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

摘要

Crystal structures of three fluorides (NH4)(2)NaFeF6, (Fe), (NH4)(2)NaGaF6, (Ga), and (NH4)(2)NaCrF6, (Cr), as well as a substituted Compound vertical bar(NH4)(1-x)K-x vertical bar 2KAlF6 (x approximate to 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 Fm (3) over barm with lattice constants a = 8.483(3), 8.450 (3), 8.4472(2) and 8.724(3)angstrom 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 l? = 1.729 A for CN = 12. An ultraviolet-visible absorption spectrum of (NH4)(2)NaCrF6, 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 (AI), and interpreted to arise from four strong hydrogen bonds (F...H-N) that are distributed in a square form around each fluorine atom. (C) 2008 Elsevier Inc. All rights reserved.