Syntheses and characterization of elpasolite-type ammonium alkali metal hexafluorometallates(III)

Crystal structures of three fluorides (NH4)2NaFeF6, (Fe), (NH4)2NaGaF6, (Ga), and (NH4)2NaCrF6, (Cr), as well as a substituted compound [(NH4)1−xKx]2KAlF6 (x≈0.17), (Al), have been refined using single-crystal and powder X-ray diffraction techniques. All these four ammonium hexafluorides have a cubic elpasolite-type structure and crystallize in the space group Fm3¯m with lattice constants a=8.483(3), 8.450 (3), 8.4472(2) and 8.724(3) Å for compounds (Fe), (Ga), (Cr) and (Al), respectively. The effective ionic radius of the ammonium ion calculated from those compounds has a mean value of R=1.729 Å for CN=12. An ultraviolet–visible absorption spectrum of (NH4)2NaCrF6, measured at room temperature, gives a crystal field (Dq=1575 cm−1) and Racah parameters (B=758 cm−1 and C=3374 cm−1). Abnormal anisotropic thermal parameters of fluorine atoms have been observed in the compound (Al), and interpreted to arise from four strong hydrogen bonds (F…H–N) that are distributed in a square form around each fluorine atom.