RAMAN-SPECTRA OF HIGH-PRESSURE POLYMORPHS OF SIO2 AT VARIOUS TEMPERATURES

Raman spectra of the two high-pressure polymorphs of SiO2 (coesite and stishovite) were investigated in the temperature range 105-875 K at a tmospheric pressure. Coesite remained intact after the highest tempera ture run, but stishovite became amorphous at temperatures above about 842 similar to 872 K. Most Raman modes exhibit a negative frequency sh ift with temperature for these polymorphs, but positive trends were al so observed for some modes. Except for some weak modes, nonlinear temp erature variation were established for these poly morphs within the ex perimental uncertainty and temperature range spanned. The slopes of th e variation (delta V-i/delta T)(p) for these polymorphs were compared with the published values. When compared with quartz and stishovite, t he four-membered rings of SiO4-tetrahedra in coesite exhibit very litt le change with both temperature and pressure. It is also suggested tha t temperature and pressure should have opposite effects on the Raman s hift of each vibrational mode.