Structural and electronic phase transitions ofThS2from first-principles calculations

Abstract

Thorium and its compounds have received considerable attention in recent years due to the renewed interest in developing the thorium fuel cycle as an alternative nuclear energy technology. There is pressing current need to explore the physical properties essential to the fundamental understanding and practical application of these materials. Here we report on a computational study of thorium disulfide (${\mathrm{ThS}}_{2}$), which plays an important role in the thorium fuel reprocessing cycle. We have employed the density functional theory and evolutionary structure search methods to determine the crystal structures, electronic band structures, phonon dispersions and density of states, and thermodynamic properties of ${\mathrm{ThS}}_{2}$ under various pressure and temperature conditions. Our calculations identify several crystalline phases of ${\mathrm{ThS}}_{2}$ and a series of structural phase transitions induced by pressure and temperature. The calculated results also reveal electronic phase transitions from the semiconducting state in the low-pressure phases of ${\mathrm{ThS}}_{2}$ in the $Pnma$ and $Fm\overline{3}m$ symmetry to the metallic state in the high-pressure phases of ${\mathrm{ThS}}_{2}$ in the $Pnma$ and $I4/mmm$ symmetry. These results explain the experimental observation of the thermodynamic stability of the $Pnma$ phase of ${\mathrm{ThS}}_{2}$ at the ambient conditions and a pressure-induced structural phase transition in ${\mathrm{ThS}}_{2}$ around 40 GPa. Moreover, the present study reveals considerable additional information on the structural and electronic properties of ${\mathrm{ThS}}_{2}$ in a wide range of pressure and temperature. Such information provides key insights into the fundamental material behavior and the underlying mechanisms that lay the foundation for further exploration and application of ${\mathrm{ThS}}_{2}$.