Open Access Research article

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

Zdeněk Trávníček1*, Radek Zbořil2, Miroslava Matiková-Maľarová1, Bohuslav Drahoš1 and Juraj Černák3

Author Affiliations

1 Regional Centre of Advanced Technologies and Materials & Department of Inorganic Chemistry, Palacký University, Tř, 17. listopadu 12, Olomouc, CZ-77146, Czech Republic

2 Regional Centre of Advanced Technologies and Materials & Department of Physical Chemistry, Palacký University, Tř, 17. listopadu 12, Olomouc, CZ-77146, Czech Republic

3 Department of Inorganic Chemistry, Institute of Chemistry, Faculty of Sciences, P.J. Šafárik University in Košice, Moyzesova 11, Košice, SK-041 54, Slovakia

For all author emails, please log on.

Chemistry Central Journal 2013, 7:28  doi:10.1186/1752-153X-7-28

Published: 8 February 2013

Abstract

Background

The Prussian blue analogues represent well-known and extensively studied group of coordination species which has many remarkable applications due to their ion-exchange, electron transfer or magnetic properties. Among them, Co-Fe Prussian blue analogues have been extensively studied due to the photoinduced magnetization. Surprisingly, their suitability as precursors for solid-state synthesis of magnetic nanoparticles is almost unexplored.

In this paper, the mechanism of thermal decomposition of [Co(en)3][Fe(CN)6] ∙∙ 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.

Results

The course of thermal decomposition of 1a in air atmosphere up to 600°C was monitored by TG/DSC techniques, 57Fe Mössbauer and IR spectroscopy. As first, the topotactic dehydration of 1a to the hemihydrate [Co(en)3][Fe(CN)6] ∙∙ 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(μ-NC)CoIII(CN)4], FeIII(NH2CH2CH3)2(μ-NC)2CoII(CN)3] and FeIII[CoII(CN)5], which were suggested mainly from 57Fe Mössbauer, IR spectral and elemental analyses data. Thermal decomposition was completed at 400°C when superparamagnetic phases of CoFe2O4 and Co3O4 in the molar ratio of 3:1 were formed. During further temperature increase (450 and 600°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) Mössbauer spectrum revealed canting of Fe(III) spin in almost fully inverse spinel structure of CoFe2O4.

Conclusions

It has been found that the thermal decomposition of [Co(en)3][Fe(CN)6] ∙∙ 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°C and the ongoing heating to 600°C results in the formation of CoFe2O4-Co3O4 nanocomposite particles as the final decomposition product.

Keywords:
Hexacyanidoferrate; Crystal structure; Thermal behavior; Mössbauer spectroscopy; Topotactic dehydration; Nanocomposite particles; CoFe2O4

Graphical abstract