College Park, Maryland      June 6 - 10 , 2004

MP38: Nature of Jahn-Teller distortion in LaMnO3: order to disorder

X. Qiu, S. J. L. Billinge (Department of Physics and Astronomy, Michigan State University, E. Lansing, MI, 48824), Th. Proffen (Los Alamos Neutron Science Center, Los Alamos National Laboratory)

One fully Jahn-Teller (JT) distorted MnO6 octahedron in the undoped LaMnO3 contains two long (2.16 ┼), two medium (1.96 ┼), and two short (1.92 ┼) Mn-O bonds. The low temperature orthorhombic phase results from the long range cooperative ordering of the JT distorted orbitals (long and short Mn-O bonds alternating in ab plane). Excellent agreement between local and average structure studies confirms the static nature of the JT distortion. At high temperatures across the Jahn-Teller structural phase transition, average structure studies found that the MnO6 octahedron has six almost equidistant Mn-O bonds (delta d < 0.05 ┼), and the high temperature orthorhombic phase is pseudo cubic. However, evident from local structure studies, the MnO6 octahedra are fully JT distorted at all temperatures, suggesting they are oriented along all directions with equal probability at high temperatures. Here presented are experimental evidences from the combined local and average structure analysis using the atomic pair distribution function (PDF) technique. We further argue that the JT distorted orbitals are also dynamic, explaining the observed drastic drop of the resistivity across JT transition. The ôlocalö ordering length of MnO6 will also be discussed.

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