Crystal structures of synthetic Ge-leucite analogues.

Crystal structures of synthetic Ge-leucite analogues. A.M.T.Bell (Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB, UK). The crystal structure of the anhydrous synthetic analogue of the silicate framework mineral leucite (KAlSi2O6) is I41/a tetragonal [1], K cations sit in extraframework channels in the crystal structure. Replacement of K by Rb gives a similar I41/a tetragonal structure for RbAlSi2O6 [2]. However, replacement of K by Cs gives an Ia-3d cubic structure for CsAlSi2O6 [3]. This cubic structure has the same topology as the tetragonal leucite structures, in all these structures silicon and aluminium cations are disordered over tetrahedrally coordinated sites (T-sites). Many more leucite analogue crystal structures are known with general formulae A2BSi5O12 and ACSi2O6, where A is a monovalent extraframework cation, B is a divalent cation and C is a trivalent cation [4]. However, it is possible to make leucite analogues where Si is replaced by Ge in germanate framework crystal structures. Synthetic Ge-leucite analogues, with the general formulae AAlGe2O6 (A = K, Rb, Cs), have been studied by X-ray powder diffraction and Rietveld refinement [5]. Crystal structures are reported for these three Ge-leucite analogues [6], the A = K and Rb structures have not yet been published, the A = Cs structure is similar to a previously published structure [7]. AAlGe2O6 is isostructural with I41/a tetragonal KAlSi2O6. However, RbAlGe2O6 and CsAlGe2O6 are not isostructural with their silicate leucite analogues. Both have the I-43d cubic structure which is isostructural with that for KBSi2O6 [8] [1] Mazzi F., Galli E., Gottardi G. Am. Mineral. 1976;61:108-115. [2] Palmer D.C., Dove M.T., Ibberson R.M., et al. Am. Mineral. 1997;82:16-29. [3] Beger R.M. Z. Krist. 1969;129:280-302. [4] Bell A.M.T. submitted to Crystallography Reviews, 2023. [5] Rietveld H.M. J. Appl. Cryst. 1969;2:65-71. [6] Bell A.M.T. Powder Diffraction (in preparation) 2024. [7] Tripathi A., Parise J.B. Micro. Meso. Mat. 2002;52:65–78. [8] Millini R., Montanari L., Bellussi G. Microporous Mat. 1993;1:9-15.