Crystal structure of the Fe-member of usovite

Abstract

Crystals of the title compound, with the idealized composition Ba2CaFeAl2F14, dibarium calcium iron(II) dialuminium tetradecafluoride, were obtained serendipitously by reacting a mixture of the binary fluorides BaF2, CaF2and AlF3in a leaky steel reactor. The compound crystallizes in the usovite structure type (Ba2CaMgAl2F14), with Fe2+cations replacing the Mg2+cations. The principal building units are distorted [CaF8] square-antiprisms (point group symmetry 2), [FeF6] octahedra (point group symmetry -1) and [AlF6] octahedra that are condensed into undulating2∞[CaFeAl2F14]4−layers parallel (100). The Ba2+cations separate the layers and exhibit a coordination number of 12. Two crystal structure models with a different treatment of the disordered Fe site [mixed Fe/Ca occupation, model (I),versusunderoccupation of Fe, model (II)], are discussed, leading to different refined formulae Ba2Ca1.310 (15)Fe0.690 (15)Al2F14[model (I)] and Ba2CaFe0.90 (1)Al2F14[model (II)].