An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite |
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Authors: | Bull Ivor Lightfoot Philip Villaescusa Luis A Bull Lucy M Gover Richard K B Evans John S O Morris Russell E |
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Institution: | School of Chemistry, University of St. Andrews, The Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, UK. |
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Abstract: | Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Sibond]Si distances related to an increase in Sibond]Obond]Si bond angles. However, in the high-temperature phase the Sibond]Obond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Sibond]Obond]Si units. |
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