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

Abstract

The crystalline structures of thorium dihydride, ThH 2 , 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 ThH 2 up to 200 GPa, including an experimentally observed I4/mmm phase and two newly predicted monoclinic phases (C2/m and C/2c phase). ThH 2 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.

Crystal Structure and Mechanical Properties of ThBC2

“…The interactions between electron and core were described by the frozen-core projector augmented wave (PAW) method [20], where the 6s 2 6p 6 6d 2 7s 2 , 2s 2 2p 1 , and 2s 2 2p 2 are valence electrons for Th, B, and C atoms, respectively. The PAW pseudo potential has been successfully used to study the compounds including Th atoms, such as thorium dicarbide [21], thorium dioxide [22], thorium dihydride [23], and thorium mononitrides [24]. The cutoff energy of 600 eV for plane wave and the 9 × 9 × 9 k-point meshes with Monkhorst-Pack method [25] were adopted, which can ensure that the total energy was converged to 1 meV per atom.…”

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

“…Thorium and its hydrides are promising materials for advanced nuclear reactors with a high level of safety [1][2][3][4][5][6][7][8] and have attracted much attention. This is mainly due to their strength and stability at elevated temperatures while maintaining the desirable properties for neutron moderation.…”

“…[9] In general, thorium hydrides crystalize in ThH 2 and Th 4 H 15 phases, which are both metallic solids. [2] Under ambient conditions, the stable thorium dihydride crystallizes in a body-centered tetragonal (bct) ionic structure with space group I4/mmm (No. 139).…”