Ab initio investigation of pressure-induced structural transitions and electronic evolution of Th3N4

The crystal structures, lattice dynamics, mechanical, electronic properties, and electron–phonon coupling of Th3N4 under environmental conditions and high pressures have been studied by merging first-principles calculations and particle-swarm optimization algorithm. Four structures are identified for Th3N4, including the R3¯m, I4¯3d, R3¯, and C2/m phases, in which the I4¯3d, R3¯, and C2/m phases are newly predicted. Their mechanical properties, including the Poisson's ratio σ, the elastic anisotropy factor AU, and the Pugh's ratio B/G, have been calculated and discussed. The results show that the R3¯m, I4¯3d, and R3¯ phases of Th3N4 behave ductile nature, while the C2/m phase behaves brittle nature. Among them, the I4¯3d phase of Th3N4 almost exhibits completely anisotropic nature. Besides, our electronic band structure calculations show that the pressure-induced semiconductor-metal transition occurs following the I4¯3d to R3¯ phase transition. Further, the electron-phonon coupling of the R3¯ phase has been analyzed. The results we obtained are of significance to further understand the physical essence of Th3N4 and its practical engineering applications.