The role of network rigidity on crystallization behaviour of glasses

it is well known that oxide glasses build up continuous networks. Therefore the rigidity of the latter plays a key role in understanding glass propert ies. The network switches from floppy to rigid if the number of covalent br idges between the network formers exceeds a certain threshold value. One an d the same structure could behave as rigid network in respect to some prope rties and could be floppy networks in respect to some other properties. Kin etics of phase transition (and most the nucleation processes) is very sensi tive to the rigidity of the network. Some tiny floppy regions still exist inside the rigid network close above t he threshold concentration, They can serve as active centers for the beginn ing of nucleation process, Much later, nucleation can appear in the rigid p art of the network. Therefore, we predict a certain interval of concentrati on of rigid bounds in which a bimodal size distribution function of the for med phase will be observed. This prediction is confirmed by experimental da ta we find in literature.