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

Abstract

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 (αa and αb) and negative (αc) coefficients than those before calcination. This indicates a greater structural flexibility once free pore vo...