Orbital order–disorder transition in La 1− xBa xMnO 3 in the low-doping region

Abstract

We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La 1− x Ba x MnO 3 by high temperature powder neutron diffraction in the low-doping region (0≤ x≤0.1). The Jahn–Teller transition temperature which is about T JT ≈750 K for the stoichiometric LaMnO 3 decreases drastically on doping with Ba. We have shown that the previously identified metrically cubic orthorhombic O-phase for x=0 is metrically orthorhombic and the orthorhombicity increases with doping. The Jahn–Teller transition is accompanied only by the partial reduction of the distortion of the MnO 6 octahedra. From the distortion of the MnO 6 octahedra we have determined the orbital mixing coefficients. Negative thermal expansion has been observed in stoichiometric LaMnO 3 in a temperature range of about 75 K just below T JT ≈750 K. The negative thermal expansion has also been observed in La 1− x Ba x MnO 3 below T JT but the effect is reduced with doping.