The α—β transition in the cristobalite phases of SiO2and AIPO4I. X-ray studies

Abstract

The α—β transition in the cristobalite phases of SiO₂ and AlPO₄ has been studied using X-ray powder diffraction. The transitions show marked hysteresis but also occur over a range of about 20°C in which the α and β phases coexist. The lattice parameters show a smooth and slow variation with temperature for each phase and a discrete difference between phases corresponding to volume differences of about 4%, showing that the transitions are of first order. The hysteresis has been studied and it is concluded that the phase coexistence in the bulk sample occurs because different crystallites undergo the transition at different temperatures. The structural aspects of the transformations are discussed and it is shown that the phase change involves a small movement of the atoms around four fold screw axes together with a disordering of oxygen atom positions