Influence of Ce doping on electrical and thermal properties of La0.7-xCexCa0.3MnO3 (0.0 ≤ x ≤ 0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La0.7-xCexCa0.3MnO3 (0.0 <= x <= 0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x <= 0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T-C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity rho(T) separates the well-define metalsemiconducting transition (T-MS) for low Ce doping concentrations (0.0 <= x <= 0.3) consistent with magnetic transitions. For the samples with 0.4 <= x <= 0.7, rho(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron-phonon and electron-electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x <= 0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron-magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T-C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between rho(T) and S(T) activation energies and a positive slope in thermal conductivity kappa(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites. (C) 2012 Elsevier B.V. All rights reserved.