In situ study of the mechanism of formation of pressure-densified SiO2 glasses

The volume of glassy a-SiO2 upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO2 glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO2. A nonequilibrium phase diagram is suggested for glassy SiO2. It accounts for all the presently available experimental data and is confirmed by the existing modeling data. (C) 2002 MAIK "Nauka / Interperiodica".