Fabrication of La _x Nd_0.67−x Sr_0.33MnO₃ polycrystalline ceramics by sol–gel method

A systematic experimental study for La _x Nd_0.67−x Sr_0.33MnO₃ polycrystalline ceramic samples (x = 0.00, 0.07, 0.08, 0.09, 0.1, 0.2 and 0.3) were prepared by sol–gel method. The crystal structures were analyzed by X-ray diffraction, which reveals that all the samples are single-phase without any detectable secondary phases. The grain size and surface morphology were investigated by scanning electron microscope, which indicates that the ceramic samples with better crystallization have high density and chemical uniformity. The relation between resistivity and temperature shows that the resistivity decreases and the metal–insulator transition temperature shifts to higher temperature with the substitution La ion for Nd ion, which gives rise to increasing transition broadening temperature, and consequently decreases the temperature coefficient of resistivity. It is mainly due to the substitution La for Nd increases the A-site average cationic radius and thus the Mn–O–Mn angle increases, which gives rise to the double exchange between Mn³⁺ and Mn⁴⁺.