Influence of Gd-content on the electrical transport properties of La_(0.67-x)Gd_xSr_(0.33)MnO_3 polycrystalline ceramics by sol-gel method

摘要

Polycrystalline perovskite manganites La_(0.67-x)Gd_xSr_(0.33)MnO_3 (x=0.15, 0.20, 0.25, 0.30, 0.35, and 0.40) were prepared by sol-gel method. The crystal structure examined by X-ray powder diffraction indicates that the samples were single phase and crystallize in a rhom-bohedral structure with increasing Gd content. The grain size and surface morphology were investigated by scanning electron microscope, which reveals that that all the ceramic samples with better crystallization have high density. The relation between resistivity and temperature studied by the standard four-probe method shows that the metal-insulator transition temperature (T_P) shifts to the lower temperature and temperature coefficient of resistivity decrease with increasing Gd doping. It is mainly due to Gd doping decreases the A-site average cationic radius and thus the Mn~(3+)-O-Mn~(4+) angle decrease, which result in the reduction of double exchange between Mn~(3+) and Mn~(4+).