Anisotropic Thermal Expansion of SiO2 and AlPO4 Clathrasils with the AST-Type Structure

The thermal expansion behavior of silica and aluminophosphate clathrasil materials that adopt the AST-type structure has been studied in both as-made and calcined forms using X-ray diffraction (single crystal or powder) over the temperature range 100-400 K The four materials studied all adopt body-centered tetragonal variants of the ideal cubic AST-type structure. In the as-made form, where quinuclidinum cations fill the larger cages and fluoride anions occupy interstitial sites at the center of double four-ring units, both materials have net positive volume thermal expansion coefficients, larger than dense forms of SiO2, but both show highly anisotropic thermal expansivity with positive expansion in the ab plane and negative expansion parallel to c The anisotropy becomes more pronounced upon removal of the extra-framework species, with larger magnitudes of positive (alpha(a) and alpha(b)) and negative (alpha(c)) coefficients than those before calcination This indicates a greater structural flexibility once free pole volume is available, although still net positive thermal expansion is seen In the case of as-made SiO2-AST (that contains template and fluoride Ions) and the fully calcined AlPO4 analogue, full structure refinements as a function of temperature have been performed, this reveals that whether or not template and fluoride are present. the double-foul-ring composite building units show a volume contraction with temperature, whereas an expansion of linking T-O-T bonds is seen These opposing effects are responsible for the anisotropy in thermal expansion behavior