First-principles calculations of phase transition, elasticity, phonon spectra, and thermodynamic properties for ThO2 polymorphs

Abstract

First-principles calculations based on density generalized function theory (DFT) were used to determine the structural transformation, phonon spectra, thermodynamic properties, and elastic behavior of ThO2 under compressive and tensile loading in this work. In compressive conditions, the transition pressures and calculation of structural parameters agree with the experiment results. Phase transitions of ThO2 from the Fm3‾m phase to the Pnma, P42/mnm and Pnnm phases under tensile conditions were anticipated to happen at 23.2 GPa, −7.69 GPa, and −7.75 GPa, respectively. Within the range of pressures analyzed, both the Fm3‾m and Pnma phases of ThO2 exhibit mechanical stability, but the P42/mnm phase and Pnnm phase are unstable at high pressures, according to the mechanical stability criteria, which is validated concerning elastic constants. Under ambient pressure, the calculated phonon spectra of the ThO2 polymorphs in this study demonstrate that all of the phases taken into consideration are dynamical stability. Also, the predicted heat capacities, isothermal bulk modulus, Gibbs free energies, and thermal expansion coefficients of ThO2 polymorphs as functions of temperature are analyzed and are in comparison with relevant experiments.