LAUSR

Abstract A series of polycrystalline ceramic samples of La 0.7 Ca 0.25 X 0.05 MnO 3 (X = Sr, Pb, and Ba) were prepared using sol-gel method with methanol as the solvent. Resistivity-temperature curves show sharp metal-insulator transitions; furthermore, magnetization-temperature curves reveal a transition from ferromagnetism to paramagnetism. After doping with Sr, Pb, and Ba, both average ionic radius of A site and tolerance factor increase. Double-exchange also increases, which supports the metallicity of polycrystalline ceramic. However, larger lattice distortion is also introduced via substitution of larger ions at A site, and this tends to localize carriers. There is competing relationship between tolerance factor enhancement and lattice distortion; in combination, they determine electronic and magnetic properties of polycrystalline ceramics. As a result, after doping with Sr, Pb, and Ba, resistance temperature coefficient decreases, and metal-insulator transition temperature ( T IM ) increases. Meanwhile, ferromagnetic-paramagnetic transition temperature shows the same tendency as T IM .