Polymorphism and phase transitions in NaUO from density functional perturbation theory.

Polymorphism and phase transitions in sodium diuranate, NaUO, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-NaUO polymorphs are predicted from DFPT phonon calculations, , the first time for the high-temperature γ-NaUO phase (3̄ symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for 2/ α-NaUO and 2/ β-NaUO, respectively. Gibbs free energy calculations reveal that α-NaUO (2/) and β-NaUO (2/) are almost energetically degenerate at low temperature, with β-NaUO becoming slightly more stable than α-NaUO as temperature increases. These findings are consistent with XRD data showing a mixture of α and β phases after cooling of γ-NaUO to room temperature and the observation of a sluggish α → β phase transition above 600 K. A recently observed α-NaUO structure with 2 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.