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

17 December 2018, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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.


crystal structures
Magnetic Structures


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.