Evolutionary prediction and experimental synthesis of SiOS at high pressure*

SiOS represents a ternary generalization of the well-known and highly important family of binary AB₂ compounds including CO₂, SiO₂, GeO₂, CS₂, SiS₂, etc. The presence of two different bond lengths suggests that its crystal structures may be different from those found in SiO₂ and SiS₂. We applied evolutionary search based on DFT ab initio calculations to determine crystal structures of SiOS for pressures up to 100 GPa. We predict the SiOS phase diagram at zero temperature and examined the structural, electronic and vibrational properties of the stable phases. At low pressure the stable phase is a tetrahedrally coordinated layered orthorhombic Cmc2₁ structure. This is predicted to transform at 14 GPa to an octahedrally coordinated layered monoclinic C2/m structure similar to the P-3m1 phase of SiS₂. The system remains insulating up to 100 GPa with band gap above 1.8 eV. Following the theoretical prediction we synthesized SiOS by laser heating elemental Si, O and S in the diamond anvil cell at pressure of 8 GPa. The observed XRD pattern is in very good agreement with the theoretical prediction for the Cmc2₁ structure.