Pressure-induced phase transformations in amorphous selenium by x-ray diffraction and Raman spectroscopy

High pressure x-ray diffraction (XRD) and Raman studies have been carried out at room temperature on amorphous selenium (Se) samples up to 40 and 20 GPa, respectively, in the diamond anvil cell. The XRD measurements reveal that the amorphous (Am) {→} crystalline phase transition occurs at 11 GPa followed by a sequence of other pressure-induced crystal to crystal phase transitions with increasing pressure. The data obtained in the pressure range 1 bar to 40 GPa fit four different crystalline phases out of the Am Se; we find between 22 and 27 GPa a phase with a new different monoclinic unit cell and near 30 GPa a phase with a new orthorhombic cell. With these the transformation sequence with increasing pressure is hexagonal Se(I) {→} monoclinic Se(II) {→} monoclinic Se(III) {→} orthorhombic Se(IV) at 14, 21.5, and 28 GPa, respectively. The volume changes across these three transitions are 12, 3, and 0 %, respectively. The zero-pressure bulk modulus of Se(I), Se(II), Se(III), and Se(IV) is computed to be 22.45, 27.08, 112.2, and 112.2 GPa, respectively. Raman spectroscopy studies are consistent with the observation of a phase transition near 14 GPa by XRD. {} 1996 The American Physical Society.