VPI-5 and AIPO,-H2 are constructed from t h e s a m e type of chain building unit which contains octahedral Al atoms (coordinated with four framework oxygens and two water molecules). The bonded water molecules are believed to stabilize the chain building unit and play an important role in the crystallization of these materials. This chain building unit limits the possibilities of incorporating silicon into t h e VPI-5 framework and synthesizing aluminosilicate or pure-silica versions of t h e

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... PI-5 structure. VPI-5 and AIPO,-H2 can irreversibly transform to AIPO,-8 and AIP0,-tridymite, respectively, via similar bond rearrangements. Several new aluminophosphates