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

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.