SILICA GLASS: A BENT-BOND MODEL

Abstract

from distorted hexagonal rings. These grains are joined together by rings other than 6-membered, probably mostly 7- and 5-membered, but 8- and 4-membered rings would not seem impossible. In the example shown in Figure 1, the tetrahedra represent Si atoms and the bent links 0 atoms. The model was built mostly from 6membered rings, but contains several 5- and 7-membered rings. When the Si-O bonds in the cristobalite lattice bend and rotate to the extents indicated, long-range order ceases and the structure melts. Relatively few bonds need to be broken, but some are. These are rejoined in a largely random way, and require the migration of some SiO2 molecules in order to preserve electrical neutrality. On the basis of Eyring's significant structure theory of liquids,4 the holes in the liquid which reflect the structure of the solid are those extra positions in the neighborhood of each Si and 0 atom about their equilibrium positions in the solid which can be filled by the atoms when the bonds are bent and rotated to form the liquid. These can be considered to provide solidlike degrees of freedom and are to be associated with 6-membered rings. The gaslike degrees of freedom are used by the migrating SiO2 molecules which result from formation of 5-membered, 7-membered, and possibly other kinds, but not 6-membered rings. The bent and rotated