Crystal structure and thermal behavior of a new borosilicate with the CAS framework type

The crystal structure of the new microporous framework borosilicate |Cs4|[B4Si20O48]-CAS has been studied using X-ray diffraction data on powders prepared by solid state reaction as well as by glass crystallization. It is isostructural with |Cs4|[Al4Si20O48]-CAS and belongs to the zeolite framework type CAS. Non-random distribution of the Si and B atoms over the tetrahedral positions has been revealed.The behavior of |Cs4|[B4Si20O48]-CAS on heating was studied using high-temperature X-ray diffraction HTXRD, differential thermal analysis (DTA) and annealing series. According to HTXRD data the thermal deformation of |Cs4|[B4Si20O48]-CAS has anisotropic character with the maximum expansion along the infinite channels, where the Cs atoms are located.The DTA and heat treatment studies showed that during the solid state reaction the first phase to crystallize at about 800 °C is the boropollucite CsBSi2O6, which beyond 900 °C transforms into |Cs4|[B4Si20O48]. Around 1100 °C a final decomposition occurs according to the reaction |Cs4|[B4Si20O48] → 20SiO2 + 4CsBO2↑.