Checkerboard-ordered pattern of Bi0.5Sr0.5MnO3 low-temperature phase probed by x-ray resonant scattering

Abstract

Resonant x-ray scattering (RXS) at the Mn K edge has been used to characterize the low temperature ordered phase of Bi0.5Sr0.5MnO3. Strong resonances were observed for the (030), (050), (05/20), (0 7/2 0), (4 0), and (3 0) reflections as the photon energy is tuned through the Mn K edge. The reported azimuthal and polarization dependence of the resonant intensities indicates that the low temperature phase is described as a checkerboard ordering of two types of Mn sites with different local geometrical structures. One of the sites is anisotropic, a tetragonal distorted oxygen octahedron and the other one is isotropic, a nearly undistorted oxygen octahedron. The distinction of these two Mn sites is accompanied by a displacement of the Mn atoms transverse to the b axis. We conclude that this checkerboard-ordered pattern is a common ground state in half-doped manganites. Intermediate valence states according to fractional charge segregation were deduced for the two nonequivalent Mn atoms, far from the localized Mn3+ and Mn4+ ionic species. Hence, the experimental data discard the ionic model of charge and orbital ordering of localized 3d (or 3d-like) states at the Mn site (or the MnO6 cluster). We suggest a description of the RXS in terms of band structure effects and lattice dynamics.