The TEM micrograph of a very small amount of as obtained samples placed on the carbon grid shows spherical particles of ~100-300 nm size as reported in our earlier papers [13,14]. However, after agitating ultrasonically and taking the suspended particles on the carbon grid shows well separated particles of size ~10 nm. The micrographs for sample annealed at 1200 ^oC shows that the particle size increases with annealing temperature. It is widely believed that the net decrease in free energy of solid-solid and solid-vapour interface provides the necessary driving force for particle growth during annealing process [15].

The powder X-ray diffractographs of as obtained sample and that of annealed at various temperatures is given in Figure 1. The diffraction pattern of as obtained samples confirms the amorphous nature of the samples. Peaks start appearing and the lines become sharper as the annealing temperature increases due to the grain growth at higher temperatures. The crystallite size was calculated from the most intense peak (311) using Sherrer equation [16]. It is seen that the particle size increases upto ~80 nm as the annealing temperatures are raised to 1200 ^oC. The lattice parameters were calculated using Powley as well as Le Bail refinement methods (built in TOPAS V2.1 of BRUKER AXS) and found to be a = b = c = 8.33103 Å and V = 578.22 ų. The data confirm the presence of a-LiFe₅O₈ formation. Rietvald analysis of the annealed sample further confirms a single phase cubic spinel structure, a-LiFe₅O₈, as given in Figure 2.

X-ray diffraction intensity calculations were carried out using the formula suggested by Buerger [17]

I_hkl = │F_hkl│²PL_p

where, notations have their usual meanings. It is well known that, the intensity ratio of planes I(220)/I(400), I(220)/I(440) and I(400)/I(440) are considered to be sensitive to the cation distribution, therefore, these ratios were used in estimating cation distribution. Finally, the cation distribution is estimated from X-ray intensity calculations for the best fit cation distribution and found to be [Fe₂]^A[LiFe₃]^BO₈.

Magnetic hysteresis loops, at room temperature were recorded for the as obtained as well as annealed sample. Typical loops for as obtained LiFe₅O₈ and after annealing at 1200 ^oC are shown in Figure 3. The as obtained sam-

ple exhibits no hysteresis, which may be attributed to superparamagnetic relaxation as confirmed by XRD. The values of saturation magnetization and magnetron n_B (the saturation magnetization per formula unit in Bohr magnetron) at 300 K for the annealed sample were obtained from the hysterisis loop and found to be 66.92 emu/g and 4.99 respectively. According to Neel’s two sublattice model of ferrimagnetism, the magnetic moment per formula unit (m_B), n_B^N(x) is expressed as:

n_B^N(x) = M_B(x) – M_A(x)

where M_B and M_A are the Band Asublattice magnetic moment in m_B respectively. The n_B^N(x) value was calculated using the ionic magnetic moments of Fe³⁺ and Li⁺ which are 5 m_B and 0 m_B respectively. The calculated value (5 m_B) clearly indicates the inverse spinel structure with lithium and three fifth of the total Fe³⁺ occupying octahedral sites and two fifth occupying tetrahedral site.

Figure 4 shows the typical Mössbauer spectra of LiFe₅O₈ at room temperature for the as obtained sample. The presence of broad doublet in this figure indicates the superparamagnetic nature of the sample. Since in the small particles (<10 nm) a substantial fraction of atoms resides on the surface, a different environment is experienced by them as compared to those inside the particle. Therefore, this broad doublet was fitted with two sets of doublets, one for atoms in the surface region (d = 0.45 mms^-1 and DE_Q = 1.02 mms^-1) and the other for those in the internal region (d = 0.45 mms^-1 and DE_Q = 0.60 mms^-1) of the particles. Larger quadrupole splitting due to surface region atoms compared to internal region atoms can be attributed to the existence of a broader distribution of interatomic spacing and partly disordering in the surface region of the ultrafine particles. It is clear that the contri-

bution of surface region atoms is 42 % of total area of the experiment spectrum. This is in agreement with the percentage (44 %) of the volume of surface region atoms to the total volume of the particle with an average grain size (8 nm) by Ma et al. [18].

The Mössbauer spectra recorded at 300 K after annealing at 1200 ^oC (Figure 4b) exhibit two normal zeeman split sextets due to the A-site Fe³⁺ ions and other due to B-site Fe³⁺ having hyperfine fields 500 and 484 kOe respectively, which indicates ferrimagnetic behaviour of the sample. The cation distribution was also calculated using the intensity ratio of the sextets corresponding to the tetrahedral and octahedral sites. The intensity ratio was found to be 2 : 3 which further confirms that the three fifth iron atoms goes in to octahedral site. The cation distribution obtained from the X-ray intensity data and magnetic moment data agrees fairly well with the cation distribution estimated from Mössbauer data.