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Magnetically Induced Rotating Rayleigh-Taylor Instability
Published on: March 3, 2017
Oxo-Hydroxoferrate K2- Fe4O7- (OH) : Hydroflux Synthesis, Chemical and Thermal Instability, Crystal and Magnetic
Ralf Albrecht1, Jens Hunger1, Theresa Block2
1Faculty of Chemistry and Food Chemistry TU Dresden 01062 Dresden Germany.
Abstract:
The reaction of Fe(NO3)3⋅9 H2O with KOH under hydroflux conditions at about 200 °C produces red crystals of K2- Fe4O7- (OH) 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 (P 1m) and the hexagonal (P63/mcm) polytypes of K2- Fe4O7- (OH) show oriented intergrowth. The sub-stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K2- Fe4O7- (OH) is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K2- Fe4O7- (OH) hydrolyzes and K2CO3 ⋅ H2O forms gradually on the surface of the K2- Fe4O7- (OH) 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- Fe4O7- (OH) proceeds via a topotactic transformation into K1+ 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.
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