K5[FeO4] and K17[Fe5O16]:: Two new potassium oxoferrates(III)

The title compounds were synthesized from potassium rich mixtures of Fe2O3, elemental potassium and the hyperoxide KO2 by applying short reaction times, a maximum temperature of 875 K and subsequent quenching of the samples. The structures of the two new oxoferrates(III) have been determined by single crystal X-ray diffraction. The orthoferrate(III) K-5 [FeO4] (Na-5 [GaO4] structure type, space group Pbca, a = 1124,0(2), b = 667,95(9), c = 2034,8(3) pm, Z = 4, R1 = 0,0585) exhibits isolated ortho-anions [FeO4](5-) with nearly ideal tetrahedral geometry and Fe-O distance in the narrow range of 189 to 192 pm. The pentaferrate K-17[Fe5O16] (space group CM, a = 671,71(5), b = 3560,8(3), c = 670,81(5) pm, 0 = 119,687(5)degrees,Z = 2,R1 = 0,0291) crystallizes with a new structure type. Its building units are isolated novel penta-nuclear anions composed of five corner sharing [FeO4] tetrahedra. These linear chain pieces [Fe5O16] are arranged in a hexagonal rod packing, with a stacking sequence according to vertical bar:AB:vertical bar along the large monoclinic b axis. The structure is thus related to that of the tetra-ferrate K-14[Fe4O13] with a comparable packing of tetra-nuclear ferrate(III) anions.