Structural and thermoelectric power studies of citrate-gelprepared cobalt-substituted lithium nanoferrites

Nanocrystalline lithium cobalt ferrites of compositions [Li0.5Fe0.5]1-x Cox Fe2O4 (where x = 0.0, 0.2, 0.4, 0.6, 0.8, and, 1.0) were synthesized at lower processing temperature (180 C) by non-conventional citrate gel autoignition method using citric acid as a fuel and oxidant. Structural characterization of the sintered samples was carried out by X-ray diffraction (XRD) analysis, FTIR spectroscopy, and transmission electron microscopy (TEM). XRD studies of the prepared samples confirmed the homogeneous single-phase cubic spinel structure with the crystallite sizes ranging from 37 to 43 nm. FTIR studies also revealed the formation of ferrites showing two significant absorption bands. The high-frequency band (t1) around 600 cm-1 was attributed to the intrinsic vibration of tetrahedral complexes, and the low-frequency band (t2) around 400 cm-1 was due to the octahedral complexes. TEM images showed the agglomeration of the particles with an average particle size of 30 nm. Thermoelectric power (TEP) studies of the prepared samples were carried out from 200 to 600 C using a differential method. The variation of Seebeck coefficient (S) with temperature indicated p-type semiconducting nature of the sample at lower temperature and n-type semiconducting nature at higher temperature indicating the conduction due to the electrons. The Curie temperature (Tc) of the prepared samples measured from TEP studies showed a decreasing trend with the increasing Cobalt concentration.