Temperature-dependent structural study of microporous CsAlSi5O12

Abstract: CsAlSi5O12 crystals were synthesized at high temperature by slow cooling of a vanadium oxide flux. Single-crystal X-ray diffraction structure analysis and electron microprobe analyses yielded the microporous CAS zeolite framework structure of Cs0.85Al0.85Si5.15O12 composition. High-temperature single-crystal and powder X-ray diffraction studies were utilized to analyze anisotropic thermal expansion. Rietveld refined cell constants from powder diffraction data, measured in steps of 25°C up to 700°C, show a significant decrease in expansion above 500°C. At 500°C, a displacive, static disorder–dynamic disorder-type phase transition from the acentric low-temperature space group Ama2 to centrosymmetric Amam (Cmcm in standard setting) was found. Thermal expansion below the phase transition is governed by rigid-body TO4 rotations accompanied by stretching of T–O–T angles. Above the phase transition at 500°C all atoms, except one oxygen (O6), are fixed on mirror planes. Temperature-dependent polarized Raman single-crystal spectra between −270 and 300°C and unpolarized spectra between room temperature and 1000°C become increasingly less resolved with rising temperature confirming the disordered static–disordered dynamic type of the phase transition. [Copyright &y& Elsevier]