XRD and DSC study of the formation and the melting of a new zeolite like borosilicate CsBSi5O12 and (Cs,Rb)BSi5O12 solid solutions

Polycrystalline CsBSi5O12 was prepared from a stoichiometric mixture by solid-state reaction above 1000 C. The solid solutions Cs1-xRbxBSi5O12 were obtained at 1000 C during a long heat treatment of polycrystalline Cs1-xRbxBSi2O6 boropollucites (xRb = 0, 0.05, 0.2, 0.4). A new borosilicate compound and its solid solutions were studied using X-ray powder diffraction (XRD), annealing, differential scanning calorimetry (DSC), and thermogravimetry (TG). For Cs,Rb-boropollucites the new phase formation is accompanied by significant mass losses detected by DSC and TG. The following mechanism of phase transformations is assumed: (Cs,Rb)BSi2O6 → (Cs,Rb)BSi5O12 + (Cs,Rb)BO2 ↑. The zeolite phase forms as a result of the boropollucite decomposition over 1000 C. Zeolite decomposes also on further heating and the SiO2 reflections are observed in the XRD pattern only. Thus above 1000 C both boropollucite and zeolite phases are unstable presumably due to the ability of the alkali cations to leave the structure. Using XRD the unit cell parameters of CsBSi5O12 have been determined in the orthorhombic crystal system: a = 16.242(4) A, b = 13.360(4) A, c = 4.874(1) A. The compound is isostructural with the zeolite compound CsAlSi5O12. In the crystal structure of Cs1-xRbxBSi5O12 solid solutions the changes of cell parameters are insignificant under the substitution of Cs by Rb atoms that indicates a very limited substitution range. (orig.)