Oxo-Hydroxoferrate K2- x Fe4O7- x (OH) x : Hydroflux Synthesis, Chemical and Thermal Instability, Crystal and Magnetic Structures

Abstract

The reaction of Fe(NO₃)₃⋅9 H₂O with KOH under hydroflux conditions at about 200 °C produces red crystals of K_2-x Fe₄O_7-x (OH) _x in a quantitative yield. In the crystal structure, edge-sharing [FeO₆] octahedra form ${}_{\infty }{}^2[$ 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 (P $\bar{3}$ 1m) and the hexagonal (P6₃/mcm) polytypes of K_2-x Fe₄O_7-x (OH) _x show oriented intergrowth. The sub-stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K_2-x Fe₄O_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 Fe₄O_7-x (OH) _x hydrolyzes and K₂CO₃ ⋅ H₂O forms gradually on the surface of the K_2-x Fe₄O_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 Fe₄O_7-x (OH) _x proceeds via a topotactic transformation into K_1+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.

Keywords: antiferromagnet; crystal structure; magnetic properties; oxo-hydroxoferrate; topochemistry.