Laboratory synthesis and characterization of Knasibfite K₃Na₄[SiF₆]₃[BF₄] and the homologous Ge compound K₃Na₄[GeF₆]₃[BF₄]

Abstract Herein we present the synthesis, crystal structure, and the infrared and Raman spectra of K3Na4[SiF6]3[BF4]. The compound also occurs in nature as the mineral Knasibfite. We obtained it from the reaction of stoichiometric amounts of SiO2, Na[BF4], K2CO3, and Na2CO3 in hydrofluoric acid at room temperature. Hydrofluorothermal synthesis at 200 °C lead to a product of higher purity. Knasibfite is colorless and crystallizes in space group Im 2m with a = 5.546(2), b = 9.261(2), c = 17.184(4) Å, V = 882.5(4) Å3, Z = 2, at T = 400(2) K. At 342 K a phase transition to a monoclinic structure was observed (I 2, a = 5.5003(6), b = 9.0890(9), c = 17.0048(2) Å, β = 90.041(9)°, V = 852.2(2) Å3, Z = 2, T = 100(2) K). By replacing SiO2 with GeO2 as a starting material K3Na4[GeF6]3[BF4] was obtained. This compound also crystallizes in form of two polymorphs, both of which are isotypic to the respective ones of K3Na4[SiF6]3[BF4].