Reactivity of FeMoO₄ in CsCl Fluxes and Formation of the Salt-Inclusion Type of Compounds

The reactivity of FeMoO₄ in CsCl fluxes has been investigated by thermal analysis and chemical reactions in evacuated silica ampules. The products have been characterized by ex situ X-ray diffraction methods. Metathesis reactions involving CsCl lead to the formation of Cs₂Fe₂(MoO₄)₃ and the salt adduct Cs₂FeCl₄·CsCl. A side reaction has been observed, which is associated with a decomposition of [MoO₄]^2- in CsCl fluxes yielding Cs₂Mo₂O₇·CsCl, which contains the rare pyromolybdate anion, [Mo₂O₇]^2-, located in the center of a _∞²[CsCl] hetero-honeycomb arrangement. This salt-inclusion type of compound has been studied further in terms of its formation starting from Cs₂MoO₄, MoO₃, and CsCl. The intermediate adduct phase, Cs₂MoO₄·MoO₃, contains uncharged _∞¹[MoO₂O_2/2] chains that react with CsCl at elevated temperatures to Cs₂Mo₂O₇·CsCl. Furthermore, the site preference for alkaline-metal cations (K⁺, Rb⁺, and Cs⁺) has been evaluated for a mixed substitution series. In accordance with the Pearson concept, the polarizability of the respect cation outweighs any size differences for the occupancy of the salt-intergrowth motif, the honeycomb part of the structure.