Negative Thermal Expansion in the Siliceous Zeolites Chabazite and ITQ-4:  A Neutron Powder Diffraction Study

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We present the results of quantitative variable-temperature neutron powder diffraction experiments performed on the OSIRIS instrument at the ISIS facility on the pure silica zeolites chabazite and ITQ-4. Chabazite has been found to be one of the most strongly contracting materials known, with a linear expansion coefficient α v /3 varying from −0.5 × 10 - 6 to −16.7 × 10 - 6 K - 1 over the temperature range 293−873 K. Full Rietveld refinement has been carried out using three different refinement strategies: free isotropic refinements, refinement using geometrical restraints, and a rigid body refinement. The free isotropic refinement was found to be the most successful. In agreement with previous studies on ZrW 2 O 8 and Sc 2 (WO 4 ) 3 type materials, we suggest that changes in Si−O−Si interpolyhedral bond angles are the driving force for the contraction mechanism. ITQ-4 has also been found to contract over the temperature range 95−510 K, with α v /3 varying from −2.2 × 10 - 6 to −3.7 × 10 - 6 K - 1 over this range.