High-pressure phase transformation and metastable phases of a Si0.29Ge0.71 alloy using a diamond anvil cell

We investigated the high-pressure phase transformation of a Si0.29Ge0.71 alloy grown by a traveling liquidus-zone (TLZ) method. The TLZ-grown Si0.29Ge0.71 alloy was subjected to high pressures of up to ∼15 GPa using a diamond anvil cell. In situ synchrotron X-ray diffraction (XRD) and micro-Raman measurements revealed the appearance of a tetragonal β-Sn phase at 12.1 GPa. A weakened residual diamond-cubic (dc) diffraction peak together with dominant β-Sn peaks was present at 15.2 GPa. Upon depressurization, a metastable rhombohedral r8 phase appeared at 8.4 GPa. When depressurized to ambient pressure, the XRD profile showed an almost body-centered-cubic bc8 phase with residual weak r8 and dc diffraction peaks. No appreciable Raman peaks were observed upon depressurization down to 4.1 GPa, but weak peaks appeared at ∼283 and ∼396 cm−1 at ambient pressure. Theoretical calculations based on density functional perturbation theory indicated that the observed peaks were not related to bc8 but rather to hexagonal diamond (hd). These results indicate that a bc8 → hd phase transformation was induced by laser heating during the Raman measurement.