First principles calculation of UO2 polymorphs and phase transitions under compressive and tensile loading

Abstract

We present a study of the structural, phase transition, electronic, elastic and thermodynamic properties of UO2 in the fluorite-type (space group Fm3-m), cotunnite-type (space group Pnma), marcasite-type (space group Pnnm) and rutile-type (space group P42/mnm) phases based on density functional theory (DFT). By GGA + U approach, ground state properties of UO2 polymorphs including structural parameters, electronic structure and mechanical properties were obtained and compared with reported numerical and experimental results. The calculated phase transition pressures for the structure transition from fluorite-type to cotunnite-type, marcasite-type and rutile-type phases are 20.6 GPa, −9.2 GPa and −7.0 GPa respectively. The dependence of electronic properties on pressure implies that there will be narrowing of band gap for phase transitions under both compressive and tensile loading. The calculations of elastic constants indicate the fluorite and cotunnite-type structures are mechanically stable in the studied pressure range, but the marcasite-type and rutile-type phases are unstable at high pressure. All the phases of UO2 were predicted to be dynamically stable under ambient pressure by the phonon dispersions calculations. The heat capacities, bulk moduli, Gibbs free energies and thermal expansion coefficients of the four UO2 polymorphs as a function of temperature are given, and those of fluorite UO2 by GGA + U calculation yield good agreement with reported experimental data.