A facile gel-combustion route for fine particle synthesis of spinel ferrichromite: X-ray and Mössbauer study on effect of Mg and Ni content
Graphical abstract
Introduction
Ferrites are ferrimagnetic oxides; crystallize into two magnetic sub-lattices, tetrahedral (A) site and octahedral [B] site. The properties, upon which their application depends, are influenced by the cation distribution among these two sites. Ferrites are high-resistivity materials with low eddy current losses which make them potential materials for high-frequency applications such as microwave devices. The properties of ferrites have been normally found to vary on substituting the other constituent ions [1], [2]. Superparamagnetism is a unique feature of ferrites and is crucially related to many modern technologies, including ferrofluid technology, magnetic refrigeration etc. [3], [4].
Nickel-magnesium ferrite is a soft magnetic material having low magnetic coercivity and high resistivity values. The high electrical resistivity and excellent magnetic properties make this ferrite an automatic choice as a core material for power transformers in electronic and telecommunication applications in megahertz frequency regions. The wet chemical method reported on here, overcomes all drawbacks of conventional ceramic process and produce ultra-fine, homogeneous and reproducible ferrite powders using aqueous solutions of nitrate salts of constituent ions.
Mössbauer spectroscopy uses the nuclear properties to get information regarding the environment surrounding the nucleus. The very high sensitivity of the Mössbauer spectrum to the oxidation state and site occupancy of Fe3+ in cubic spinel makes Mössbauer spectroscopy valuable for addressing the fundamental relationships between crystal chemistry and structure. The technique can also detect the relative percentage of different charged states of the same atom like Fe2+ and Fe3+ present in the material. Being that the iron occupies two local environments, the A-site and B-site, and two species (Fe2+ and Fe3+) occupy the B-site, one might expect the spectrum to be a combination of three spectra. However, delocalization of electrons or electron hopping between Fe2+ and Fe3+ in the B- site causes the nuclei to sense an average valence in the B-site and thus the spectrum are fitted with two curves accordingly. The isomer shift of the fitted curves can be used to determine which curve corresponds to which valence ions [5], [6].
In this work, attempts to substitute magnesium by nickel were undertaken through sol–gel route. The precipitation methods always have the disadvantage that the stoichiometry of the precipitate may not be exact if one or more ions are left in solution. The sol–gel route overcomes this because the reactants never precipitate out. X-ray and Mössbauer spectral studies were carried out in order to make phase analysis. It was established that iron ions were located in two different interstitials from which oxidation state of iron was confirmed.
Section snippets
Experimental details
The A.R. grade nitrate salts (Merck Specialties Pvt. Ltd., India) of Fe, Mg, Cr and Ni were taken by the stoichiometric calculations for the synthesis of each compounds of the system Mg1−xNixFeCrO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). As the metal nitrates are hygroscopic they were preserved in moisture free dessicator. After dissolving the metal nitrates separately in minimum de-ionized water, solutions were mixed well and a solution of citric acid was added along with a small amount of
X-ray diffraction study
XRD patterns of Mg1−xNixFeCrO4 samples sintered at 600 °C for 8 h are shown in Fig. 1 which confirms the single cubic spinel phase. The diffraction lines are found to be sharp and no other phases has been detected for all the samples. The material has a well-defined polycrystalline nature, belonging to the fcc Bravais lattice; which confirm the spinel structure [9].
The irregular variation of lattice parameter ‘a’ with Ni content can be explained on the basis of ionic radii. Both Mg (0.65 Å) and Ni
Conclusions
The system Mg1−xNixFeCrO4 is successively synthesized by the facile sol–gel auto-combustion method at controlled pH and exhibits the single phase cubic spinel structure. The variation of lattice parameter ‘a’ with Ni content is due to structural change, atomic weight and the replacement of larger radius ion Mg2+ by smaller radius ion Ni2+. The calculated site radii and bond lengths show that the octahedral site radius is larger than the tetrahedral site. The isomer shift ‘δ’ and the values of QS
Acknowledgements
Author V.T. Vader wish to acknowledge his sincere gratitude to UGC-WRO, Pune (India) for financial assistance through ‘Minor Research Scheme F.No.47-2038/11 (WRO), 22.02.2012’. Author is also thankful to BARC, Mumbai for providing Mössbauer facility for samples analysis.
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