The structures and thermal responses of the calcium uranates α-Ca3UO6 and Ca2UO5 were investigated using in situ synchrotron x-ray diffraction and neutron powder diffraction. Rietveld refinement yielded accurate structural models, with α-Ca3UO6 adopting the rhombohedral R3̅ space group and Ca2UO5 crystallizing in monoclinic P21/c. Thermal behavior was examined using thermogravimetric analysis under varied atmospheres combined with in situ diffraction measurements. α-Ca3UO6 exhibits oxygen loss on heating, manifested as changes in U─O bond lengths within the UO6 octahedra. These changes induce progressive distortion and strain in the CaO6 polyhedra, preceding a distinct structural event observed by synchrotron x-ray diffraction at ∼530°C. Ca2UO5 undergoes two thermally driven events, including oxygen loss at ∼600°C. The presence of trans short U─O (uranyl-like) bonds is considered to play a key role in constraining UO6 octahedral distortion at elevated temperatures. In contrast, the CaOx polyhedra display substantial bond-length variability in response to the rigidity of the U─O framework. At higher temperatures under vacuum, anomalous U─O bond expansion correlates with mass-loss events. These results highlight the complex, environment-dependent thermal behavior of calcium uranates, and the structural significance of short trans U─O bonds.
Nicholas et al. (2026) studied this question.