High temperature phase transitions in synthetic RbGaSi2O6 and RbFeSi2O6 leucite analogues

High temperature phase transitions in synthetic RbGaSi2O6 and RbFeSi2O6 leucite analogues. A.M.T.Bell (Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB, UK). Leucite (KAlSi2O6) [1] is a tetrahedrally coordinated silicate framework mineral. Synthetic analogues of leucite can be synthesised with stoichiometries of A+2B2+Si5O12 or A+C3+Si2O6, 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. Ambient temperature structures of synthetic anhydrous leucite analogues (where A = K or Rb and C = Al, Ga or Fe3+) all have I41/a tetragonal structures [1-5] with disordered tetrahedrally coordinated sites (T-sites). On heating these tetragonal leucites can undergo phase transitions to Ia-3d cubic. Phase transitions have been reported for KCSi2O6 [2, 4] and RbAlSi2O6 [4]. Two more I41/a tetragonal to Ia-3d cubic phase transitions are reported after high temperature X-ray powder diffraction studies on RbGaSi2O6 (773K) and RbFeSi2O6 (673K). [1] Mazzi, F., et al. (1976). American Mineralogist, 61, 108-115. [2] Bell, A.M.T. & Henderson, C.M.B. (2020). Journal of Solid State Chemistry, 284, 121142. [3] Bell, A.M.T. & Henderson, C.M.B. (1994). Acta Cryst., C50, 1531-1536. [4] Palmer, D.C., et al. (1997). American Mineralogist, 82, 16-29. [5] Bell, A.M.T. & Stone A.H. (2021). Powder Diffraction, 36(4), 273–281.