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

Orange‐colored crystals of the oxoferrate tellurate K 12+6 x 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+6 x Fe 6 Te 4− x O 27 was obtained. K 12+6 x Fe 6 Te 4− x O 27 crystallizes in the acentric cubic space group I 4 ‾ 3 d . [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+6 x 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.