GeS Phases from First-Principles: Structure Prediction, Optical Properties, and Phase Transitions upon Compression

The structures of GeS beyond the orthorhombic α-Pnma phase are explored by density functional theory calculations of the energetics and phonon spectra guided by an evolutionary algorithm. We found six new, dynamically stable structures at ambient conditions, close to the ground state. Our study of the electronic and optical properties of these unconventional phases indicates distinct differences from α-Pnma GeS. The band gaps of the new phases vary in the range of 0.39-1.59 eV, and their optical properties are attractive for optoelectronic applications. Upon compression, the new GeS structure denoted as γ-Pnma (analog to γ-Pnma SnSe) is thermodynamically preferred over α-Pnma at 15 GPa. At higher pressures, the GeS structure converges to a Cmcm-like structure at 30 GPa. Finally, cubic Pm3¯ m is the favorable and stable phase at 70 GPa. The mechanism of the pressure-induced transition correlates with the delocalization of the lone pairs.