Structural and electronic phase transitions of thorium monoxide from first-principles calculations

Abstract

Because of the innovative concepts for creating the thorium fuel cycle as an alternative nuclear energy source, thorium and its compounds have attracted a lot of attention. It is essential to study the physical properties that are necessary for the basic understanding and practical application of these materials. Thorium monoxide has already drawn researchers' attention as a metal oxide. Under different pressure conditions, the crystal structures, electronic band structures, phonon dispersions, mechanical properties, and thermodynamic properties of ThO have been determined using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Previous experiments agree with our results for the lattice constants of the phase of Fm3‾m. Moreover, a novel structure of R-3m is predicted to be thermally stable at a pressure of roughly 22 GPa. The phase is then also mechanically and dynamically stable, according to the findings of our computed elastic constants and phonon dispersion. The phase change from Fm3‾m to R-3m has not, however, been associated with any electronic transitions, and as both phases are mental, they may have excellent thermal conductivity. We further calculated the results of the Helmholtz free energy, entropy, specific heat, internal energy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature based on the stable dynamics and mechanical properties, which reveal that they all could be good fuel. The results provide key insights into understanding the structural and electronic behaviors of ThO under the condition of external pressure.