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

Orange-colored crystals of the oxoferrate tellurate K_12+6xFe₆Te_4?xO₂₇ 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 K_12+6xFe₆Te_4?xO₂₇ was obtained. K_12+6xFe₆Te_4?xO₂₇ crystallizes in the acentric cubic space group I 3d. Fe^IIIO₅] pyramids share their apical atoms in Fe₂O₉] groups and two of their edges with Te^VIO₆] 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 <111> directions and cross in cavities acting as nodes. The ion conductivity of cold-pressed pellets of ball-milled K_12+6xFe₆Te_4?xO₂₇ is 2.3×10^?4 S ? cm^?1 at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the Fe₂O₉] groups.