Volume 225, Issue 10 (October 2010)
Hannes Krüger (2010). Ca5.45Li3.55[SiO4]3O0.45F1.55 and Ca7K[SiO4]3F3: single-crystal synthesis and structures of two trigonal oxyfluorides. Zeitschrift für Kristallographie: Vol. 225, No. 10, pp. 418-424.
doi: 10.1524/zkri.2010.1268

Ca5.45Li3.55[SiO4]3O0.45F1.55 and Ca7K[SiO4]3F3: single-crystal synthesis and structures of two trigonal oxyfluorides

Hannes Krüger*

* Correspondence address: Institut für Mineralogie und Petrographie, Innrain 52, 6020 Innsbruck, Österreich,
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Keywords: X-ray diffraction, Single crystal structure analysis, Twinning, Bond valence sum calculations, Alkaline-earth silicate oxyfluorides



Abstract

Single crystals of the two title compounds have been synthesised utilising fluoride-based fluxes. X-ray diffraction (XRD) revealed that Ca5.45Li3.55[SiO4]3O0.45F1.55 crystallises in space group R-3m (a = 7.1365(6), c = 41.459(3)Å) and Ca7K[SiO4]3F3 exhibits space group P31m (a = 10.8161(5), b = 6.6469(3)Å). 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.