Oxo‐Hydroxoferrate K(2−x)Fe(4)O(7−x)(OH)(x): Hydroflux Synthesis, Chemical and Thermal Instability, Crystal and Magnetic Structures

The reaction of Fe(NO(3))(3)⋅9 H(2)O with KOH under hydroflux conditions at about 200 °C produces red crystals of K(2−x)Fe(4)O(7−x)(OH)(x) in a quantitative yield. In the crystal structure, edge‐sharing [FeO(6)] octahedra form [Formula: see text] Fe(2)O(6)] honeycomb nets. Pillars consisting of pairs of vertex‐sharing [FeO(4)] tetrahedra link the honeycomb layers and form columnar halls in which the potassium ions are located. The trigonal (P [Formula: see text] 1m) and the hexagonal (P6(3)/mcm) polytypes of K(2−x)Fe(4)O(7−x)(OH)(x) show oriented intergrowth. The sub‐stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K(2−x)Fe(4)O(7−x)(OH)(x) is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K(2−x)Fe(4)O(7−x)(OH)(x) hydrolyzes and K(2)CO(3) ⋅ H(2)O forms gradually on the surface of the K(2−x)Fe(4)O(7−x)(OH)(x) crystals. Upon annealing at air at about 500 °C, the potassium atoms in the columnar halls start to order into a superstructure. The thermal decomposition of K(2−x)Fe(4)O(7−x)(OH)(x) proceeds via a topotactic transformation into K(1+x′)Fe(11)O(17), adopting the rhombohedral β’’ or the hexagonal β‐aluminate‐type structure, before γ‐Fe(2)O(3) is formed above 950 °C, which then converts into thermodynamically stable α‐Fe(2)O(3).