In situ study of the mechanism of formation of pressure-densified Sio$_2$ glasses

The volume of glassy $a$-Sio$_2$ 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 $a$-Sio$_2$ 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 $a$-Sio$_2$. A nonequilibrium phase diagram is suggested for glassy Sio$_2$. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.