In the course of a series of KF-flux synthesis experiments aiming at the preparation of new compounds in the ternary system K2O–CaO–SiO2, of interest to e.g. the glass and fertiliser industries and to recycling of ashes of biomass combustion, a novel potassium-calcium silicate-fluoride was obtained whose composition was determined as K7Ca9[Si2O7]4F using electron-microprobe analysis. Polycrystalline material of the same compound could be also prepared by solid-state reactions. Applying direct methods it was possible to solve the crystal structure of this compound from single-crystal diffraction data. Structure solution was complicated due to complex pseudo-merohedral twinning. K7Ca9[Si2O7]4F crystallises in the monoclinic space group P21/m, with a = 5.6122(2) Å, b = 13.8143(5) Å, c = 9.8016(4) Å, β = 90.255(4)°, V = 759.90(5) Å3, Z = 1. Additional characterisation has been performed by Raman spectroscopy.
The compound belongs to the group of sorosilicates and is built up from Si2O7 groups. Potassium and calcium cations are distributed among five crystallographically independent cation positions, which are coordinated in form of distorted octahedra, trigonal prisms or more irregular polyhedra. By sharing common faces, octahedra and trigonal prisms form infinite columns running parallel to . These columns are connected to the Si2O7 groups through common corners. Additional fluorine positions are located in voids between the Si2O7 groups and the columns. A detailed comparison with structurally related compounds is given, including a group-theoretical discussion of the symmetry relationships.