The α—β transition in the cristobalite phases of SiO2and AIPO4II. Calorimetric studies

Heat capacity measurements have been made on α- and β-cristobalite phases of SiO2 and AIPO4. Detailed measurements by both heating and cooling methods have been made in the transition region. These are consistent with the idea that the apparently gradual nature of the transition is due to individual crystallites transforming at different temperatures with no correlation between α → β and β → α temperatures. Enthalpies and entropies of transition have been determined and the latter confirm that short-range order in the β-phases extends over only a few unit cells. Strain effects in both α- and β-phases due to this disorder are suggested as the cause of the variation of transition temperature between crystallites