Crystallization and transformation of aluminum orthophosphates in organic solvent containing a small amount of water

Aluminum orthophosphate (AlPO4) samples were synthesized by high-temperature (200-300 degrees C) hydrolysis of a mixture of aluminum triisopropoxide and triethyl phosphate with a small amount of water dissolved in an organic solvent from the gas phase. At 250 degrees C, cristobalite-type AlPO4, which is the metastable phase at that temperature, was first crystallized from the amorphous phase and then transformed to stable quartz-type AlPO4 (berlinite) with a prolonged reaction time. When the amount of water charged into the system was increased, berlinite was formed immediately. The formation of the metastable phase could be attributed to the low solubility of AlPO4 in the organic solvent containing a small amount of water.