Thermal behaviour of siliceous faujasite: Further structural interpretation of negative thermal expansion

• Si-Y thermal behaviour was studied between 298 and 1123 K by in situ synchrotron XRPD.
• A strong negative thermal expansion is confirmed up to about 923 K.
• Above 923 K, positive thermal expansion is observed.
• The main structural changes undergone by Si-Y during heating involve the D6R units.

The high-temperature behaviour of siliceous faujasite (Si-Y) was investigated by in situ synchrotron X-ray powder diffraction from room temperature up to 1123 K. This porous phase is remarkably stable when heated, and no phase transitions or changes in symmetry are observed. A marked negative thermal expansion (NTE), already reported in literature for a heating range from 25 to 573 K, was confirmed up to about 923 K. Above this temperature a positive thermal expansion was observed. Si-Y’s thermal behaviour was interpreted on the basis of the transverse thermal vibrations of the oxygen atoms involved in the T–O–T linkages and a series of other structural features characterizing the faujasite structure, namely the T–T distances between adjacent tetrahedral sites, the thickness of the double 6-membered rings, and the ditrigonal index of the 6-membered rings. Moreover, the thermal behaviour of several other anhydrous porous materials with NTE behaviour is discussed and compared to that of Si-Y.

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