Cs₆[Fe₂O₆] and Rb₄[Fe₂O₅]: New oxoferrates(III) with edge sharing FeO₄ tetrahedra

Abstract

The two new oxoferrates(III) Cs₆[Fe₂O₆] and Rb₄[Fe₂O₅] were synthesized at temperatures of 500 and 600 °C via reaction of Fe₂O₃ with the elemental alkali metals (A) and their hyperoxides AO₂ in the form of dark-red crystals. Their crystal structures have both been determined by single crystal X-ray diffraction. The structure of Cs₆[Fe₂O₆] (monoclinic, P2₁/c, a = 724.6(2), b = 1212.1(5), c = 767.6(3) pm, β = 105.03(4)°, Z = 2, R1 = 0.0272, K₆[Mn₂O₆] structure type) contains isolated bi nuclear anions [O₂FeO₂FeO₂]^6- composed of two edge sharing FeO₄ tetrahedra, which are also known from the potassium and rubidium compounds A₆[Fe₂O₆] that crystallize in the klassengleiche super group C2/m. Comparable building blocks are connected via common O vertices to form chains ¹_∞[O_1/2OFeO₂FeOO_1/2]^4– in the rubidium ferrate(III) Rb₄[Fe₂O₅] (monoclinic, C2/c, a = 711.43(14), b = 1750.0(5), c = 725.3(2) pm, β = 104.72(2)°, Z = 4, R1 = 0.0611). As a consequence of the edge sharing of FeO₄ tetrahedra, both compounds show short Fe—Fe distances of 273.3(2) pm in Cs₆[Fe₂O₆] and 265.6(3) pm in Rb₄[Fe₂O₅] respectively. Based on similarities in the packing of the (Fe^III/A^I) cations, a structural relationship between the two compounds can be derived. The influence of the different counterions on the structure of the oxoferrate units in the phases of general composition A₆[Fe₂O₆] and A₄[Fe₂O₅] is discussed.