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CanpuIational techniques predict thal several microprous zeolitic srmctures have unusual negative ccefhcienfs of lhennal expansion. High-resolution powder diffraction techniques confirm the qualitative a s p & of these simulations.Zeolites are an important class of framework-structured microporous solids which have attracted widespread interest because of their intriguing and varied crystal architectures and their widespread applications as heterogeneous catalysts in cracking, isomerization and other reactions. Typical framework structures are shown in figure 1. The materials are normally aluminosilicates with the negative charge of the framework being compensated by extra framework cations and/or protons present as bridging OH groups. Purely siliceous samples (i.e. Si02 polymorphs) can, however, be prepared for several zeolitic structures.