Structural studies and magnetic and transport properties of Cr-substituted La0.67Ba0.33Mn1-xCrxO3 (0 ≤ x ≤ 0.15) perovskites

We have investigated the structural, magnetic and electrical transport properties of a series of AB(0)O(3)-type perovskite compounds, La0.67Ba0.33Mn1-xCrxO3 (0 <= x <= 0.15), which strongly depend on the doping level x. The slight difference between the ionic radii of Cr3+ and Mn3+ causes no change in the structure when x <= 0.1, remaining rhombohedral (space group R-3C), while for x = 0.15 the structure becomes cubic (space group Pm-3m). Energy dispersive X-ray analysis (EDAX) confirms the expected stoichiometry of all samples. Upon Cr doping on the Mn site, the lattice parameters, the unit cell volume and the B-O-B bond angle are reduced. All samples present a single magnetic transition from ferromagnetic to paramagnetic phase, showing a decrease of the Curie temperature T-c and the magnetization M when x increases (x <= 0.15). However, Cr doping makes the saturation magnetization at 5K to decrease, which indicates that the Cr3+ moments tend to be antiparallel to the Mn3+ moments at low temperature. The Cr-doped manganites exhibit a large variation in resistivity values. The increase of Cr doping (x <= 0.15) leads to an increase of the electrical resistivity. Below 10 at.% of Cr3+, the electrical resistivity shows a metallic behavior, which is well fitted by the relation rho = rho(0) + rho T-2(2) + rho T-4.5(4.5), indicating the importance of the grain/domain boundary, the electron-electron scattering effects and, to a lesser extent, the electron-(magnon, phonon) scattering effects in the mechanism of conduction. On the other hand, the 15 at.% of Cr3+ doping makes the material to exhibit a semiconductor behavior, for which the electronic transport can be explained by a variable range hopping (VRH) and small polaron hopping (SPH) models. Results are consistent with a reduction of the number of available hopping sites for the Mn e(g) (up arrow) electron due to the substitution of Mn3+ by Cr3+, which suppresses the double exchange (DE) interactions. (C) 2012 Elsevier B.V. All rights reserved.