Studies On Physical Properties Of Electron-doped Manganites La_1-xTe_xMnO₃ And Ca_1-xCe_xMnO₃

The electron (1e) doped La_1-xTe_xMnO₃ (x=0.05, 0.1, 0.2, 0.3) and the two-electron (2e) doped Ca_1-xCe_xMnO₃ (x=0.05, 0.1, 0.2, 0.3) polycrystal samples have been successfully fabricated by sol-gel method in the present thesis. The structural, magnetic, electrical transport, thermal transport and infrared spectra properties of the La_1-xTe_xMnO₃ series have been systematically studied, and the structures, magnetic properties, and electrical transport investigations were also done on Ca_1-xCe_xMnO₃ samples.X-ray diffraction patterns of La_1-xTe_xMnO₃ show that all the samples crystallized in single phase with rhombohedral structure and R 3c space group. The Curie temperature and the metal-insulator transition temperature increase with increasing of x for the 0.05≤x≤0.2 samples. While the Curie temperature and the metal-insulator transition temperature of the x=0.3 sample is less than that of the x=0.2 sample. The resistivity of low doped manganites (x=0.05, 0.1) does not present metallic properties.While for the x=0.2, 0.3 samples, their resistivity versus temperature curves reveal prominent metal-insulating transition, and the magnetoresistances for them under 30 kOe magnetic field are 59% at 249 K and 56% at 245 K, respectively. For La_1-xTe_xMnO₃ manganites, a prominent feature of thermopower is that it exhibits a peak around Curie temperature. When a 70 kOe magnetic field is applied, magnetothermopower is up to about 70% around Curie temperature. At low temperatures, strong phonon-phonon Umklapp scattering suppresses the increasing of thermal conductivity with increasing temperature. While above Cure temperature, the thermal conductivity increases with increasing temperature. For x=0.05 compound, the effective carriers concentration n^*_eff increases monotonously with increasing temperature, while for x=0.3 compound n^*_eff decreases sharply around Curie temperature with increasing temperature. This is comparable with the variation of resistivity with temperature for these samples. For Ca_1-xCe_xMnO₃, X-ray diffraction patterns show that the structure of samples is orthorhombic lattice for x≤0.1, while it converts to monoclinic lattice for x≥0.2. For the x=0.1 and 0.2 samples, it is observed that the Ce doping induces a charge ordering transition with a broad peak at 173 K and 256 K, respectively. The temperature dependence of resistivity shows the insulating behavior in the whole studied temperature range for all samples. For the 0.1 and 0.2 samples, a magnetoresistance peak occurs around the charge ordering temperature. For Ca_0.95Ce_0.05MnO₃, the resistivity vs magnetic field curve reveals"butterfly"shape, which indicates the spin-dependent tunneling plays a key role on the transport properties of this sample.