PRESSURE-DEPENDENCE OF RAMAN PHONONS OF SOME GROUP-IVA (C, SI, AND GE) ELEMENTS

The Raman spectra of diamond, silicon and germanium were measured at room temperature for pressures up to 180 kbar in a diamond-anvil cell. Attempts to use the Raman spectra from the diamond anvils themselves as a pressure calibration standard were unsuccessful because of the experimental difficulty of recording only the high stress in the immediate vicinity of the centre of the diamond anvil. Raman spectra recorded from silicon and germanium showed that they change from the cubic diamond structure to the metallic beta-Sn structure at pressures slightly greater than 140 and 100 kbar, respectively, under a hydrostatic environment. When the pressure is released the metallic phase of both silicon and germanium does not revert to the diamond structure but forms a metastable phase at pressures below 70 kbar. The Raman spectrum of the metastable Si phase is consistent with the body-centred cubic structure proposed in the literature. The proposed structure of metastable Ge should possess several Raman bands, yet only one strong Raman band at 292 cm-1 (at ambient conditions) was observed in the present study. The bulk moduli of the diamond structures of C, Si and Ge were calculated from the Raman spectra of these elements and compared favorably with those determined by ultrasonic experiments.