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potassium ferrate V2.pdf (1.26 MB)

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

preprint
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.

Funding

Deutsche Forschungsgemeinschaft, Collaborate Research Center 1143

History

Email Address of Submitting Author

thomas.doert@tu-dresden.de

Institution

Technische Universitaet Dresden

Country

Germany

ORCID For Submitting Author

0000-0001-7523-9313

Declaration of Conflict of Interest

No conflict of interests

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