Improved electrical and dielectric properties of La-doped Co ferrite
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We report on the enhanced dielectric constant and electrical resistivity of the Co-ferrite (CoO. Fe2O3) by partially substituting Fe with La. Structural characteristics of La-doped Co ferrite namely CoO.Fe1.925La0.075O3 indicate the cubic inverse spinel phase with a small amount of LaFeO3 additional phase. The lattice parameter obtained is 8.401 Å (60.001 Å), which is higher than that reported for Co ferrite (8.387 Å, 60.001 Å). The dielectric constant and electrical resistivity of CoO. Fe1.925La0.075O3 are higher compared with pure Co ferrite. The dielectric constant dispersion of CoO. Fe1.925La0.075O3 in the frequency range of 100 Hz to 1 MHz fits to the modified Debye’s function with more than one ion contributing to the relaxation. Temperature-dependent electrical resistivity curves exhibit two distinct regions indicative of two different types of conduction mechanisms. Analysis of the data indicates that the small polaron and variable-range hopping mechanisms are operative in the 220 to 300 K and 160 to 220 K temperature regions, respectively.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.37
proved electrical properties, specifically enhancement in the electrical and dielectric property of Co ferrites, would be advantageous in such applications. Based on the recent observations that the Co ferrites with better electronic and magnetic performances also served as better candidates for Li batteries and supercapacitors, such enhancement is expected to improve the efficiency if used for electrochemical device applications. However, the electrical properties of a Co ferrite at room temperature are dependent on the exact chemical composition, firing temperature (if any), reactive/processing atmosphere, and the ions that substitute for Fe3+/Fe2+ ions.1,2 In Co ferrite, the relatively large oxygen ions form a face-centered cubic (fcc) lattice. In this cubic closepacked structure two kind of interstitial sites occur, the tetrahedral (A) and the octahedral (B) sites, which are surrounded by four and six oxygen ions, respectively (Fig. 1). In the aforementioned cubic unit cell, 64 tetrahedral sites and 32 octahedral sites are present, of which only 8 and 16, respectively, are occupied by metal ions [Fig. 1(a)].1,2 The tetrahedral (A) sites [Fig. 1(b)] are occupied by the Fe3+ ions, and the octahedral (B) sites [Fig. 1(c)] are occupied by the Co2+ and Fe3+, in equal proportions. The angle A–O–B is closer to 180° than the angles B–O–B and A–O–A, and therefore the AB pair (Fe–Fe) has a strong superexchange (antiferromagnetic) interaction.1–3 Partial substitution of Fe3+ by rare earth ion in the spinel structure has been reported to lead to structural distortion6,8,25 that induces strains and significantly modifies the electrical and dielectric properties. It has been reported that inclusion of Zn, Cu, Co, and Cd in ferrites26,27 leads to an increase in the dielectric constant because of the formation of excess Fe2+, which leads to increase in
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