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

The structures of GeS beyond the orthorhombic alpha-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 alpha-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 gamma-Pnma (analog to gamma-Pnma SnSe) is thermodynamically preferred over alpha-Pnma at 15 GPa. At higher pressures, the GeS structure converges to a Cmcm-like structure at 30 GPa. Finally, cubic Pm (3) over barm is the favorable and stable phase at 70 GPa. The mechanism of the pressure-induced transition correlates with the delocalization of the lone pairs.

Automated summary

Using density functional theory calculations guided by an evolutionary algorithm, we investigated the structures of GeS beyond the orthorhombic alpha-Pnma phase. We discovered six new energetically stable structures at ambient conditions, which exhibit distinct electronic and optical properties compared to alpha-Pnma GeS. The band gaps of these new phases range from 0.39 to 1.59 eV, making them attractive for optoelectronic applications. Under compression, the preferred structure changes from alpha-Pnma to gamma-Pnma and then to a Cmcm-like structure at higher pressures. Finally, the stable phase at 70 GPa is cubic Pm (3) over barm, with the transition mechanism related to the delocalization of lone pairs.