Chromium Substituted Cobalt Ferrites by Glycine-Nitrates Process

Chromium substituted cobalt ferrites (CoFe2–xCrxO4, 0  x  2) were synthesized through solution combustion method using glycine as fuel, named glycine-nitrates process (GNP). As evidenced by X-ray diffraction data (XRD), single cubic spinel phase was formed for all CoFe2−xCrxO4 (0  x  2) series. The cubic lattice parameter (a) decreases with increasing chromium content. Room temperature 57Fe Mössbauer spectra revealed the Fe3+ and Cr3+ site occupancy, the local hyperfine magnetic fields and the substitution of Fe3+ by Cr3+ in the lattice. Scanning electron microscopy (SEM) showed a refinement of particle size with the increase of Cr3+ content. Magnetic measurements from 5 K to 120 K have shown a dropping in the saturation magnetization as the chromium content increases. This behaviour has been explained in terms of substitution of Fe3+ by Cr3+ in the cubic lattice of cobalt ferrite.


INTRODUCTION
[3][4][5] The spinel ferrites exhibit two types of structures: a normal spinel structure and a partially/completely inverse structure. [4,6,7]he cobalt ferrite (CoFe2O4) is an inverse spinel in which the degree of inversion depends on the synthesis parameters and the thermal history. [8]The cobalt chromite (CoCr2O4) crystallizes in a normal spinel structure.All the Co 2+ ions located in the tetrahedral positions form a diamondtype sublattice, whereas all the Cr 3+ ions located in the octahedral positions build a pyrochlore-type sublattice. [4,9]t is well known that the electric, optical and catalytic properties of the ferrites can be improved through the variation of the cations in the tetrahedral and/or the octahedral sites. [4,10,11]For example, the substitution of Fe 3+ ions with Cr 3+ ions at B site contributes to enhancing the electrical resistivity for sensors and actuator applications. [10,11][14] The critical requirements of obtaining well controlled uniformity and high-purity spinel ferrites/substituted ferrites led to the development of soft chemistry routes, such as the coprecipitation, [7] the microwave assisted route, [15] the reverse and normal micelles, [16] the sol-gel method, [17] the hydrothermal route, [6] the precursor route (the thermal decomposition of multimetallic compounds). [12,13,18,19]ne of the most interesting and attractive methods for tailoring nanosized spinel ferrites is the self-propagating combustion -the solution combustion synthesis (SCS).Discovered in 1988, during the reaction between aluminium nitrate and urea, [20] the solution combustion N synthesis, a simple and low cost method, uses the energy produced by the self sustained reaction between the homogeneous solution of metal nitrates as oxidizer, and urea, hydrazide, derivates of hydrazide, glycine or citric acid as fuels.The solution combustion process is characterized by high temperature, fast heating rates and very short reactions times. [21]The structural and morphological characteristics of the final oxides depend on the nature of the fuel and the oxidizer/fuel ratio.The ratio of the nitrates to fuel in the initial mixture was calculated using propellant chemistry concepts.[24][25] In order to obtain nanosized chromium substituted cobalt ferrites, many research groups have used the solution combustion method with various fuels, like urea, [26] citric acid [21,27] etc. P. P. Hankare et al. have developed a sol-gel autocombustion method to obtain nanocrystalline CoFe2-xCrxO4 powders (0  x  2) using citric acid as fuel. [14,28,29]t is the aim of this paper to report on the synthesis of nanosized chromium substituted cobalt ferrite CoFe2-xCrxO4 (0  x  2) through the solution combustion method using glycine as fuel; this is known as the glycine-nitrates process (GNP).The structure, morphology, site occupancy and magnetic properties of the synthesized samples are presented and discussed.

Preparation of CoFe2-xCrxO4 (0  x  2)
The ratio of cobalt, iron and chromium nitrates to glycine in the initial mixture was derived from the total oxidation number of the oxidizer and fuel.The "stoichiometric composition" of the redox mixture 2(-15) + 1(-10) + n(+9) = 0 requires n = 4.5.Thus, the reactants were combined in the molar proportion 2 : 1 : 4.5.The reactants were mixed in an agate mortar until a honey-like homogeneous solution was formed.The hydration water from the nitrates was the only solvent.The mixed solution was placed on a heater at 250-300 °C.Initially, the viscous solution melted and then it decomposed by spontaneous self-ignition, foaming and puffing, leaving behind a voluminous fluffy powder (Scheme 1).
Under complete combustion, the chemical reaction can be written as follows: The powders obtained as described above were calcined at 700 °C / 1h in order to obtain well-crystallized chromium substituted cobalt ferrites.

Characterization Techniques
X-ray powder diffraction data were recorded using Rigaku's Ultima IV multipurpose diffraction system.The diffractometer (operating at 40 kV and 30 mA) was set in parallel beam geometry, using CuKα radiation (λ = 1.5406Å), CBO optics and graphite monochromator.The measurements were performed in θ-2θ mode, 0.02° step size and 5° min -1 scan speed.Phase identification was performed using Rigaku's PDXL software, connected to ICDD PDF-2 database.The lattice constants were refined using Whole Powder Pattern Fitting (WPPF) and crystallite size was calculated by Williamson-Hall method.The microstructure of the chromium substituted cobalt ferrites was investigated by Scanning Electron Microscopy (SEM) using a FEI Quanta 3D FEG operating between 2 and 30 kV, equipped with an Energy Dispersive X-ray (EDX) spectrometer for elemental analysis.The IR spectra of the chromium substituted cobalt ferrites were recorded on KBr pellets with a JASCO FTIR 4100 spectrophotometer in the 4000-400 cm -1 range.Room temperature Mössbauer measurements were performed by means of a WissEL-ICE Oxford Mössbauer cryomagnetic system and a 10 mCi 57 Co(Rh) source.The sample thickness was 0.7 mg cm -2 .The magnetic measurements of heattreated chromium substituted cobalt ferrites were made using a superconducting quantum interference device (SQUID).The hysteresis loops have been obtained at three different temperatures (5, 70 and 120 K) into a magnetic field of 5 T.
The nitrates salts are preferred as "raw materials" because they can serve as nitrogen source for the synthesis and are water soluble at room temperature.The glycine is a very good, gentle fuel for the combustion reaction, being oxidized by nitrate ions.At the same time, it can act as a very good chelating agent for many metal ions. [18,19]. Sharma et al. [30] describe the solution combustion reaction as a two step process with the first step corresponding to the formation of a complex compound precursor and the second step corresponding to the self-ignition.
The formation of the precursor as a single molecular compound influences the homogeneity and the stoichiometry of the final oxide.In a series of previous papers, we succeeded to isolate and characterize the precursors from GNP. [18,19,24,28]

X-ray Diffraction
The XRD patterns of all the samples CoFe2-xCrxO4 (0 ≤ x ≤ 2) calcined at 700 °C / 1h show the formation of the single phase cubic spinel structure belonging to the space group Fd3m (ICDD 022-1086 for CoFe2O4; ICDD 22-1084 for CoCr2O4).No additional lines corresponding to any other phase were detected (Figure 1).
The lattice parameter a decreases with the increase of the Cr 3+ content from 8.384 Å to 8.328 Å (Table 1).This decrease in a with x can be explained based on the difference in ionic radii between the larger ionic radius Fe 3+ ion (0.67 Å) and the smaller ionic radius Cr 3+ ion (0.63 Å). [14,21] The average crystallite size of all the ferrite samples was

Scanning Electron Microscopy
SEM measurements emphasize a clear refinement of particle size with the increase of Cr 3+ content (x).CoFe2O4 (x = 0, Figure 2a-c), exhibits a sintered porous structure of faceted grains sized ~ 1 micron, while CoCr2O4 (x = 2, Figure 2d-f), exhibits a structure of fine non-agglomerated nanoscale powder.The gradual change of the microstructure and the elemental composition in the intermediate samples containing both iron and chromium (x = 0.5, 1 and 1.5) can be observed in Figure 3.The presence of iron seems to ease sintering and the particle growth of the ferrite powders at relatively moderate temperature.This effect can be attributed to the fact that the magnetic interaction between   crystallites can facilitate their advantageous self-alignment, promoting low temperature sintering.The effect gradually diminishes and disappears with the growth of the Cr 3+ content.The same effect was previously observed in the chromium substituted copper ferrites. [13]-IR Spectroscopy The formation of the spinel structure of CoFe2-xCrxO4 (0 ≤ x ≤ 2) is also supported by the FT-IR analysis.The IR spectra of all the samples are recorded in the range 4000-400 cm -1 .
The two very intense metal-oxygen bands observed in these IR spectra represent characteristic features of the single phase spinel ferrites (Figure 4).The higher band, 1, in the range 615-570 cm -1 , corresponds to the stretching vibrations of the metal from the tetrahedral site, MtetraO, whereas the lower band (2), in the range 508-395 cm -1 , is assigned to the octahedral metal stretching, MoctaO.It is well known that Co 2+ and Cr 3+ ions are preferentially located in the octahedral site, while Fe 3+ ions can occupy both octahedral and tetrahedral sites.
In the IR spectra of the CoFe2-xCrxO4 (0 ≤ x ≤ 2) samples the bands (1) and (2) shift towards higher frequencies with the increase of the Cr 3+ content: 1 shifts from 578 to 615 cm -1 , while 2 shifts from 395 to 508 cm -1 .According to literature, the increase of the Cr 3+ leads to the gradual transition from an inverted spinel structure to a normal one. [4,14,31,32]

Mössbauer Spectroscopy
Figure 5 (a-d) shows the room temperature Mössbauer spectra of the samples CoFe2-xCrxO4 (x = 0; 0.5; 1.5; 2) together with the computer fit (continuous) lines, under the hypothesis of Lorentzian line shape.In order to understand the Mössbauer results we have to remember that in AB2O4 spinels every A-site (tetrahedral) iron ion is surrounded by 12 B-site (octahedral) next nearest neighbours and each B-site iron ion is bounded by six A-site nearest neighbours. [33,34]e hyperfine parameters (isomer shift IS, quadrupole splitting ΔEQ and magnetic fields Hhf at the iron nucleus) for all Mössbauer spectra are listed in Table 1.In the computer fit, the line intensities were fixed at 3 : 2 : 1 (theoretical values) for all magnetic sublattices, the line widths were considered equal and kept fixed for all magnetic sextets and free for the central quadrupole doublet.
At x = 0, the Mössbauer spectrum (Figure 5a) consists of a magnetic hyperfine pattern that was deconvoluted in two sextets corresponding to Fe 3+ ions in tetrahedral and octahedral sites, respectively. [33][35][36][37] More recent studies consider that the IS values correlated with the structural data can be better used for site assignments in cobalt ferrite.Consequently, the sextet with lower IS (~ 0.19 mm s -1 , relative to -iron) is assigned to iron in tetrahedral sites (from inter-nuclear distance arguments) [35] and the sextet with higher IS (0.32 mm s -1 ) corresponds to octahedral iron sites.
At x = 0.5, the Mössbauer spectrum is rather complex (Figure 5b), showing a magnetic hyperfine pattern with large line width accompanied by a central quadrupole doublet.Taking into account the strong preference of chromium ions for octahedral sites, the probability for Cr 3+ to occupy the 12 positions of B sites, could be mathematically described by a binomial distribution.We have to note that the binomial (random) distribution is a pure mathematical approach to describe the unresolved structure of complex systems.In the hypothesis of random distribution of Cr 3+ in octahedral positions, the best fit was obtained by considering six magnetic sublattices and a central quadrupole contribution.In Table 2, Bn denotes the relevant magnetic sublattice (spectral area > 5 %) corresponding to the presence of ‚n' Cr 3+ ions in the B sites of the spinel structure.The same refinement procedure was applied for the sample at x = 1.0 (Figure 5c).The best fit was given by seven magnetic sextets in the fitting run.At x = 1.5, the Mössbauer spectrum (Figure 5d) displays a single quadrupole doublet with IS = 0.263 mm s -1 and ΔEQ = 0.414 mm s -1 , parameters very close in value to those at x = 0.5 and 1.0.These values are characteristic to the Fe 3+ ions.The quadrupole doublets in the spectra at x = 0.5, 1.0 and 1.5 can be associated with the contribution to the Mössbauer spectra of iron ions with very reach Cr 3+ neighbouring.No Fe 2+ was evidenced in the analysed spectra.

Magnetic Measurements
Figure 6 displays the magnetic hysteresis loops for all chromium substituted cobalt ferrite samples calcined at 700 °C / 1h.The magnetic parameters were determined from the individual M-H curves and were presented in Table 3.
Table 3 clearly shows that the saturation magnetization (Ms) at 5 K decreases with the increase in the Cr 3+ ions content from 82.34 emu g -1 to 42.38 emu g -1 in the chromium substituted cobalt ferrites.This decrease is due to the replacement of the Fe 3+ ions (magnetic moment 5 µB) by the less magnetic Cr 3+ ions (magnetic moment 3 µB) in the octahedral (B) sites of the ferrite lattice.This behaviour is also observed in the evolution of coercivity (Bc); the presence of Cr 3+ ions induces a decrease in the anisotropy field which in

Table 2 .
Mössbauer fit results for the chromium substituted cobalt ferrite samples.