Thermal decomposition of [Co(en)3][Fe(CN)6][bullet operator]2H2O: Topotactic dehydration process, valence and spin exchange mechanism elucidation

In this paper, the mechanism of thermal decomposition of [Co(en)3][Fe(CN)6][bullet operator]2H2O (1a) is elucidated, including the topotactic dehydration, valence and spins exchange mechanisms suggestion and the formation of a mixture of CoFe2O4-Co3O4 (3:1) as final products of thermal degradation.The course of thermal decomposition of 1a in air atmosphere up to 600[degree sign]C was monitored by TG/DSC techniques, 57Fe Mossbauer and IR spectroscopy. As first, the topotactic dehydration of 1a to the hemihydrate [Co(en)3][Fe(CN)6][bullet operator]1/2H2O (1b) occurred with preserving the single-crystal character as was confirmed by the X-ray diffraction analysis. The consequent thermal decomposition proceeded in further four stages including intermediates varying in valence and spin states of both transition metal ions in their structures, i.e. [FeII(en)2(mu-NC)CoIII(CN)4], FeIII(NH2CH2CH3)2(mu-NC)2CoII(CN)3] and FeIII[CoII(CN)5], which were suggested mainly from 57Fe Mossbauer, IR spectral and elemental analyses data. Thermal decomposition was completed at 400[degree sign]C when superparamagnetic phases of CoFe2O4 and Co3O4 in the molar ratio of 3:1 were formed. During further temperature increase (450 and 600[degree sign]C), the ongoing crystallization process gave a new ferromagnetic phase attributed to the CoFe2O4-Co3O4 nanocomposite particles. Their formation was confirmed by XRD and TEM analyses. In-field (5 K / 5 T) Mossbauer spectrum revealed canting of Fe(III) spin in almost fully inverse spinel structure of CoFe2O4.It has been found that the thermal decomposition of [Co(en)3][Fe(CN)6][bullet operator]2H2O in air atmosphere is a gradual multiple process accompanied by the formation of intermediates with different composition, stereochemistry, oxidation as well as spin states of both the central transition metals. The decomposition is finished above 400[degree sign]C and the ongoing heating to 600[degree sign]C results in the formation of CoFe2O4-Co3O4 nanocom