Variable Temperature Studies of Two Calcium Uranates α-Ca₃UO₆ and Ca₂UO₅

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 CaO_x 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.