Structural and thermal properties ofLaMnO3 from neutron diffraction and first principles studies

Abstract

Neutron diffraction experiments have been performed on powder samples ofLaMnO3 below and above the Jahn–Teller transition temperature of 750 K. Experimentalinvestigations are assisted by density functional theory calculations.Theoretical studies are carried out for the orbitally ordered state ofLaMnO3 which allows one to compare the behavior of the orbitally ordered and disorderedstructures as a function of temperature. The temperature dependences of the structuralparameters characterizing the Jahn–Teller distortions are reported and discussed. Agradual departure of the experimental data from theoretical predictions is observed above650 K. In this range of temperatures, anions surrounding the Jahn–Teller active cationsperform more isotropic thermal motion. The onset of structural phase transition induces areduction of the crystal volume by about 0.4% which follows from the structuraltransformations yielding more regular oxygen octahedra formed above the phasetransformation. It is found that above the Jahn–Teller transition the distortions of theMnO6 octahedraare not completely removed. The non-vanishing distortions are accompanied by the lifted degeneracy ofthe Mn eg states. Weak residual distortions can be assigned to the short-range orbital orderthat persists within a local scale but it seems quenched on average giving riseto a disappearance of the long-range order coherency of the Jahn–Teller effect.