Observation of orbital ordering and Jahn-Teller distortions supporting the Wigner-crystal model in highly dopedBi1−xCaxMnO3

We report on the experimental characterization of orbital ordering and the associated lattice distortions in highly doped Bi_1−xCa_xMnO₃. Resonant x-ray diffraction was used at the Mn L-edge for the direct observation of the ordered localized states, and at the Mn K-edge for the sensitivity to the distortions of the manganese-oxygen octahedra. The orbital ordering on Mn atoms was directly observed at x=0.69; the analysis and the numerical simulations of the K-edge spectra allow us to characterize the pattern of the distorted octahedra at x=⁴/₅. These observations support the Wigner-crystal-type model at both dopings; the bi-stripe model is ruled out at x=0.69.