Preparation and Characterization of Chromium Doped NiCu-Zn Nano Ferrites

Chromium doped Ni-Cu-Zn nano ferrites with chemical formula Ni0.2Cu0.2Zn0.6Fe2-xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by using sol-gel auto combustion method. The prepared precursors of Chromium substituted Ni-CuZn ferrites were sintered at 500 °C for 4h. Compositional stoichiometry were confirmed from EDAX patterns. The XRD data revealed that the all samples possess a single phase cubic spinel structure. The Lattice constant, X-ray density, hopping lengths and crystallite size determined from XRD data decreases with increase in Cr3+ concentration. The IR spectra show two major absorption bands, high frequency band ν1 ≈ 600 cm –1 and low frequency band ν2 ≈ 450 cm –1 attributed to the stretching vibration of tetrahedral and octahedral sites respectively. The surface morphology of the prepared samples was studied by Scanning Electron Microscopy and Transmission Electron Morphology.


Introduction
Nano ferro spinels are of great interest for addressing relationship between physical properties and their crystal structure.Due to their reduced sizes, these nanoparticles may possess novel and/or improved properties in comparison to the bulk materials.This has renewed interest to study different properties of pure and mixed spinel ferrite systems in nanocrystalline regime.Transition metal ferrites both doped and undoped are attractive candidates for a wide range of applications including catalysis, [1][2][3][4] and several devices such as antennas, permanent magnets, memory storage devices, microwave devices, telecommunication, computer etc. 5 The polycrystalline ferrites such as Ni-Cu-Zn ferrites have very important structural properties dependent on several factors such as method of preparation, substitution of cations, sintering temperature, sintering time, sintering atmosphere, porosity and microstructure. 6,7Ni-Cu-Zn ferrites were considered as one of the most versatile magnetic materials to manufacture Multilayer chip inductors (MLCIs) mainly because of their high electrical resistivity, low sintering temperature and high permeability. 8,91][12][13][14][15][16] These oxides can be sintered at relatively low temperatures with a wide range of compositions.3][24][25] The sol-gel auto-combustion synthesis method have many advantages as compared to the conventional methods such as low temperature processing and/or better homogeneity for the synthesis of multi-component materials.Considering the importance of Ni-Cu-Zn ferrite, we investigated the preparation of chromium doped Ni-Cu-Zn nano ferrite by sol-gel auto-combustion method using citric acid as fuel at low temperature with a view to study the influence of the substitution of Cr 3+ ion on the structural properties of the system.During evaporation, a very viscous brown gel was formed.When all of the water molecules were removed from the mixture, the viscous gel began to froth.After few minutes, the gel was ignited and burnt with glowing flints.The decomposition reaction continued until the entire citrate complex was consumed.The auto-combustion was completed within a minute, yielding brown-colored ash as the precursor.Sintering temperature was determined from TGA / DTA and prepared powders of all the precursor samples were sintered at 500 °C for 4h to obtain the final product.

Characterization of Samples
Simultaneous thermo gravimetric (TGA) and differential thermal analysis (DTA) of precursors were carried on SDT Q600 V20.9 Build 20 instrument in air atmosphere at heating rate 10 °C / min, within temperature range 25 °C to 800 °C.The compositional stoichiometry was determined by energy dispersive X-ray spectroscopy (EDAX, Inca Oxford, attached to the SEM).The crystallographic structures were identified by X-ray powder diffraction with Cu-K α radiation (λ = 1.5405Å) by Phillips X-ray diffractometer (Model 3710).The infrared spectra of all the samples were recorded at room temperature in the range 300 to 800 cm -1 using Perkin Elmer infrared spectrophotometer.Morphology and structure of the powder samples were studied on JEOL-JSM-5600 N Scanning Electron Microscope (SEM) and on Philips (model CM 200) Transmission Electron Microscope (TEM).

1. Thermal Analysis (TGA / DTA)
The typical TGA/DTA curves of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2- x Cr x O 4 (x = 0.0; Figure 1 and x = 1.0; Figure 2), illustrates two weight loss steps.The first weight loss step in the temperature range of 30-100 °C, corresponding to endothermic peak around 80 °C, which is due to the loss of coordination water in the precursor.The second weight loss step observed in the temperature range of 310-425 °C corresponding to the exothermic peak around 380 °C, is as a result of the decomposition of unreacted starting citric acid remained after combustion.The released heat in the process of exothermic decomposition has been observed to be sufficient for complete conversion of the metal precursors to metal oxides. 26

2. 1. Elemental Analysis
The compositional stoichiometry of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2- x Cr x O 4 ferrite nanoparticles was determined by EDAX.
The EDAX confirmed the homogeneous mixing of the Fe, Ni, Cu, Zn, Cr and O atoms in pure and doped ferrite samples.Figure 3 shows the plots of observed and theoretical percentage of Fe, Ni, Cu, Zn, Cr and O values versus composions (x).The observed elemental % (obtained from EDAX) values are in close agreement with theoretical % (the starting composition used for the preparation) values.The EDAX analysis is affected by the surface crystalline defects of the nanoparticles.This can also be taken into account to explain the difference between the values of the atomic ratio as determined by EDAX and the expected value. 27,28Lattice parameter (a) of all the samples was determined by using the following equation: 29 (1) Lattice constant (a) values with an accuracy of ± 0.002 Ǻ were calculated for each sample using XRD pattern and are listed in Table 1.It is observed that the lattice constant (a) decreases with increase in Cr 3+ concentration.In the present ferrite system Fe 3+ ions (0.67Ǻ) ions are replaced by the relatively small Cr 3+ ions (0.64Ǻ).[32] The X-ray density of all the samples was obtained by the following relation:

2. 2. X-ray Diffraction
( Where, '8' is formula unit, 'M' is molecular weight, 'N' is Avogadro's number, 'a' is lattice constant.The values of X-ray density are presented in table 1.It is seen from Table1that, like lattice parameter, X-ray density also decreased with increasing Cr 3+ content 'x'.The decrease in X-ray density is attributed to decrease in lattice constant.It is observed that X-ray density increase for x = 1.0.This is related to the molecular weight of the sample overtakes the volume (a 3 ).
The average crystallite diameter 'D XRD ' of powder estimated from the most intense (311) peak of XRD and using the Debye-Scherrer method, 29 (3) Where, β 1/2 is the full width of half maximum in (2θ), 'θ' is the corresponding Bragg angle and C = 0.9.The values of the crystallite size are given in Table 1.The crystallite size is decreases from 30.3 nm to 8.9 nm with increasing Cr 3+ substitution.The decrease in the crystallite size indicates that the addition of the Cr 3+ obstruct the crystal growth. 33Due to the surface temperature and the molecular concentration at the surface of the crystal results into decrease in the crystal growth.
The percentage porosity 'P' of all the samples was calculated using the values of X-ray density and bulk density: (4) It is observed that porosity increased from 25.4% (x = 0.0) to 32% (x = 1.0) with the Cr 3+ substitution.In the present series of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4 , both the molecular weight and the volume of the unit cell decrease with increasing Cr 3+ substitution, but the rate of the decrease of molecular weight is more than that of volume.Therefore, the density decreases with Cr 3+ substitution, this resulted in increase in porosity.Apart from this, the increase in porosity is mainly attributed to decrease in crystallite size, which increases the grain boundaries of the particle and accordingly the porosity. 27he distance between the magnetic ions i.e. hopping lengths (L A and L B ) in the tetrahedral A-site and octahedral B -site was calculated using following relation: 34 (5) (6)   It has been observed from Table 1 that the hopping lengths (L A and L B ) decreased with Cr 3+ substitution.Decrease in both the hopping lengths with Cr 3+ substitution is due to the decrease in lattice constant.

2. Infrared Spectroscopy
The infrared spectroscopy is an important tool to probe various ordering phenomena that provide information on the position of ions and vibrational modes of crystals.The substitution of metal ion in ferrites may give rise to structural change within the unit cell without affecting the structure as a whole.Such structural changes brought about by metal ions strongly influence the lattice vibrations. 35The IR spectra as shown in Figure 5, were recorded at room temperature in the frequency range 300-800 cm -1 .For ferrites, generally it is found two assigned absorption bands appear around 600 cm -1 : ν 1 , which is attributed to stretching vibration of tetrahedral groups Fe 3+ -O 2-and around 400 cm -1 : ν 2 , which is attributed to the octahedral groups complex Fe 3+ -O 2-.It is observed from Table 2 and Figure 6 that the higher frequency band (ν 1 ) is appeared in the range of 568-610 cm -1 whereas lower frequency band (ν 2 ) is appeared in the range of 388-491 cm -1 .These bands are characteristics features of spinel structure.It explains that the normal mode of vibration of tetrahedral cluster is higher than that of octahedral cluster.It should be attributed to the shorter bond length of tetrahedral cluster and longer bond length of octahedral cluster.Where, K O is the force constant of octahedral site, K t is the force constant of tetrahedral site, M 1 molecular weight of tetrahedral site, M 2 molecular weight of octahedral site, ν 1 the corresponding center frequency on tetrahedral site, and ν 2 the corresponding center frequency on octahedral site.
The molecular weights M 1 and M 2 for each sample are calculated from the cation distribution.The force constant is the second derivative of the potential energy with respect to the site radius with the other independent parameters kept constant.The bond lengths R A and R B have been calculated using the formula given by Gorter. 37The molecular weights of the tetrahedral M 1 and octahedral M 2 sites have been calculated using the cation.The values of R A , R B and the force constants K t and K O are listed in Table: 2.

2. 4. Scanning Electron Microscopy (SEM)
Typical Scanning electron micrograph (SEM) of the sample x = 0.6 is shown in Figure 6.Each composition is characterized by a typical porous structure and small rounded grains.It is observed from SEM images that the structure is affected by the Cr 3+ substitutions.It can be observed from the SEM images that the prepared samples are amorphous and porous in nature.The decrease in the grain size and an increase in porosity are observed with increasing Cr 3+ substitutions.The observed changes in grain size suggest that the substitution of Cr 3+ in Ni-Cu-Zn ferrite solid solution occurs during sol-gel combustion process which enables a better homogeneity in the powders and, hence, a more controlled microstructure is obtained.

5. Transmission Electron Microscopy (TEM)
TEM image of the typical sample x = 0.4 is presented in Figure 7.The particles were well distributed and slightly agglomerated.The agglomeration is the indication of high reactivity of the prepared sample with the heat treatment and it may also be come from the magnetostatic interaction between particles.Since Cr 3+ ions provide stability to the Ni-Cu-Zn lattice; it is believed that they also inhibit the process of grain growth through coagulation at the stage where the sol-gel is formed and hence samples of small particle size are produced. 38Selected area electron diffraction (SAED) patterns of the respective TEM image is also shown in Figure 8.

Conclusion
Nanocrystalline Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4 ferrites with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 were prepared by sol gel auto combustion method using citric acid as a fuel.The EDAX pattern confirmed the homogeneous mixing in pure and doped ferrite samples with desired composition.Structural analysis with XRD reveals that the system confirms the formation of single phase cubic spinel structure of Chromium doped Ni-Cu-Zn Nano Ferrites.Lattice constant and X-ray density decreased with Cr 3+ substitution.The crystallite size is observed in the range of 8.9-30.3nm, which is in close agreement with crystallite size obtained from TEM.It is concluded from IR spectra that higher frequency band (ν 1 ) is appeared in the range of 568-610 cm -1 whereas lower frequency band (ν 2 ) is appeared in the range of 388-491 cm -1 confirming characteristics features of spinel structure.It is observed from SEM that the prepared samples are amorphous and porous in nature.TEM images of the samples confirmed the particle size of obtained ferrite samples is in nm dimensions.The prepared Chromium doped Ni-Cu-Zn Nano Ferrites may be used as catalyst for organic transformations and in several devices such as antennas, memory storage devices, microwave devices etc.

1 .
Materials and Synthetic Procedure Nanocrystalline chromium substituted Ni-Cu-Zn ferrites, with composition Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were synthesized by the sol-gel auto-combustion method.Analytical reagent grade nickel nitrate (Ni(NO 3 ) 2 • 6H 2 O), copper nitrate (Cu(NO 3 ) • 6H 2 O), zinc nitrate (Zn(NO 3 ) 2 • 6H 2 O), chromium nitrate (Cr(NO 3 ) 3 • 9H 2 O) and iron nitrate (Fe(NO 3 ) 3 • 9H 2 O) were used for synthesis.Citric acid (C 6 H 8 O 7 • H 2 O) was used as fuel.The reaction procedure was carried out in an air atmosphere without the protection of inert gases.Metal nitrates and citric acid were used in the molar ratio 1:3.The metal nitrates were dissolved in desired stoichiometric proportions together in the minimum amount of double-distilled water to obtain a clear solution.An aqueous solution of citric acid was mixed with the metal-nitrate solution, and ammonia solution was slowly added to adjust the pH ≈ 7. The mixed solution was placed on a hot plate with continuous stirring at 90 °C.

Figure 5 .
Figure 5. IR spectra for the series Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4

Figure 6 .Figure 7 .
Figure 6.Scanning electron micrograph of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4 (x = 0.6) Shinde et al.: Preparation and Characterization ... The Bragg' rings observed in these SAED patterns corresponding to specific 'd' values, that match perfectly with the 'd' values calculated from XRD.The superimposition of the bright spot with Debye ring pattern indicates polycrystalline nature of the sample which is in accordance with XRD.Like XRD; SAED also confirmed that the sample does not possess any type of impurity or second phase.

Table 1 .
Lattice constant (a), X-ray density (d x ) and hopping lengths (L A ) and (L B ), particle size (D xrd ) and porosity (P) of Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4

Table 2 .
Band position (ν 1 and ν 2 ), Force constant (K 0 and K t ) and Bond length (R A and R B ) of system Ni 0.2 Cu 0.2 Zn 0.6 Fe 2-x Cr x O 4