An x-ray powder diffraction study of the high temperature phase transitions in α-quartz-type AlPO4-GaPO4 solid solutions

Al1-xGaxPO4 solid solutions (x = 0.3, 0.7) with the α-quartz-type structure were investigated up to 1208 K by x-ray powder diffraction. The composition Al0.7Ga0.3PO4 exhibits almost simultaneous transitions to the β-quartz and β-cristobalite forms at close to 1050 K. The tendency towards the β-quartz type structure is found to be much less marked for the Ga-rich composition (x = 0.7) based on the temperature dependence of the cell parameters, molar volume, fractional atomic coordinates and tetrahedral tilt angle. Direct transformation to the β-cristobalite form begins close to 1123 K. The β-quartz form exists as a stable phase only for values below x = 0.3