Effect of Li substitution on the crystal structure and magnetoresistance of LaMnO3

The crystal structure and magnetoresistance of the polycrystalline La1-xLixMnO3 (x=0.10, 0.15, 0.20, 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that the room temperature structural transition from rhombohedral (R (3) over barC) to orthorhombic (Pbnm) symmetry occurs at the Li-doped level x greater than or equal to0.2. Accompanying the occurrence of the structural transition, the lattice distortion and the bending of the Mn-O-Mn bond increase and the ferromagnetic transition temperature T-C decreases. For x=0.10 and 0.15 samples, double metal-insulator (M-I) transitions accompanying a single ferromagnetic transition and a negative magnetoresistance as high as 26% in a magnetic field of 0.8 T are observed. For x=0.20 and 0.30, the samples manifest nonmetallic behavior throughout the measured temperature range. We suggest that the double M-I transitions phenomena of low Li-doped samples originate from the magnetic inhomogeneity due to the formations of the Mn3+ and Mn4+-rich regions induced by partial substitution of the monovalent Li1+ ions for the trivalent La3+ ions. The transport property of high Li-doped samples (x=0.20 and 0.30) can be explained according to the additional localization of e(g) electrons induced by a static coherent Jahn-Teller distortion of the MnO6 octahedra stemming from the structural transition from rhombohedral (R (3) over barC) to orthorhombic (Pbnm) and the reduced bandwidth of e(g) electrons due to the increased bending of the Mn-O-Mn bond. (C) 2000 American Institute of Physics. [S0021-8979(00)04423-6].