Polymorphism and phase transitions in layered uranium(VI) hydroxides: <i>Ab initio</i> lattice dynamics simulations of UO2(OH)2

The phase transitions and thermodynamics of stoichiometric α-, β-, and γ-UO2(OH)2 polymorphs are investigated using density functional perturbation theory. The pressure-induced β(Pbca) → α(Cmca) phase transition is reproduced by calculations, with a volume reduction of ΔV/V = −14.7% similar to experiment. Consistent with observation, a temperature-driven γ(P21/c) → β(Pbca) phase transition is predicted near 533 K. At 298.15 K, the computed standard molar heat capacity of α-UO2(OH)2 is Cp0 = 112.1 J mol−1 K−1, only 1.6% smaller than the value of Cp0 = 113.96 ± 0.12 J mol−1 K−1 measured by calorimetry. Cp0 = 112.4 and 104.8 J mol−1 K−1 are predicted for the β- and γ-UO2(OH)2 polymorphs, respectively. The calculated molar enthalpy and Gibbs energy functions of the α-, β-, and γ-UO2(OH)2 polymorphs are also reported.