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

Abstract

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium <i>A</i>UO₄ oxides using a combination of high-resolution structural and spectroscopic measurements supported by <i>ab initio</i> 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 <i>Cmmm</i> whereas MnUO₄, CoUO₄, and MgUO₄ are described by the lower symmetry <i>Ibmm</i> space group and are isostructural with the previously reported β-NiUO₄ [Murphy et al. <i>Inorg. Chem.</i> <b>2018</b>, <i>57</i>, 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 <i>A</i>²⁺ and U⁶⁺ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the <i>A</i>O₆ polyhedra, leading to a change in symmetry from <i>Cmmm</i> to <i>Ibmm</i>. Such tilts are absent in the <i>Cmmm</i> structure. Heating the <i>Ibmm A</i>UO₄ oxides results in reduction of the tilt angle. This is demonstrated for MnUO₄ where <i>in situ</i> S-XRD measurements reveal a second-order phase transition to <i>Cmmm</i> near <i>T</i> = 200 °C. Based on the extrapolation of variable temperature <i>in situ</i> S-XRD data, CoUO₄ is predicted to undergo a continuous phase transition to <i>Cmmm</i> at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+<i>U</i> 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 <i>A</i>UO₄ family of oxides.