Phase Relations in the System SiO2-AlPO4 up to 400°C

Abstract

The phase relationships along binary join of SiO2-AlPO4 were investigated up to 400°C using several starting materials made by a partial solution route. Precursors used were boehmite (AlOOH), H3PO4, non crystalline silica (Ludox, Cab-O-Sil), and quartz. Studies up to 400°C showed that SiO2, AlPO4, and its hydrate were the only crystalline and non-crystalline phases present along the binary join, and no substantial crystalline solution of any ternary phase was observed. Three polymorphic forms of AlPO4, namely, berlinite, tridymite form, and cristobalite form, coexisted at as low as 200°C. The nature of the silica precursors greatly influenced the development of the polymorphic phases of AlPO4. The low quartz precursor suppressed the formation of the cristobalite form of AlPO4 and favored berlinite production. On the other hand, non crystalline silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhanced that of the cristobalite form of AlPO4. The silica precursors acted as structural seeds for the growth of AlPO4. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AlPO4 phases on the surface of SiO2 particles even in these low temperature reactions. The influence of other precursors and route during the syntheses of AlPO4 and other phases in the SiO2-AlPO4 are discussed.