Phase transition and thermodynamic properties of ThO2 from first-principles calculations

Within the framework of the quasi-harmonic approximation, the thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are studied using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon density of states and the Debye–Grüneisen model. The calculated results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.49 GPa, which is in agreement with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly above room temperature. The predicted heat capacity and linear thermal expansion coefficient of cubic ThO2 are in good consistence with the experimental data. By comparing the experimental results with the calculation results from the first-principles and Debye–Grüneisen model, it is found that the thermodynamic properties of ThO2 are depicted well by the first-principles phonon treatment after including the an-harmonic correction to quasi-harmonic free energy.