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

Orange-colored crystals of the oxoferrate tellurate K12+6x Fe6 Te4-x O27 [x=0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water-base ratio n(H2 O)/n(KOH) of 1.5 starting from Fe(NO3 )3  ⋅ 9H2 O, TeO2 and H2 O2 at about 200 °C. By using (NH4 )2 TeO4 instead of TeO2 , a fine powder consisting of microcrystalline spheres of K12+6x Fe6 Te4-x O27 was obtained. K12+6x Fe6 Te4-x O27 crystallizes in the acentric cubic space group I 4 ‾ 3d. [FeIII O5 ] pyramids share their apical atoms in [Fe2 O9 ] groups and two of their edges with [TeVI O6 ] 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 K12+6x Fe6 Te4-x O27 is 2.3×10-4 S ⋅ cm-1 at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe2 O9 ] groups.