Raman modes of metastable phases of Si and Ge

We have measured Raman spectra of metastable phases of Si and Ge prepared in a diamond anvil cell for pressures up to 12 GPa. For Si we observe eight Raman lines with mode Grüneisen parameters varying between -0.3 and 1.5. These lines can be assigned to the zone-center phonons of the cubic BC8-structure (Z = 8) by assuming a violation of Raman selection rules presumably due to disorder. In the case or Ge we observe two phases with different Raman spectra. The spectrum of the first phase is strikingly similar to that of BC8-Si. Below about 5 GPa this phase transforms into a second phase, which can be identified as Ge in the hexagonal diamond structure.