A systematic investigation of lanthanum-based manganite, La0.67Sr0.33MnO3, has been undertaken with a view to understand the influence of varying particle sizes on electrical transport properties. With a view to obtain materials with varying particle size, they were prepared by sol-gel route, sintering at four different temperatures. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD data has been analyzed by Rietveld refinement technique and it has been confirmed that the materials have rhombohedral crystal structure with R3c space group. Metal-insulator transition temperatures (Tp) were found to decrease continuously with decreasing particle size where as ferro to paramagnetic transition temperatures (Tc) are found to remain constant. The magnetoresistance (MR) values are found to increase with decreasing particle size. With a view to understand the conduction mechanism, the electrical resistivity data have been analyzed both in the ferromagnetic metallic (T < Tp) as well as high temperature paramagnetic insulating (T > Tp) regions.

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With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken. These materials were prepared using the citrate–gel route by sintering at 900 °C. All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements. The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained. The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively. Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region.The influence of average A-site cation radius ⟨rA⟩ and cation mismatch parameter σ2 on thermoelectric power (TEP), MR, electrical and magnetic transitions has been analysed. The TEP data at low temperatures (T < TP) have been analysed by considering the magnon concepts, while the high temperature (T > TP) data were explained by the Mott's small polaron conduction mechanism. The effects of ⟨rA⟩ and σ2 on polaron activation and hopping energies were also studied systematically.

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A systematic investigation of La 0.67 Ca 0.33 MnO 3 manganites has been undertaken, mainly to understand the influence of varying crystallite size (nano meter range) on electrical resistivity, magnetic susceptibility and thermoelectric power. The materials were prepared by the sol-gel method of sintering at four different temperatures between 800-1100°C. The samples were characterized by X-ray diffraction and data were analyzed using Rietveld refinement. The metal-insulator transition temperatures (T P ) are found to increase with increasing sintering temperatures, while the magnetic transition temperatures (T C ) decrease.The electrical resistivity and thermoelectric power data at low temperatures (T < T P ) have been analyzed by considering various scattering phenomena, while the high temperature (T > T P ) data were analyzed with Mott's small polaron hopping conduction mechanisms.