Structural characterization of new hydridofluorides KcaH_(3-x)F_x with x = 1; 1.5; 2; 2.5

摘要

The direct reaction of hydrogen on a mixture KF, CaH_2 and CaF_2 in stoichiometric proportions allowed to synthesize a series of discrete phases KcaH_(3-x)F_x with x = 1, 1.5, 2 and 2.5. All phases were indexed with an orthorhombic unit cell [e.g.: a=623.47 (5), b = 881.05(8), c = 620.83(6) pm for KcaHF_2]. Their crystal structures are isotypic with KcaF_3 and have been refined by a X-ray Rietvely study. They derive from perovskite type by a Pm-3m -> Pnma distortion. Contrary to the homologous NaMgH_2F in which H~- and F~- anions are equally distributed in a disordered way in the two anionic sites 4c and 8d, in the KcaH_(3-x)F_x series, the hydride ions preferentially occupy the 8d site and begin to occupy the 4c site only for the two compositions richer in hydrogen.