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

Orange‐colored crystals of the oxoferrate tellurate K(12+6x )Fe(6)Te(4−x )O(27) [x=0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water–base ratio n(H(2)O)/n(KOH) of 1.5 starting from Fe(NO(3))(3) ⋅ 9H(2)O, TeO(2) and H(2)O(2) at about 200 °C. By using (NH(4))(2)TeO(4) instead of TeO(2), a fine powder consisting of microcrystalline spheres of K(12+6x )Fe(6)Te(4−x )O(27) was obtained. K(12+6x )Fe(6)Te(4−x )O(27) crystallizes in the acentric cubic space group I [Formula: see text] 3d. [Fe(III)O(5)] pyramids share their apical atoms in [Fe(2)O(9)] groups and two of their edges with [Te(VI)O(6)] 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 LiNbO(3).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+6x )Fe(6)Te(4−x )O(27) is 2.3×10(−4) S ⋅ cm(−1) at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe(2)O(9)] groups.