Melting of the cooperative Jahn-Teller distortion in LaMnO_3 single crystal studied by Raman spectroscopy

摘要

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO_3 single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to th tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T_c. The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T_c and be explained by local melting of the orbital order that begins quite below T_c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions.