High temperature intrinsic defects in fused silica

The equilibrium concentrations of intrinsic defects in fused silica are estimated in the temperature range 1000-2000 K, assuming that thermal SiO bond breaking in a regular glass network creates a pair of moveable intrinsic defects: (a) 3-coordinated Si atoms and (b) non-bridging O atoms. Besides the recombination of moveable defects, which restores the regular network (R ⇆ a + b), two 3-coordinated Si atoms create an O vacancy (A ⇆ a + a) and two non-bridging O atoms create a bridging peroxy linkage (B ⇆ b + b). With the use of single SiO, SiSi and OO bond energies as activation energies for these reactions, the differential equations for the bridging peroxy linkage equilibrium defect concentrations are solved for different environments. There is good correlation between the temperature dependence of the equilibrium concentration of bridging peroxy linkages and the dependence of the intensity of the Raman 606 cm^-1 mode on fictive temperatures. On this basis, bridging peroxy linkages are proposed as the source of the 606 cm^-1 Raman mode.