Abstract

Powder X-ray and neutron diffraction studies of the calcined siliceous zeolites ITQ-7, ITQ-9 and CIT-5, and the aluminophosphates AlPO₄-31 and Mg-doped AlPO₄-17 have been carried out as a function of temperature in order to determine their thermal expansivity properties. Of these, ITQ-7, ITQ-9 and MAPO-17 show negative thermal expansivity over a wide temperature range, in common with several other previously studied microporous materials. CIT-5 and AlPO₄-31 are anomalous in showing an expansion on heating. A detailed study of CIT-5 highlights the key structural parameters which determine its positive expansivity. From expansivity data currently available on seventeen microporous materials, it is clear that negative thermal expansivity must be considered the norm rather than the exception. Positive expansivity is encouraged by the structure having a relatively high framework density and a one-dimensional channel system.