These are preliminary reports that have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information. For more information, please see our FAQs.
potassium ferrate V2.pdf (1.26 MB)

Hydroflux Synthesis, Chemical and Thermal Instability, Crystal and Magnetic Structures of the Oxoferrate(III, IV) K2−xFe4O7

revised on 17.12.2018, 15:09 and posted on 17.12.2018, 17:28 by Ralf Albrecht, Jens Hunger, Theresa Block, Rainer Pöttgen, Anatoliy Senyshyn, Thomas Doert, Michael Ruck
The reaction of Fe(NO3)3∙9 H2O with KOH under hydroflux
conditions at about 200 °C produces red crystals of K2−xFe4O7 in a quantitative yield. In the crystal structure, edge-sharing [FeO6]
octahedra form [ ∞Fe2O6] honeycomb nets. Pillars consisting of pairs of vertex-sharing [FeO4] tetrahedra link the honeycomb layers and form columnar halls in which the potassium ions are located. The trigonal (P3 1m) and the hexagonal (P63/mcm) polytypes of K2−xFe4O7 show oriented intergrowth. The sub-stoichiometric potassium content (x ≈ 0.3) is compensated by iron(IV), as supported by 57Fe Mößbauer spectroscopy, besides the dominating iron(III). K2−xFe4O7 is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K2CO3·H2O forms gradually on the surface of K2−xFe4O7 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 K2−xFe4O7 proceeds via a topotactic transformation into K1+x’Fe11O17, adopting the rhombohedral β’’ or the hexagonal β-aluminate-type structure, before γ-Fe2O3 is formed above 950 °C, which then converts into thermodynamically stable α-Fe2O3.


Deutsche Forschungsgemeinschaft, Collaborate Research Center 1143


Email Address of Submitting Author


Technische Universitaet Dresden



ORCID For Submitting Author


Declaration of Conflict of Interest

No conflict of interests