A complete set of transverse acoustic phonon frequencies of Ge has been measured at pressures up to 9.7 GPa by inelastic neutron scattering and studied by ab initio calculations using density functional perturbation theory. At the X and L points, the pressure dependence of the phonon frequency ω in the cubic structure is shown to be that of a classical soft mode with ${\mathrm{\omega }}^2$ strictly linear in pressure P. This finding is in contrast with previous interpretations that associated the pressure dependence of the zone boundary modes with the occurrence of the first order transition at $\sim 10$ GPa and represented the pressure dependence of its frequencies by a polynomial in P.

• Received 15 October 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.1313