| Summary: | 碩士 === 國立東華大學 === 物理學系 === 105 === In this thesis, we report the investigation of the temperature-dependent magnetization (χ), electrical resistivity (ρ), Seebeck coefficient, thermal conductivity (κ), and specific heat (CP) on the Ba-doped manganite compounds La0.8Ca0.2-xBaxMnO3 (0 ≦ x ≦ 0.2) in the temperature range of 10 K to 300 K. It was observed from electrical and magnetization measurements that with increasing barium content both the metal-insulator transition temperature and the ferromagnetic-paramagnetic transition temperature shift towards higher temperatures. The value of electrical resistivity is reduced with increasing the barium content. This is attributed to the fact that the substitution of smaller calcium ions (1.34 Å) by larger barium ions (1.61 Å) increases the A-site radius and in turn decreases the distortion of MnO6 octahedron. As a consequence, a structural transformation from rhombohedral to cubic structure occurs that leads to an enhancement of double exchange interaction. From specific heat measurements, the entropy change increases slightly with increasing barium content, indicating that strength of magnetic ordering increases with barium substitution which is consistent with the magnetization measurements. It is revealed that the electron-electron scattering dominates the electrical transport in the metallic region below the temperature 100 K, while the ρ(T) data is well-described by the small polaron hopping (SPH) model in the insulating region. Analyses of Seebeck coefficient and thermal conductivity data confirm that the SPH model is operative in the insulating regime.
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