Effect of A-site cationic radius on ceramic La_0.67−xDy_xSr_0.33MnO₃ prepared by sol–gel technique

La_0.67−xDy_xSr_0.33MnO₃ (x = 0.1, 0.12, 0.15, 0.20, 0.25, 0.30, 0.32) ceramics were prepared by sol–gel method. The samples were characterized by scanning electron microscopy, X-ray diffractometry, and resistance–temperature test (ρ–T) method. The experimental results showed that all of the samples were composed of a single phase with the orthorhombic perovskite structure and Pnma space group. The samples had high density, the average size of the grain decreased from 11.83 μm to 7.24 μm, while the number of grain boundaries increased. The one reason why resistance (ρ_max) increases was supposed to be the boundary scattering enhancement, while the temperature corresponding to the peak resistivity (T_P) decreases. The average cation radius of the A-site decreased as the amount/concentration of doping with Dy³⁺ increased, changing the bond length and bond angle. Meanwhile, the double exchange was suppressed, this being another and more important reason why the resistance increased and the T_P decreased. The metal–insulator transition temperature wide degree (ΔT) and resistivity (ρ) are the important factors in the temperature coefficient of resistance. In the competition of the two factors, the ρ occupied an advantageous place, therefore the electrical properties of the materials can be further optimized by adjusting the average cation radius of the A-site.