CsAlSi 5 O 12 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 Cs 0.85 Al 0.85 Si 5.15 O 12 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 1C up to 700 1C, show a significant decrease in expansion above 500 1C. At 500 1C, 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 TO 4 rotations accompanied by stretching of T-O-T angles. Above the phase transition at 500 1C all atoms, except one oxygen (O6), are fixed on mirror planes. Temperature-dependent polarized Raman single-crystal spectra between Γ270 and 300 1C and unpolarized spectra between room temperature and 1000 1C become increasingly less resolved with rising temperature confirming the disordered static-disordered dynamic type of the phase transition.