High-temperature phase transitions of gallium orthophosphate (GaPO4)

Phase transitions of gallium orthophosphate (GaPO4) have been studied by thermal analysis between ambient temperature and 1100 degrees C. In situ X-ray powder diffraction at selected temperatures confirmed the three known polymorphic structure types of low-quartz, low-cristobalite and high-cristobalite 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-quartz heating transformation was found to be kinetically hindered. Isothermal in situ X-ray powder diffraction revealed a remarkable temperature dependence of this reaction kinetics.