Volume contraction and resistivity drop at the Jahn-Teller transition in La_1−xCa_xMnO₃

Abstract

We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transition in La_1−xCa_xMnO₃ by high temperature x-ray powder diffraction with synchrotron radiation and resistivity measurements. The unit cell volume of LaMnO₃ decreases with increasing temperature in a narrow temperature range below TJT≈750 K and then undergoes a volume collapse at TJT. The transition is first order. Similar behavior is also obtained in Ca-doped La_1−xCa_xMnO₃ for x=0.025 and 0.075. The amount of volume collapse, however, decreases with the doping and also the first order discontinuous transition crosses over to a quasi-continuous transition with doping. We interpret the volume contraction at the transition is due to a more efficient packing of the MnO6 octahedra in the orbitally liquid state and the crossover from the discontinuous to the quasi-continuous transition is due to the change in the anharmonic coupling parameter with the hole doping. The resistivity of LaMnO₃ decreases as a function of temperature and then shows abrupt drop at TJT becoming almost temperature independent at higher temperature. The resistivity of La_1−xCa_xMnO₃ also decreases at TJT but the abrupt drop becomes smeared out at higher doping. The similar behavior of the unit cell volume and the resistivity at the Jahn-Teller transition suggests that the volume contraction at TJT causes delocalization of eg electrons.