Ca_(5.45)Li_(3.55)[SiO_4]_3O_(0.45)F_(1.55) and Ca_7K[SiO_4]_3F_3: single-crystal synthesis and structures of two trigonal oxyfluorides

摘要

Single crystals of the two title compounds have been synthesised utilising fluoride-based fluxes. X-ray diffraction (XRD) revealed that Ca_(5.45)Li_(3.55)[SiO_4]_3O_(0.45) F_(1.55) crystallises in space group R3m (a = 7. 1365(6), c = 41.459(3)A) and Ca_7K[SiO_4]_3F_3 exhibits space group P31m (a = 10.8161(5), b = 6.6469(3) A). Both compounds show a non-stoichiometric composition, related to a coupled substitution of mono- and divalent cations compensated by the oxygen–fluorine contents. The Ca/Li ratio in the first compound is derived from the structural refinement, as well as from bond-valence sum ( BVS) calculations. Electron microprobe analysis was performed to reveal the K/Ca ratio in the second structure, which shows a slight enrichment in potassium compared to the ideal stoichiometry. The distribution of K and Ca over two cation sites was studied with BVS calculations. Both structures belong to the group of nesosilicates ( monosilicates) with additional anions. Furthermore, the structures are discussed in the context of related materials.