Structural aspects of volume nucleation in silicate glasses

An interrelationship between parameters of short and intermediate range order in silicate glasses and the tendency to nucleate homogeneously in the volume is tested. Changes in the average coordination number and metal-oxygen distance of network modifying cations as well as changes in the concentration of constitutive silica tetrahedra accompanied with the crystallization of 18 stoichiometric glass compositions into their crystalline analogs are determined. The intermediate range structure of the glasses is investigated by configurational entropy and flow birefringence. The changes in structural parameters are analyzed in terms of the reduced glass transition temperature Trg, which is negatively correlated with the maximal rate of volume nucleation. The results indicate that the short-range structure in stoichiometric glasses is, in general, very similar to the corresponding crystal structure but independent of the Trg-scale and for this reason independent of nucleation properties. In contrast to the short range of the glass structure, birefringence induced by a forced flow above the glass transition temperature and configurational entropy are positively correlated with increasing Trg. The results indicate increased structural order in the intermediate range for melts with a high supercool limit (Trg < 0.58). It is concluded that this order phenomena may promote nucleation events and may help to explain the tendency to volume nucleation of silicate glasses with Trg < 0.58.