KGaSi2O6, an I41/a tetragonal leucite analogue with possible tetrahedral site cation ordering?

KGaSi2O6, an I41/a tetragonal leucite analogue with possible tetrahedral site cation ordering? A.M.T.Bell (Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB, UK) and C.M.B.Henderson (School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK). Leucites are tetrahedrally coordinated silicate framework structures with stoichiometries of A+2B2+Si5O12 or A+C3+Si2O6 with some of the silicon framework cations partially replaced by divalent (B) or trivalent (C) cations. A monovalent extraframework alkali metal (A) cation is also incorporated in these structures to balance the charges. Our previous work on ambient temperature structures of synthetic anhydrous leucite analogues has shown that Ia-3d cubic (e.g. Rb2ZnSi5O12 [1]) and I41/a tetragonal (e.g. KFeSi2O6 [2]) structures have disordered tetrahedrally coordinated sites (T-sites). However, our previous work on lower symmetry leucite structures has shown that Pbca orthorhombic (e.g. Cs2CdSi5O12 [3]) and P21/c monoclinic (e.g. K2MgSi5O12 [4]) structures have ordered T-sites. 41 years ago an I41/a tetragonal leucite analogue structure with stoichiometry K0.8Rb0.2GaSi2O6 was reported [5]. This structure has partial T-site cation ordering, the mean T-O distances and T-site occupancies for the three crystallographically distinct T-sites are:- T1 1.68(1)Å, 1/3Ga:2/3Si; T2 1.65(3)Å, 1/4Ga:3/4Si; T3 1.71(2)Å, 1/2Ga:1/2Si. We have recently synthesised a leucite analogue with stoichiometry KGaSi2O6. Using the Rietveld method [6] a crystal structure has been refined from X-ray powder diffraction data collected on this material. The initial Rietveld refinement used the I41/a tetragonal crystal structure of KAlSi2O6 [7] as a starting model, Ga replaced Al on the T-sites and complete T-site disorder was assumed, 3wt.% KGaSiO4 [8] was found as impurity phase. The refined crystal structure had the following mean T-O distances:- T1 1.61(2)Å, T2 1.68(2)Å, T3 1.65(2)Å; there may be some partial T-site ordering. However, no chemically sensible site occupancies could be refined so the T-site occupancies were fixed at 1/3Ga:2/3Si, any partial T-site ordering is not conclusive. [1] Bell, A.M.T. & Henderson, C.M.B. (1994). Acta Cryst. C50, 984-986. [2] Bell, A.M.T. & Henderson, C.M.B. (1994). Acta Cryst. C50, 1531-1536. [3] Bell, A.M.T. et al. (2010). Acta Cryst. B66, 51-59. [4] Bell, A.M.T. et al. (1994). Acta Cryst. B50, 560-566. [5] Klaska, R. (1978). Naturwissenschaften 65, 592-593. [6] Rietveld, H.M. (1969). J. Appl. Cryst. 2, 65-71. [7] Mazzi, F. et al. (1976). American Mineralogist 61, 108-115. [8] Barbier J. & Fleet M.E., (1987). J. Solid State Chem., 71, 361-370.