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

Abstract

Polycrystalline 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 6s2 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 370K down to 330K as x increases to 0.3 from 0, and down to 270K when La is totally replaced by Bi. With an increasing substitution level, the saturation moment (Ms) decreases from the near theory value of 3.67μB down to 2.5μB for x=0.3, and 0.08μB for the totally substituted sample. The sample with x=0.67 shows a very weak ferromagnetic property. However, Ms should not change if only the unchanged Mn3+/Mn4+ ratio is considered in the La0.67−xBixSr0.33MnO3 system. The decrease in Ms can only be explained by an enhanced anti-ferromagnetic coupling that is simultaneous with the weakening of the ferroelectric 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.