First-principles study of pressure-induced phase transitions, mechanical and thermodynamic properties of ThBC

First-principles calculations and particle swarm optimization algorithm are combined to predict the crystal structures in the pressure range from 0 to 100 GPa. Four phases of ThBC are determined, including the P4₁22, Cmcm, Cmce and Immm phases, in which the Cmcm, Cmce and Immm phases are newly predicted structures. The mechanical, electronic and thermodynamic properties of the four phases are investigated. According to the enthalpy-pressure and volume-pressure curves, the phase transition pressure from the P4₁22 phase to the Cmcm phase is 15 GPa, the Cmcm to the Cmce is 36 GPa and the Cmce to the Immm is 69 GPa. All the transitions belong to the first-order phase transition. Based on the calculated elastic constants, the P4₁22, Cmcm, Cmce and Immm phases exhibit brittle nature. The Young's moduli show that the P4₁22 phase has the largest degree of anisotropy, and the Immm phase has the smallest. The calculated density of states reveal that the P4₁22, Cmcm, Cmce and Immm phases are all metallic.