Structural Evolution and Medium Range Order in Permanently Densified VitreousSiO2

Positron annihilation lifetime spectroscopy is employed to measure the size of the interstitial void spaces characterizing the structure of a set of permanently densified ${\mathrm{SiO}}_{2}$ glasses. The average volume of the voids is markedly affected by the densification process and linearly shrinks by almost an order of magnitude after a relative density variation of 22%. In addition, x-ray diffraction shows that this change of density does not modify appreciably the short range order, which remains organized in ${\mathrm{SiO}}_{4}$ tetrahedra. These results strongly suggest a porous medium description for $v\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$ glasses where the compressibility and the medium range order are dominated by the density variation of the voids volume up to densities close to that of $\ensuremath{\alpha}$-quartz.