Effects of Ag addition on the structural and electrical properties of La0.67Sr0.33MnO3 ceramics

La0.67Sr0.33MnO3: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 R3C. 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 ((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, 367K 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 ((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.