Negative thermal expansion in the siliceous zeolites chabazite and ITQ-4: A neutron powder diffraction study

We present the results of quantitative variable-temperature neutron powder diffraction experiments performed on the OSIRIS instrument at the ISIS faci lity on the pure silica zeolites chabazite and ITQ-4. Chabazite has been fo und to be one of the most strongly contracting materials known, with a line ar expansion coefficient alpha(v)/3 varying from -0.5 x 10(-6) to -16.7 x 1 0(-6) K-1 over the temperature range 293-873 K. Full Rietveld refinement ha s been carried out using three different refinement strategies: free isotro pic refinements, refinement using geometrical restraints,and a rigid body r efinement. The free isotropic refinement was found to be the most successfu l. In agreement with previous studies on ZrW3O8 and Sc-2(WO4)(3) type mater ials, we suggest that changes in Si-O-Si interpolyhedral bond angles are th e driving force for the contraction mechanism. ITQ-4 has also been found to contract over the temperature range 95-510 K, with alpha(v)/3 varying from -2.2 x 10(-6) to -3.7 x 10(-6) K-1 over this range.