Elsevier

Journal of Molecular Structure

Volume 1015, 16 May 2012, Pages 112-117
Journal of Molecular Structure

Y[Fe1−xCox(CN)6]·4H2O (0  x  1) solid solutions: Synthesis, crystal structure, thermal decomposition and spectroscopic and magnetic properties

https://doi.org/10.1016/j.molstruc.2012.02.023Get rights and content

Abstract

The series of solid solutions Y[Fe1−xCox(CN)6]·4H2O (0  x  1) were prepared and characterized by means of powder X-ray diffraction (PXRD), Infrared and Mössbauer spectroscopy and magnetic measurements. The thermal decomposition process has been followed by thermogravimetric and differential thermal analysis (TGA–DTA). The crystal structure of the complexes was refined by means of Rietveld analysis. The Y[Fe1−xCox(CN)6]·4H2O complexes crystallize in the orthorhombic crystal system, space group Cmcm. The Y3+ ion is eight-coordinated forming a bicapped distorted trigonal prism YN6O2. The Fe3+ and Co3+ ions are six-coordinated in the form of an irregular octahedra (Fe,Co)C6 group and cyanide linkages between YN6O2 and (Fe,Co)C6 groups build an infinite polymeric array.

The Mössbauer spectra of all solid solutions Y[Fe1−xCox(CN)6]·4H2O present quadrupolar splittings with negative isomer shifts values due to the existence of a strong π back-bonding effect from the Fe3+ ion towards the CN ligands.

All compounds follow the Curie–Weiss law showing anti-ferromagnetic interactions at very low temperatures.

Highlights

► The series of solid solutions Y[Fe1−xCox(CN)6]·4H2O were prepared and characterized. ► The crystal structures were refined using Rietveld analysis with PXRD data. ► Y[Fe1−xCox(CN)6]·4H2O complexes crystallize in the orthorhombic crystal system, space group Cmcm. ► Magnetic measurements reveal that all the complexes show anti-ferromagnetic interactions at low temperatures. ► According to thermal analysis, YFe1−xCoxO3 oxides with perovskite-type structure were obtained.

Introduction

The rare-earth hexacyanometallates (III) hydrates, Ln[M(CN)6nH2O (Ln = lanthanide; M = transition metal; n = 4, 5) are precursors for the synthesis of perovskite-type oxides which have a variety of applications such as chemical sensors, catalysts [1], [2], [3], [4], colossal magnetoresistant materials [5] and multiferroism [6]. Traditionally, the ceramic method has been employed to synthesize mixed oxides. However, this method usually needs very high temperatures to reach chemical homogeneity, producing very low surface areas and oxygen deficient materials. The thermal decomposition of heteronuclear complexes was proposed by Gallaguer in 1968 to prepare LaFeO3 and LaCoO3 from hexacyanometallates as precursors [7]. The oxides obtained by this method were formed at shorter annealing times and lower temperatures than ceramic methods. In addition, the use of soft chemical routes can yield homogeneous phases with small grain size [7], [8], [9], [10], [11], [12], [13], [14], [15].

The crystal structure of hexacyanometallates consists of alternating cyanide-bridged MC6 and LnN6Oy (y = 2, 3) polyhedra in an orthorhombic or hexagonal lattice, depending on the degree of hydration at the lanthanide centers [8], [16]. The CN ligand has the ability to serve as bridge group between neighboring metal centers, removing electron density from the metal linked at its C end, through a π back-bonding interaction, to increase the charge density on the N end that is the coordination site for the other metal. This leads to the overlapping between the electron clouds of neighboring metal centers and to their spin coupling and, thereby, a magnetic ordering is established. This supports the role of hexacyanometallates as prototype of molecular magnets [17].

In the present article we report the synthesis, structural, spectroscopic and magnetic characterization of solid solutions Y[Fe1−xCox(CN)6]·4H2O. The crystal structure refinement of Y[Fe1−xCox(CN)6]·4H2O complexes were performed by means of Rietveld analysis using standard powder X-ray diffraction (PXRD). These measurements were complemented by thermogravimetric and differential thermal analysis, Infrared (IR) and Mössbauer spectroscopy. The magnetic properties of Y[Fe1−xCox(CN)6]·4H2O have also been investigated.

Section snippets

Synthesis

The polycrystalline samples of the series Y[Fe1−xCox(CN)6nH2O were prepared by the co-precipitation method, mixing aqueous solutions of equimolar amounts of K3[Fe(CN)6], K3[Co(CN)6] and Y(NO3)3·6H2O under continuous stirring at 60 °C for 2 h. Y(NO3)3·6H2O was prepared from the evaporation of a solution of concentrated HNO3 and Y2O3. The color of the precipitates changes from orange to white as we move from Y[Fe(CN)6]·4H2O to Y[Co(CN)6]·4H2O in the series. The resulting precipitates were

Crystal structures refinement

The PXRD patterns of all the complexes are very similar (Fig. 1a), as expected for isostructural compounds. Fig. 1b shows the refined PXRD pattern for one representative of the series, Y[Fe0.5Co0.5(CN)6]·4H2O. The refined PXRD patterns of the other complexes are shown in Supplementary information. Crystallographic parameters and discrepancy factors for Y[Fe0.5Co0.5(CN)6]·4H2O after Rietveld refinement are summarized in Table 1. The series of complexes Y[Fe1−xCox(CN)6]·4H2O crystallizes in the

Conclusions

The series of solid solutions Y[Fe1−xCox(CN)6]·4H2O was synthesized for the first time and their crystal structures were refined with the Rietveld method from PXRD data. All the complexes crystallize in the orthorhombic crystal system, space group Cmcm. In these structures, Fe3+ and Co3+ ions are octahedrally coordinated to six cyano groups and the Y3+ ion is eight-coordinated to six N atoms from CN ligands and two O from coordinated water molecules. Cell parameters and cell volume change

Acknowledgements

R.E.C. thanks FONCYT for PICT2007 303, CONICET for PIP #11220090100995 and SECyT-UNC for the Project 159/09. D.M.G. thanks CONICET for a fellowship. D.M.G and M.I.G. thank CIUNT for financial support, Project 26D-428. R.E.C and A.P. Jr. would like to thank CONICET and CNPq for the CIAM collaboration project. J.G. acknowledges support from grants FONCyT PICT 2007-824 and SECyT-UNCuyo 06/C301.

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