Polymorphism and Phase Transitions in Layered Uranium(vi) Hydroxides: Ab Initio Lattice Dynamics Simulations of UO2(OH)2

JOURNAL OF APPLIED PHYSICS（2024）

Sandia Natl Labs

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Abstract

The phase transitions and thermodynamics of stoichiometric alpha-, beta-, and gamma-UO2(OH)(2) polymorphs are investigated using density functional perturbation theory. The pressure-induced beta(Pbca) -> alpha(Cmca) phase transition is reproduced by calculations, with a volume reduction of Delta V/V = -14.7% similar to experiment. Consistent with observation, a temperature-driven gamma(P2(1)/c) -> beta(Pbca) phase transition is predicted near 533 K. At 298.15 K, the computed standard molar heat capacity of alpha-UO2(OH)(2) is C-p(0) = 112.1 J mol(-1) K-1, only 1.6% smaller than the value of C-p(0) = 113.96 +/- 0.12 J mol(-1) K-1 measured by calorimetry. C-p(0) = 112.4 and 104.8 J mol(-1) K-1 are predicted for the beta- and gamma-UO2(OH)(2) polymorphs, respectively. The calculated molar enthalpy and Gibbs energy functions of the alpha-, beta-, and gamma-UO2(OH)(2) polymorphs are also reported.

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