High-temperature phase transitions of gallium orthophosphate (GaPO4)

Phase transitions of gallium orthophosphate (GaPO4) have been studied by th ermal analysis between ambient temperature and 1100 degrees C. In situ X-ra y powder diffraction at selected temperatures confirmed the three known pol ymorphic structure types of low-quartz, low-cristobalite and high-cristobal ite GaPO4, respectively. Whereas the as-synthesized powder retained its low -quartz structure up to 970 degrees C, powders cycled up to 1100 degrees C exhibited three phase transitions (low-cristobalite --> (approximate to 620 degrees C) high-cristobalite --> (approximate to 800 degrees C) low-quartz --> (approximate to 970 degrees C) high-cristobalite) while heating up and one phase transition (high-cristobalite --> (approximate to 590 degrees C) low-cristobalite) while cooling down. However, the high-cristobalite/low-q uartz heating transformation was found to be kinetically hindered. Isotherm al in situ X-ray powder diffraction revealed a remarkable temperature depen dence of this reaction kinetics.