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