On α– β phase transition in cristobalite-type Al 1−xGa xPO 4 (0.00⩽ x⩽1.00)

Abstract

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic ( α) low-cristobalite to cubic ( β) high-cristobalite phase transition for Al 1− x Ga x PO 4, (0.00⩽ x⩽1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO 4 is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO 4 transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO 4 slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO 4 transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO 4 reverts to trigonal quartz type phase. However, annealing of GaPO 4 at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.