Orbital order–disorder transition in La1−xBaxMnO3 in the low-doping region

We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La1−xBaxMnO3 by high temperature powder neutron diffraction in the low-doping region (0≤x≤0.1). The Jahn–Teller transition temperature which is about TJT≈750 K 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 TJT≈750 K. The negative thermal expansion has also been observed in La1−xBaxMnO3 below TJT but the effect is reduced with doping.