Effect of progressive substitution of La3+ by Bi3+ on the structure, magnetic and transport properties of La0.67Sr0.33Mno3

yPolycrystalline La0.67-xBixSr0.33MnO3 (x = 0, 0.1, 0.2, 0.3 and 0.67) was prepared by solid-state reaction. The effects of La-substitution by Bi with a typical polarized lone pair electron character on the structure, magnetic and transport properties are presented. The results show that the polarized lone pair 6s(2) electrons have a dramatic effect on these properties. Structure refinements by the Rietveld method show that the substitution elongates the a- and c-axes, and eventually changes the structure from rhombohedral to tetragonal. At the same time, the elongated Mn3+-O-Mn4+ chain weakens the double exchange between adjacent Mn3+ and Mn4+ ions via the O bridge. As a consequence, the ferromagnetic coupling temperature decreases from 370 K down to 330 K as x increases to 0.3 from 0, and down to 270 K when La is totally replaced by Bi. With an increasing substitution level, the saturation moment (M-s) decreases from the near theory value of 3.67 muB down to 2.5 mu(B) for x = 0.3, and 0.08 mu(B) for the totally substituted sample. The sample with x = 0.67 shows a very weak ferromagnetic property. However, M-s should not change if only the unchanged Mn3+/Mn4+ ratio is considered in the La0.67-xBixSr0.33MnO3 system. The decrease in M-s can only be explained by an enhanced anti-ferromagnetic coupling that is simultaneous with the weakening of the ferroelectdc coupling by Bi. With increasing Bi content, the resistivity increases, and the temperature of the semiconductor to metallic transition also rises. Eventually the totally substituted sample becomes an insulator. (C) 2004 Elsevier B.V. All rights reserved.