Structural Control of Phase Formation in Low‐Tem perature AIPO4—Sio2 Reactions

Abstract

The phase relation along the binary join of AIPO4‐SiO2 were investigated up to 400°C using starting materials made by a solution route. Precursor structures used were boehmite (AIOOH), H3PO4, noncrystalline silica, and quartz. The silica precursors acted as structural seeds for the epitaxial growth of AIPO4. Studies showed that SiO2 and AIPO4 were the only crystalline and noncrystalline phases present along the binary join, and no substantial crystalline solution or any ternary phase was observed. Three polymorphic forms of AIPO4, i.e., berlinite, tridymite, and cristobalite, coexisted as low as 200°C. The nature of the silica precursor greatly influenced the development of the polymorphic phases of AIPO4. The low‐quartz precursor suppressed the formation of the cristobalite form of AIPO4 and favored berlinite (AIPO4 quartz) production. On the other hand, noncrystallin silica with a cristobalite‐like broad XRD peak suppressed the formation of berlinite and enhaned that of the cristobalite form of AIPO4. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AIPO4 phases on the surface of SiO2 particles even in these low‐temperature reactions.