Study of cation distribution in lithium doped nickel ferrite

Abstract

The structural and magnetic properties of ferrites are sensitive to the nature, the valence state and distribution of metal ions over tetrahedral (A) and octahedral (B) sites in the spinel lattice. Therefore, the knowledge of cation distribution and spin alignment is essential to understand the magnetic properties of spinel ferrites. The outstanding fact about the ferrites is that they combine extremely high electrical resistivity with good magnetic properties. This means that the ferrites are more suitable for high frequency and low loss applications as compared to ferromagnetic metals. For more than a decade, lithium ferrite materials have dominated in the field of microwave applications. Several series of lithium ferrite covering wide range of properties have become commercially available. Suitable substituents in lithium ferrites have made it possible to tailor the materials properties for a variety of diverse requirements of microwave devices. Lithium ferrites also have the inherent properties of high Neel temperatures, rectangular hysterisis loop and high dielectric constant combined with low cost. There are many research reports in the literature on the studies of Li-Zn, Li-Mg, Li-Ti, Li-Cd ferrites [1– 4]. However, no information is available in the literature regarding the Li-Ni ferrite and relative site preference of Li+1, Fe+3 and Ni+2 in the spinel lattice. The spinel system Li0.5Fe2.5−x Mx O4 (where M = V, Cr and Rh) has been studied by Blasse [5]. It has been observed that substitution of V, Cr and Rh causes migration of Li ions for B-sites to A-sites. It has also been observed that Li ions occupy B-sites in pure Li-ferrite. The above results indicate that the Li ions have B-site preference and therefore it is interesting to study the sites occupancy of Li ions in the presence of the cations having strong B-site preference. Kapitonove [6] studied the distribution of cations in Li0.5Fex Ga2.5−x O4 with varying composition and preparation conditions. Nickel ferrite (NiFe2O4, x = 0.0) is an inverse spinel with all the Ni ions on B-sites. In order to study the relative site preferences for Li1+ and Ni2+ ion for octahedral site in spinel lattice, the spinel series Li0.5x Ni1−x Fe2+0.5x O4 with x = 0.0–0.8 has been prepared. The cation distributions have been determined through X-ray diffraction and confirmed by magnetization measurements. It is found that a greater percentage of Li1+ ions occupy the A-sites in the Ni-rich samples. This is reflected in the variation of lattice constant, magnetization and Neel temperature with Li-substitution supported by IR spectroscopy. The thermal variation of low field a.c. susceptibility exhibits normal ferrimagnetic behavior. Eight compositions of Li-Ni system were prepared by standard ceramic technique where stoichiometric proportions of Li2Co3, Fe2O3 and NiO powder were ground in acetone. The homogeneous mass was pelletized using 2% solution of polyvinyl acetate as binder medium. The pellets were presintered at 1000 ◦C for 12 h. In the final sintering process the materials were held at 1200 ◦C for 12 h and furnace cooled at the rate of 2 ◦C per min to room temperature. The X-ray diffraction patterns were recorded for all the samples on a Philips diffractometer model PW 1710. The magnetization measurements were made at 300 K using the high field hysterisis loop tracer. The low field (39.8 A/m) a.c. susceptibility measurements of powdered samples were taken in the temperature range 300–900 K using the double coil set-up, operating at a frequency of 263 Hz. The infrared spectra were recorded at 300 K on Perkin-Elemer IR spectrometer in KBr medium, in the frequency range 1000 cm−1–400 cm−1. The X-ray diffraction (XRD) patterns showed that the samples were single phase spinels. No reflections other than those belonging to a spinel structure were observed in the patterns. The variation of lattice constant a (nm) with content x is shown in Fig. 1. From Fig. 1 it is seen that the lattice constant value is not much influenced by Li1+ substitution (within the error) in NiFe2O4 (x = 0.0) up to x = 0.3, thereafter, it gradually decreases with the average rate of 0.0008 nm per 10% of Li1+-substitution. This happens for the Ni-rich samples (x 0.3, a greater fraction of Li1+ migrates to octahedral (B) sites with decreasing Ni concentrations. The X-ray density (dx) decreases with Li1+ substitutions because of the fact that the rate of decrease of molecular weight is faster than that of the volume of the unit cell per 10% of lithium concentration. The XRD intensities were calculated by using the formula suggested by Burger [7].