Abstract
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 4‾ 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.