Effect of Ca-doping on electrical properties of La_0.46Sm_0.21Sr_0.33-xCa_xMnO₃ ceramics prepared by sol-gel technique

La_0.46Sm_0.21Sr_0.33−xCa_xMnO₃ (x = 0.00, 0.05, 0.10, 0.15, and 0.20) ceramics synthesized by Pechini sol–gel method were investigated by X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectroscopy. Temperature dependence of resistivity (R-T) was measured using the standard four-probe method. All the samples had single phases of orthorhombic perovskite crystal structure. By substituting Sr ion with Ca ion, the samples achieved high crystalline quality, density, and chemical uniformity. After the substitution, the resistivity of samples decreased, and the metal–insulator (M–I) transition temperature (T_P) shifted to a lower temperature, making the M–I transition sharper. Therefore, the temperature coefficient of resistance increased. Moreover, the substitution decreased the A-site average cationic radius <r_A>, thus decreasing the Mn–O–Mn bond angle. This weakens the double exchange between Mn³⁺ and Mn⁴⁺.