High-Pressure Synthesis of β- and α‑In2Se3‑Like Structures in Ga2S3

The synthesis of polymorphs of α′-Ga2S3 at room temperature on compression/decompression is studied from a joint experimental and theoretical point of view to reveal the nature of their crystalline structures. The results of Raman scattering and X-ray diffraction measurements on these polymorphs have been analyzed supported by theoretical ab initio simulations. On compression, α′-Ga2S3 undergoes a phase transition above 16 GPa to β′-Ga2S3 with a tetradymite-like (R-3m) structure, isostructural with β-In2Se3. On decompression, β′-Ga2S3 undergoes a phase transition below 9.0 GPa to φ-Ga2S3, which is isostructural with α-In2Se3 (R3m). Raman signatures of symmetry breaking as well as clear structural differences between the pressure dependence of the unit-cell volume per formula unit, zero-pressure axial compressibilities, bulk modulus, and their first pressure derivative between β′-Ga2S3 and φ-Ga2S3 have allowed us to determine the R3m nature of φ-Ga2S3. The observation of the R3m phase is also supported by theoretical total energy ab initio simulations. This result unveils a pressure-induced paraelectric-ferroelectric R-3m-to-R3m transition, like the theoretically predicted temperature-induced transition in several III–VI B 2 X 3 compounds, which could find use in technological applications. Finally, φ-Ga2S3 undergoes a phase transition below 1.0 GPa to γ-Ga2S3 with a disordered zincblende (F-43m) phase, isostructural with α-Ga2Se3 and remains metastable at room conditions. Since the disordered zincblende phase of Ga2X3 chalcogenides has also been found upon decompression in AGa2 X 4 chalcogenides, we discuss the relation between the pressure-induced phase transitions of both Ga2 X 3 and AGa2 X 4 compounds.