Ab initio study of polymorphism and phase transitions in Na₂U₂O₇

Polymorphism and phase transitions in sodium diuranate, Na₂U₂O₇, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-Na₂U₂O₇ polymorphs are predicted from DFPT phonon calculations, i.e., the first time for the high-temperature γ-Na₂U₂O₇ phase (Rm symmetry). The standard molar isochoric heat capacities are predicted within the quasi-harmonic approximation for P2₁/a α-Na₂U₂O₇ and C2/m β-Na₂U₂O₇, respectively. Gibbs free energy calculations reveal that α-Na₂U₂O₇ (P2₁/a) and β-Na₂U₂O₇ (C2/m) are almost energetically degenerate at low temperature, with β-Na₂U₂O₇ becoming slightly more stable than α-Na₂U₂O₇ as temperature increases. These findings are consistent with XRD data showing a mixture of α and β phases after cooling of γ-Na₂U₂O₇ to room temperature and the observation of a sluggish α → β phase transition above ca. 600 K. A recently observed α-Na₂U₂O₇ structure with P2₁ symmetry is also shown to be metastable at low temperature. Based on Gibbs free energy, no direct β → γ solid-solid phase transition is predicted at high temperature, although some experiments reported the existence of such phase transition around 1348 K. This, along with recent experiments, suggests the occurrence of a multi-step process consisting of initial β-phase decomposition, followed by recrystallization into γ-phase as temperature increases.