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 B2X3 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 AGa2X4 chalcogenides, we discuss the relation between the pressure-induced phase transitions of both Ga2X3 and AGa2X4 compounds.