Elastic constants of stishovite up to its amorphization temperature

Brillouin spectra of stishovite were measured up to the amorphization temperature ( K) using large (1 mm) defect-free single crystals. At room temperature the sound velocities extracted from the BLS yield all six elastic constants. The derived bulk modulus and the polycrystalline shear modulus of stishovite at ambient conditions are B = 316 GPa, G = 222 GPa. Between room temperature and 800 K only a small decrease of the elastic constants was observed. This is an indication that the amorphization process is not due to a soft, long-wavelength, acoustic phonon. Room-temperature Raman spectra from samples heated to close to the amorphization temperature show that amorphization occurs more rapidly on the (110) than on the (001) crystallographic face. These results, together with those of x-ray measurements from the literature, allow us to conclude that amorphization in stishovite is most likely a surface phenomenon initiated by large oscillations of the Si atoms within the crystal faces whose normals are perpendicular to the c-axis.