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 Fm (3) over barm and I4/mmm symmetry. The Fm (3) over barm phase is proved to be the ground-state phase of ThS2 and ThSe2. The calculated enthalpies indicate that the Fm (3) over barm 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 Fm (3) over barm, 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 B-0 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. (C) 2018 Elsevier B.V. All rights reserved.