Non-Randomness in Network Glasses and Rigidity

The continuous random network model is widely used as a realistic description of the structure of covalent glasses and amorphous solids. We point out that, in real glasses and amorphous materials, there are nonrandom structural elements that go beyond just simple chemical ordering. We propose that the network can self-organize at its formation or fictive temperature and examine some of the possible consequences of such self-organization. We find that the absence of small rings can cause the mechanical threshold to change from a second-order to a first-order transition. We show that, if stressed regions are inhibited in the network, then there are two phase transitions and an intermediate phase that is rigid but stress-free. This intermediate phase is bounded by a second-order transition, on the one hand, and a first-order transition, on the other. Recent experiments in chalcogenide glasses give evidence for this intermediate phase.