Pressure-induced amorphization of bismuth orthosilicate

High-pressure behaviour of bismuth orthosilicate (Bi4Si3O12) is investigated in a diamond anvil cell up to a pressure of 20 GPa using Raman spectroscopy. A new broad peak observed in the Raman spectra above 6 GPa is assigned to resonantly excited photoluminescence from self-trapped Frenkel excitons. Two phonons are found to have negative Gruneisen parameter, suggesting instability of the structure at high pressure. Broadening and disappearance of most of the Raman lines suggest amorphization of this system above 16 GPa. Amorphization is found to be reversible, unlike the case of Bi4Ge3O12, where it is irreversible. In light of a recent model of pressure-induced amorphization it is argued that amorphization in bismuth orthosilicates and germanates arises due to kinetic hindrance of decomposition into mixtures of daughter compounds with dense-packed structures.