Abstract
Crystal structures of three fluorides (NH 4) 2NaFeF 6, ( Fe), (NH 4) 2NaGaF 6, ( Ga), and (NH 4) 2NaCrF 6, ( Cr), as well as a substituted compound [(NH 4) 1− x K x ] 2KAlF 6 ( 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 (NH 4) 2NaCrF 6, 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.
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