The widespread occurrence of negative thermal expansion in zeolites

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.