Letter to the Editor: Structural transitions of the vitreous state

The presence of structural nanoheterogeneity means that sharp structural transitions at a well defined temperature or composition are an anathema to the vitreous state. Hence they should not be expected, but rather an extended transition, whose width is defined by the form and length scale of the relevant nanoheterogeneity. It should also be noted that all of the vitreous state transitions considered here, together with crystallisation at the melting point, are spatially non-uniform. This behaviour should be compared to sharp transitions that occur within the crystalline state, for example the displacive transition between α- and β-quartz, and those from a paramagnetic to a magnetically-ordered state. Such sharp transitions involve simultaneous co-operative movements across a large number identical unit cells, and hence cannot occur in the absence of a periodic structure. As a result, they are sensitive to disorder within individual unit cells, as may be seen from the case of cristobalite, where the presence of disorder reduces the αβ transition temperature, and in extremis inhibits the transition from β- to α-cristobalite.