Effects of Ag addition on the structural and electrical properties of La 0.67 Sr 0.33 MnO 3 ceramics

Abstract

La_0.67Sr_0.33MnO₃:mol-%Ag _x (LSMO:Ag _x , x = 0, 0.1, 0.2, 0.3 and 0.4) polycrystalline ceramics were prepared by chemical co-precipitation methods. All of the LSMO:Ag _x samples showed rhombohedral perovskite structure with a space group of . The diffraction peaks of metallic Ag did not appear in the XRD graph, due to the Ag substitution at A sites (La³⁺ or Sr²⁺ ions) and the evaporation loss of the element Ag in the process of high-temperature sintering, or the residual amount of Ag beyond the sensitivity of the XRD. With the increase of Ag addition, unit cell volume (V), metal−insulator transition temperature (T_p) and conductivity at T_p (ρ_Tp) of the LSMO:Ag _x samples increased significantly. At the same time, the temperature coefficient of resistance (TCR%), temperature peak of the TCR% (T_k) and the figure of merit (Z) reached the optimal values of 3.3% K⁻¹, 367 K and 0.049 at x = 0.3. Ag substitution at A sites (La³⁺ or Sr²⁺ ions) and gathering at grain boundaries (GBs) were proposed to explain the existence of Ag addition. All results suggested that the enhancement of electrical properties was due to silver-doping and it increased the bond angles of Mn−O−Mn (θ_Mn−O−Mn) and the bond distance of Mn−O (d_Mn−O), improving Mn⁴⁺ concentration, grain sizes and GBs connectivity in LSMO:Ag _x ceramics.