Zero-Point Effects on Phase Transitions of Thorium Dihydride under High Pressure

Abstract

The crystalline structures of thorium dihydride, ThH2, under pressure are investigated by using an unbiased structure searching method coupled with ab initio calculations. Three low-enthalpy phases are found as the thermodynamic ground states of ThH2 up to 200 GPa, including an experimentally observed I4/mmm phase and two newly predicted monoclinic phases (C2/m and C/2c phase). ThH2 is predicted to undergo I4/mmm → C2/m → C/2c phase transitions without zero-point (ZP) effects, whereas it directly transforms from the I4/mmm phase to the C2/c phase with ZP effects. Phonon calculations show that these competitive phases are thermodynamically stable. There is a strengthening of the metallic characters of the chemical bonding with increased pressure. Our results highlight the role of ZP effects in the high-pressure behaviors of metal hydrides and provide insight into further studies of other compounds containing light elements under pressure.