1982 Volume 90 Issue 1048 Pages 703-709
Additivity for the molar volumes of alkali silicate glasses (0<R2O<33.3mol%) was discussed in terms of two structural units, SiO4/2 and ROSiO3/2. The units were represented by the cristobalite-type cage skeletons composed of Si-O-Si and Si-OR⋅RO-Si bonds. The cages involved voids associated with the units and the glasses were regarded as the aggregates of those cage skeletons. The deviation from the linearity (additivity) of molar volumes against R2O molar fraction was excellently approximated by the random packing of two kinds of spheres which corresponded to the SiO4/2 and ROSiO3/2 units. Furthermore, based on the cage structure model, Si-Si distances at the bridging and non-bridging parts were evaluated. It was shown that the two Si and two O atoms at the non-bridging parts were arranged in an array of a line, ≡Si-O--O-Si≡, in rubidium and cesium glasses, while in lithium, sodium and potassium glasses the two non-bridging oxygens were located at the sites away from the Si-Si tie line, ≡Si-O--O-Si≡.
