Cooperative Jahn-Teller effect and electron-phonon coupling in La1−xAxMnO3

Abstract

A classical model for the lattice distortions of ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{A}}_{\mathit{x}}$ ${\mathrm{MnO}}_3$ is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of ${\mathrm{LaMnO}}_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely ≊0.6 eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x dependence of the transition temperature is argued to occur because added holes produce a ‘‘random’’ field which misaligns the nearby sites.