Abstract:

Based on the first-principles calculations and particle swarm optimization algorithm, the crystal structures and physical properties of Th₂N₂S are examined in the pressure range of 0~200 GPa. Our results successfully reproduce the experimental phase$P\bar {{3}}m1$ at ambient pressure and predicted two new structures at high pressure: the I4/mmm and Cmmm phases. A series of pressure-induced structural phase transitions were determined, namely from the$P\bar {{3}}m1$ phase to the I4/mmm phase, and then to the Cmmm phase, with corresponding phase transition pressures of 48.2 GPa and 156.2 GPa. The phonon dispersion curves and elastic constants of Th₂N₂S indicate that these three phases are dynamically and mechanically stable. The obtained mechanical properties demonstrate the natural ductility of the $P\bar {{3}}m1$, I4/mmm and Cmmm phases. Among them, the anisotropy degree of the Cmmm phase is the largest. Further, our electronic structure calculations show that the phase transition from the$P\bar {{3}}m1$ to I4/mmm is a semiconductor-metal phase transition.

Key words: high-pressure structural phase transition, structure prediction, crystal structure, first-principles calculation, thorium-based nuclear material