Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and...

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO₄ oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO₄, MnUO₄, CoUO₄, and MgUO₄ are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO₄ is best described by space group Cmmm whereas MnUO₄, CoUO₄, and MgUO₄ are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO₄ [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A²⁺ and U⁶⁺ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO₆ polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO₄ oxides results in reduction of the tilt angle. This is demonstrated for MnUO₄ where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO₄ is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO₄ family of oxides.