Prediction of novel gallium-sulfur compositions under pressure

Predicting the (meta)stable crystal structures in a binary phase diagram under pressure is essential for enhancing our understanding of high-pressure materials. The gallium-sulfur system is especially intriguing because of the semiconductor materials present at atmospheric pressure, with the potential for new compositions to form under compression. In this article, we employed two distinct and powerful methodologies for crystal structure prediction from ambient pressure up to 100 GPa in the Ga-S system: evolutionary algorithms as implemented in the USPEX package, utilizing the accuracy of density functional theory (DFT) for precise electronic structure calculations, and ab initio random structure searching (AIRSS), leveraging ephemeral data-derived potentials (EDDP) to achieve high-speed exploration. Our crystal structure search not only reaffirms the existence of the known GaS and Ga₂S₃ phases but also reveals eleven novel phases emerging progressively as the pressure is increased from 0 to 100 GPa, demonstrating their dynamic stability across varying pressure regimes. Our calculations predict P6₃/mmc → C2/m → R3̄m and Cc → R3m → R3̄m transitions in GaS and Ga₂S₃, respectively. Among the eleven predicted phases, C2/m GaS₂ and C2/m Ga₃S₄ persist to ambient pressure on decompression, which are dynamically stable. C2/m GaS₂ is a layered material composed of 2D sheets of Ga₂S₂ and intercalated S₂ dimers that exhibits electrical insulating properties. Upon compression, a pressure-induced polymerisation is observed in GaS₂. The S₂ dimers couple to form a linear, infinite sulphur chain with 1 electron-2 center bonds. This electronic configuration, i.e. 7 electrons per -(S^-)- repeating unit, confers the electrical metallic properties of the high-pressure Cmcm GaS₂ phase.