Pressure-induced crystallization and phase transformation of amorphous selenium: Raman spectroscopy and x-ray diffraction studies

High- pressure Raman spectroscopy studies have been carried out on amorphous Se ( a- Se) at room temperature in a diamond anvil cell with an 830 nm exciting line. Raman evidence for the pressure- induced crystallization of a- Se and the coexistence of the unknown high- pressure phase with the hexagonal phase is presented for the first time. Further experimental proof of high- pressure angle-dispersive x- ray diffraction studies for a- Se indicates that the unknown high-pressure phase is also a mixture phase of the tetragonal I4(1)/acd and Se IV structure. Our Raman and x- ray diffraction results suggest that hexagonal Se I undergoes a direct transition to triclinic Se III at about 19 GPa, which is in good agreement with the theoretical prediction.