A series of Bismuth substituted cobalt nanoferrites having the chemical formula CoBi_xFe_2-xO₄ (where x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25) were prepared by sol-gel method. The starting materials were cobaltus nitrate [Co(No₃)₂∙6H₂O] with 99% purity from sd-fine chemicals, bismuth Nitrate [Bi(No₃)₃∙5H₂O] with 98.9% purity from sd-fine chemicals, ferric nitrate [Fe(No₃)₃∙9H₂O] with 99.1% purity from sd-fine chemicals, citric acid [C₆H₈O₇∙H₂O] with 99.9% purity from sd-fine chemicals, 25% of aqueous solution of ammonia (NH₃) with A- grade from sd-fine chemicals.

The stoichiometric amount of all metal nitrates were dissolved in different glass beakers with minimum quantity of double distilled water and mixed together with the help of magnetic stirrer. Later on a solution of citric acid was added to the mixed metal solution with molar ratio of nitrates to citric acid is 1:3. Under constant stirring an aqueous solution of ammonia (NH₃) was added to this nitrate mixture drop wise to adjust the pH value about to 7. And finally a homogeneous solution was obtained. Then the mixed solution was heated about 100˚C with uniform stirring and evaporated to obtain a highly viscous gel. The resultant gel was further heated 180˚C to 200˚C. When all water molecules were removed from the mixture, the gel gave a fast flameless auto combustion reaction with the evolution of gaseous products. And after some time it starts in the hottest zones of the beaker and propagated from the bottom to the top like the eruption of a volcano. After completion of this reaction a dark brown ash powder was yielded at 250˚C with a structure similar to branched tree. All these synthesis process was outlined in the following Figure 1. At last it is cooled to room temperature and ground in an agate mortar for at least 30 minutes to get a highly dense power. Finally the powder was sintered at 600˚C for 5 hours in air and cooled to room temperature. Further the prepared compositions were characterised by XRD, SEM, EDS, TEM and FTIR.

The structural characterization of synthesized samples were carried out XRD by Phillips X-ray Diffractometer (model 3710) using with Cu Kα (λ = 1.54 Å) radiation. This was operated at room temperature by continuous scanning with 2θ values in the range of 10˚ to 80˚ to investigate phase, crystallite size and lattice parameters. The surface morphology of the samples was examined by using Scanning Electron Microscope (SEM) and the micro structural analysis of the prepared samples was carried out by Transmission Electron Microscope (TEM). The elemental analysis was carried out by using Energy Dispersive Spectrometer (EDS), which confirm the purity of all prepared samples. The infrared spectra of synthesized powders were recorded by SHIMADZU Fourier Transform Infrared Spectrophotometer (FT-IR), Model P/N-206-73500-38, in the range of 4000 to 250 cm⁻¹ with a resolution of 4 cm⁻¹, which reveals the formation of single phase cubic spinal structure.

X-ray diffraction patterns of composition CoBi_xFe_2-xO₄ (x = 0.0 to 0.25) nano ferrite particles are shown in Figure 2. Generally for a pure cobalt ferrite the XRD pattern exhibits eight peaks which were located in between

2 = 15˚ to 80˚ as follows. In which the corresponding intensities of the peaks displayed as percentage (%) of the most intensive peak located with relative miller indices are shown in Table 1 [18] . The XRD pattern mat- ches with the (JCPDS-KDD) file number of (22-1086) and the corresponding reflections of peaks (220), (311), (400), (333), (440) were observed in X-ray diffraction pattern. On substitution of Bismuth in very small amount which does not alter the spinal structure of ferrite system [19] . It is also observed that the Bragg’s peaks confirm the formation of a single phase fcc spinal structure. From the XRD pattern it is identified for x = 0.00 shows only peaks consistent with cubic spinal phase and rest of all the samples with the composition, x = 0.05 to 0.25, have additional peaks marked with the “♦” sign in Figure 2, which corresponds to the element of Bismuth. The crystallite sizes of all compositions were determined from broadening of the peak (311) of XRD pattern using the following Debye-Scherrer formula.

where, D = the average crystallite size of the phase under investigation, λ = wavelength of X-ray beam used, β = full width at half maxima (FWHM) in radians and θ = Bragg’s angle.

From Table 2, it is observed that the lattice parameter decreases as the Bismuth concentration increases. The decrease in lattice constant (a) is due to the difference in ionic radii of Bi³⁺ (0.74 Å) as compared to Fe³⁺ (0.78 Å). Therefore the smaller Bi³⁺ ions were replaces the larger Fe³⁺ ions completely in octahedral positions [20] and the crystalline size was found to be in the range of 17 to 26 nm which was good agreement with XRD result.

The lattice parameter “a” was calculated using the following equation.

Figure 3 shows the variation of lattice constant (a) with the composition (X), which indicates that the variation of lattice constant (a) is found to be decrease linearly with increase of Bi³⁺ ion content. The unit cell volume was calculated as V = a³ in atomic units (where a = lattice constant). If we speak in terms of volume, it is clear that the volume of unit cell depends on the lattice constant (a). The increase of bismuth doping in the composition, the lattice constant and the volume of unit cell were decreased linearly [21] . Several investigators were observed the similar behaviour of lattice constants with doping concentration. The decrease in lattice parameter on

the basis of relative ionic radii of Cr³⁺ and Fe³⁺ ions [22] [23] .

The X-ray densities were calculated using the following Formula (3) and were tabulated in Table 2.

where, M = Molecular weight of the sample, N = Avogadro number, a = lattice constant.

The variation of X-ray density (d_x) and porosity (P) with the composition (X) is shown in Figure 4. In any ferrite system the variation of X-ray density with composition depends on the lattice constant (a) and total molecular weight (M) of the sample [24] . In this series from the Table 3, it was clear that as the Bi³⁺ ion content increases, the total molecular weight of the sample was also increases. This is due to the increase in mass overtakes the decrease in volume of the unit cell. Therefore greater atomic weight of bismuth ions (208.98 gm/mol) were replaces completely with lesser atomic weight of ferric ions (55.85 gm/mol). From Table 2 and Table 3 it is clearly shown that, the lattice parameter decreases with the increase of bismuth ion composition. Hence from Figure 4 shows the X-ray density (d_x) increases with increase of total molecular weight and it also confirms that the porosity decreases linearly with the composition (X). A similar behaviour of X-ray density with Cr-substitu- tion was presented by Md. Javed Iqbal, Mah Rukh, Sidiqyah in Co-Cr ferrite system [25] .

The distance between magnetic ions (hopping length) in A-site (tetrahedral) and B-site (octahedral) were calculated using the relations (4) & (5) [23] .

where, d_A = hopping length for tetrahedral site, d_B = hopping length for octahedral site and a = lattice constant.

The calculated values of d_A and d_B of different compositions were tabulated in Table 3. The variation of hopping length for octahedral and tetrahedral sites with Bi³⁺ ion composition is shown in Figure 5 and it was observed that the hopping length were decreases with the increase of bismuth composition. It may be due to smaller ionic radius of the Bi³⁺ (0.74 Å) than Fe³⁺ (0.78 Å) [21] .

The Scanning Electron Microscopic (SEM) images of all the synthesized samples were shown in Figure 6. The

morphology of the prepared samples by sol-gel combustion method was studied by SEM (Scanning Electron Microscope) technique. The prepared samples of composition (x = 0.00 to 0.25) have been presented and secondary electron images were taken at different magnifications. It is clear that the SEM images show that the particles have an almost homogeneous distribution and some of them are agglomerated form. It is evidenced by SEM images that the aggregation of particles lies in nanometre region. The formation of ferrite was during sintering the pores between the particles were removed and formed strong bonds in the agglomeration form [24] . The interaction of grain boundary and porosity is important in determining the limited grain size of the particles. With this study it can be observed that the average grain size of the particles were decreasing trend with Bi³⁺ ion doping, moreover it is slightly lesser than determined by XRD and conforming the synthesized samples were crystalline structure.

Figures 7 (a)-(c) Show images of Transmission Electron Microscope (TEM) and Selected Area Electron Diffraction (SAED) patterns of cobalt ferrite of the sample x = 0.10 sintered at 600˚C. The TEM micrographs of these samples show the complete view about crystallite size, morphology and micro structure. From Figure 7(a), it can be observed that the particles were rounded in cubic shape and formed loose aggregates and the particle size was found 50 nm which was a good agreement to XRD result. Some separated particles are also seen in those samples. However some moderately agglomerated particles as well as separated particles also present in the images. Which is due to increases with sintering temperature and hence some degree of agglomeration at this 600˚C appears which unavoidable [25] [26] .

The corresponding selective area electron diffraction (SAED) analysis of the sample x = 0.10 were shown in Figure 7(b) and Figure 7(c). This indicates that Bismuth doped cobalt ferrite nano particles were found well- crystalline in nature. According to this diffraction pattern the measured lattice constant and inter planar spacing d_(hkl) agrees well with the XRD result.

The elemental analysis of the Co-Bi nano ferrite samples with different compositions was done by Energy dispersive spectrometer (EDS). The product of CoBi_xFe_2-xO₄ of all the compositions has been determined by the EDS and the patterns obtained are shown in Figure 8. All the EDS images show the presence of Co, Bi, Fe and Oxygen in the sample which did not contain the elements of Na and other elements. So the results indicated that the cation Na⁺ did not take part in the reaction. Most of the undesired precursor materials like chloride ions have been completely removed from quantification of the peaks were found to be values of 1:1:1 and the elemental % and atomic % of different elements were shown in Table 4.

FTIR spectroscopic analysis is an additional tool for the structural characterisation of the spinal structure of Co-Bi ferrite system. The Figure 9 shows FT-IR spectra of CoBi_x Fe_2-xO₄ (x = 0.00 to 0.25) nano ferrite particles at room temperature in the range of 350 to 900 cm⁻¹. The spectra of all the ferrites have been used to locate the band positions which are summarized in the Table 5. In the present study two prominent absorption bands ν₁ and ν₂ are found at around 596.15 and 393.50 cm⁻¹ respectively for all the compositions. The absorption bands observed within the specific limits reveal the formation of single phase spinal structure having two sub lattices, i.e., tetrahedral (A-site) and octahedral (B-site) [27] . The high frequency band ν₁ called strongest absorption lies in the range 591.33 to 596.15 cm⁻¹ while the low frequency band ν₂ called weakest absorption lies in the range of 384.09 to 393.50 cm⁻¹ and the both the frequency values for all the composition are furnished in Table 5.

It is observed that the differences in the band positions is expected because of the difference in bond length of Fe³⁺ ? O²⁻ ions at the octahedral and tetrahedral sites [28] -[30] . Because of tetrahedral dimensions are less with compared to octahedral site dimensions, the absorption bands have inverse relationship with the bond length. It is also observed that, the bands ν₁ and ν₂ were slightly shifted towards higher frequency side with the increase of bismuth content. Due to decrease in site radius, that increase the fundamental frequency and hence the central frequency should shift towards the higher frequency side [31] [32] .