An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite.

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 Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.