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

Abstract

The reactivity of FeMoO 4 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 Cs 2 Fe 2 (MoO 4 ) 3 and the salt adduct Cs 2 FeCl 4 •CsCl. A side reaction has been observed, which is associated with a decomposition of [MoO 4 ] 2− in CsCl fluxes yielding Cs 2 Mo 2 O 7 •CsCl, which contains the rare pyromolybdate anion, [Mo 2 O 7 ] 2− , located in the center of a ∞ 2 [CsCl] heterohoneycomb arrangement. This salt-inclusion type of compound has been studied further in terms of its formation starting from Cs 2 MoO 4 , MoO 3 , and CsCl. The intermediate adduct phase, Cs 2 MoO 4 •MoO 3 , contains uncharged ∞ 1 [MoO 2 O 2/2 ] chains that react with CsCl at elevated temperatures to Cs 2 Mo 2 O 7 •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.

Configuration of the dimolybdate in salt inclusion type of compounds, Cs2Mo2O7 ⋅ CsX (X=Cl, Br, and I)

“…While the apical Cs1 cations restrict the rotation of the dimolybdate towards a staggered conformation, the electrostatic interaction between O2 and the hetero-honeycomb forming ions may allow for a distortion towards bending. Here, we focus on the halides with different sizes (MEFIR-MEan Fictive Ionic Radii: [21] r Cl = 173 pm, [15] r Br = 185 pm, r I = 202 pm) and Pearson hardness. [22] In other words, the question arises how restrictive is the honeycomb lattice with respect to distortion and consequently a structural transition.…”

“…These contain the $${_\infty ^2 \{ {\rm{AX}}\} }$$ hetero‐honeycomb layers which restrict the conformation of the dimolybdate, namely eclipsed and linear at room temperature. Members of this family of compounds, A(1) 2 Mo 2 O 7 ⋅ A(2)Cl, with A(1)=Cs, Rb and A(2)=Cs, Rb, and K, were investigated in terms of cation sizes and Pearson hardness [15] . Previously, single crystal structures of Cs 2 Mo 2 O 7 ⋅ CsBr [16] and K 2 Mo 2 O 7 ⋅ KBr [17] have been reported and for the latter the Raman and IR bands of the dimolybdate were assigned based on D 3 h symmetry.…”

“…Members of this family of compounds, A(1) 2 Mo 2 O 7 • A(2)Cl, with A(1) = Cs, Rb and A(2) = Cs, Rb, and K, were investigated in terms of cation sizes and Pearson hardness. [15] Previously, single crystal structures of Cs 2 Mo 2 O 7 • CsBr [16] and K 2 Mo 2 O 7 • KBr [17] have been reported and for the latter the Raman and IR bands of the dimolybdate were assigned based on D 3h symmetry.…”