First-principles study on structural, mechanical and electronic properties of thorium dichalcogenides under high pressure

By merging ab initio calculations and particle-swarm optimization algorithm, we have predicted two new phases of thorium dichalcogenides ThX₂ (X = S, Se and Te) in the Fm3¯m and I4/mmm symmetry. The Fm3¯m phase is proved to be the ground-state phase of ThS₂ and ThSe₂. The calculated enthalpies indicate that the Fm3¯m to Pnma phase transition pressures are about 2.3 GPa and 0.35 GPa for ThS₂ and ThSe₂, respectively; and the Pnma to I4/mmm phase transition pressures are about 37 GPa, 17 GPa and 2 GPa for ThS₂, ThSe₂ and ThTe₂, respectively. The phonon dispersion curves and elastic constants suggest that all of the Fm3¯m, Pnma and I4/mmm phases are dynamically and mechanically stable. The electronic calculations show that a pressure-induced semiconductor to metal transitions of these three compounds will occur following the Pnma to I4/mmm phase transition. Our calculated bulk modulus B₀ and elastic constants C_ij of the various phases of these compounds show that the materials become softer one after another from ThS₂ to ThSe₂ and then to ThTe₂.