Hypersonic properties of undoped and Cr-doped bismuth germanium oxide

The velocity and attenuation of hypersonic waves propagating along the [100] or [110] axis of undoped and Cr-doped Bi₁₂GeO₂₀ have been measured at frequencies from 0.76 to 1.65 GHz and at temperatures of 4.5, 24, 77 and 293 K. The second-order elastic constants and Debye temperature have been calculated from these results. The measurements and the results of the calculations are used to interpret the attenuation in terms of collisions between low-energy phonons, constituting the sound wave, and thermal phonons. The thermal-phonon lifetime and Grüneisen constant have been estimated for undoped and doped material.