Potassium Ion Conductivity in the Cubic Labyrinth of a Piezoelectric, Antiferromagnetic Oxoferrate(III) Tellurate(VI)

Orange‐colored crystals of the oxoferrate tellurate K12+6x Fe6Te4−x O27 [x=0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water–base ratio n(H2O)/n(KOH) of 1.5 starting from Fe(NO3)3 ⋅ 9H2O, TeO2 and H2O2 at about 200 °C. By using (NH4)2TeO4 instead of TeO2, a fine powder consisting of microcrystalline spheres of K12+6x Fe6Te4−x O27 was obtained. K12+6x Fe6Te4−x O27 crystallizes in the acentric cubic space group I 4‾ 3d. [FeIIIO5] pyramids share their apical atoms in [Fe2O9] groups and two of their edges with [TeVIO6] octahedra to form an open framework that consists of two loosely connected, but not interpenetrating, chiral networks. The flexibility of the hinged oxometalate network manifests in a piezoelectric response similar to that of LiNbO3.The potassium cations are mobile in channels that run along the directions and cross in cavities acting as nodes. The ion conductivity of cold‐pressed pellets of ball‐milled K12+6x Fe6Te4−x O27 is 2.3×10−4 S ⋅ cm−1 at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe2O9] groups.
K(nockout) response: K12+6x Fe6Te4−x O27 was synthesized under hydroflux conditions. The acentric cubic crystal structure comprises two non‐interpenetrating chiral oxometalate networks formed by edge‐ or corner‐sharing [TeVIO6] and [FeIIIO5] polyhedra. The potassium ions are mobile in the labyrinth between the networks. The magnetic moments of the iron cations are pairwise antiparallel, and the compound exhibits a strong piezoelectric response.