Orbital order–disorder transition in La_1−xBa_xMnO₃ in the low-doping region

Abstract

We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La_1−xBa_xMnO₃ 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 for the stoichiometric LaMnO₃ 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 MnO₆ octahedra. From the distortion of the MnO₆ octahedra we have determined the orbital mixing coefficients. Negative thermal expansion has been observed in stoichiometric LaMnO₃ in a temperature range of about 75 K just below T_JT≈750. The negative thermal expansion has also been observed in La_1−xBa_xMnO₃ below T_JT but the effect is reduced with doping.