; Bertaut, E. F.; Blaise, A. Neutron diffraction and magnetic measurement study of ferromagnetic UFeO4. Comptes Rendus Hebdomadaires Des Seances De L Academie Des Sciences Serie B 1968, 266 (2), 45−48. (71) Bacmann, M.; Bertaut, E. F.; Blaise, A.; Chevalier, R.; Roult, G. Magnetic structures and properties of UFeO4. J. Appl. Phys. 1969, 40 (3), 1131−1132. (72) Greenblatt, M.; Hornreich, R. M.; Sharon, B. Magnetoelectric compounds with two sets of magnetic sublattices - UCrO4 and NdCrTiO5. J. Solid State Chem. 1974, 10 (4), 371−376. (73) Shannon, R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta K

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Article

Inorganic Chemistry Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32 (5), 751−767.

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DOI: 10.1021/acs.inorgchem.8b02355 Inorg. Chem. XXXX, XXX, XXX−XXX