Thermal-induced microstructural changes of nickel-iron cyanide

Molecular-based magnet nickel-iron cyanide, K0.8Ni1.1[Fe(CN)(6)]. 4.5H(2)O, was prepared by coprecipitation. The powder X-ray diffraction indicates an fee crystal structure with a unit cell constant of 10.4 Angstrom. Its Curi e temperature was determined to be 28 K from the Curie-Weiss law. The effec ts of heat treatment on the microstructures as well as magnetic properties in the compound were studied. Several structures were encountered during th e course of annealing from room temperature to 600 degrees C. The conversio n of the ferromagnetic Fe-III-CN-Ni-II linkage to a paramagnetic Fe-II-CN-N i-III linkage occurred after heat treatment at 150 degrees C in argon. Ther e was another cyanide structure with a tentative composition of (Ni, Fe)(CN )(2-3)O1-2, which possessed a Curie temperature of 50-60 K and is expected to be ferrimagnetic. Above 550 degrees C, the cyanide compound decomposed i nto a mixture of fee Fe-Ni and amorphous carbon. In a separate experiment, fairly pure fee Fe-Ni particles were prepared by microemulsion method follo wed by decomposition at 600 degrees C under a partial pressure of air. The ultrafine particles were found to have a mean particle size of similar to 1 0 nm, a saturation magnetization of 110 emu/g, and a coercivity of 0.3 kOe.