Microstructural Changes Induced by Thermal Treatment of Cobalt(II) Hexacyanoferrate(III) Compound

Molecular-based magnet cobalt–iron cyanide, K0.2Co1.4[Fe(CN)6]·7H2O, was prepared by coprecipitation. The powder Xray diffraction indicates a FCC crystal structure with a unit cell constant of 10.25 ?. It has a transition temperature of 11 K. The effects of heat treatment on the microstructures as well as magnetic properties in the compound were studied. Several structures were encountered during the course of annealing from room temperature to 600°C. In the as-prepared (freeze–dried) sample, the majority of the structure was found to be FeIII–CN–CoII, with a minor amount of FeII–CN–CoIII structure. The amount of FeIII–CN–CoII structure decreased as a function of annealing temperature. The third structure, namely CoII–CN–FeIII began to show up after annealing the sample at 100°C. Total decomposition of the cyanide sample occurred at 500°C. A mixture of BCC Fe–Co and a significant amount of (25 wt%) amorphous carbon particles was found in the sample annealed at 600°C.