Oxo‐Hydroxoferrate K_2−xFe₄O_7−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 K2−xFe4O7−x(OH)x in a quantitative yield. In the crystal structure, edge‐sharing [FeO6] octahedra form ∞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 true3‾ 1m) and the hexagonal (P63/mcm) polytypes of K2−xFe4O7−x(OH)x show oriented intergrowth. The sub‐stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K2−xFe4O7−x(OH)x is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K2−xFe4O7−x(OH)x hydrolyzes and K2CO3 ⋅ H2O forms gradually on the surface of the K2−xFe4O7−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 K2−xFe4O7−x(OH)x 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.

Antiferromagnetic Alkali Metal Oxohydroxoferrates(III) with Correlated Hydrogen Bonding Systems

“…In comparison with Mössbauer spectroscopic data of other iron oxides and hydroxides, the isomer shift is in good agreement with iron(III) in tetrahedral coordination . The 6 K spectrum shows full magnetic hyperfine field splitting with B Hf =47.5(1) T, similar to K 2−x [Fe 4 O 7−x (OH) x ] . For the spectrum at room temperature we observe similar full magnetic hyperfine field splitting but with a reduced hyperfine field of B Hf =36.8(1) T. The temperature dependence of the hyperfine field usually follows a Brillouin function.…”

“…The base concentration q ( A ) of the hydroflux turned out to be the most important factor for the synthesis of the oxohydroxoferrates. With KOH, four phases were found in different concentration ranges: α ‐Fe 2 O 3 , K 2−x [Fe 4 O 7−x (OH) x ], K , and KFeO 2 (Figure ). q (K)≤0.5 results in phase pure α ‐Fe 2 O 3 in the form of dark‐red hexagonal crystals (Figure S1, Supporting Information).…”

Tunable Potassium Ion Conductivity and Magnetism in Substituted Layered Ferrates

“…This kind of stacking creates an oxoferrate framework with large interlayer voids where the potassium cations reside. The sub‐stoichiometric potassium content is compensated by hydroxide groups, so that every iron in K 2–x Fe 4 O 7–x (OH) x is trivalent as confirmed by Mößbauer spectroscopy [1] . The [FeO 4 ] tetrahedra pairs can be arranged in a staggered or an ecliptic configuration, giving rise to different polytypes [1] .…”

“…Recently, we synthesized the potassium oxohydroxoferrate K 2–x Fe 4 O 7–x (OH) x ( x ≈ 0.3) [1] with the hydroflux method, which uses a ultrabasic reaction medium consisting of an approximately equimolar mixture of alkali metal hydroxide and water [2] . In the specific case, we used potassium hydroxide for the hydroflux and Fe(NO 3 ) 3 ⋅ 9H 2 O as iron source.…”

Hydroflux Synthesis and Characterization of the Non‐Centrosymmetric Oxomanganate(V) KSrMnO4

“…Only a few examples are known, for instance the redox system of copper compounds in a sodium and/or potassium hydroxide hydroflux or for the synthesis of potassium oxohydroxoferrates, in which the base concentration is the most important parameter. [10,25,26] Crystal Structure X-ray diffraction on a bluish-green single-crystal revealed the composition KSrMnO 4 and an orthorhombic structure in the space group P2 1 2 1 2 1 (no. 19) with the lattice parameters a = 743.26(7) pm, b = 575.72(6) pm and c = 987.2(1) pm at 100(1) K (Figure 2; Tables S1-S4, Supporting Information).…”