The checkerboard pattern of Bi0.63Sr0.37MnO3 determined using resonant x-ray scattering at the Mn K edge

A study using resonant x-ray scattering at the Mn K edge has been carried out on a Bi0.63Sr0.37MnO3 single crystal. This compound undergoes a metal-insulator phase transition to the so-called charge ordered (CO) state at about 530 K. Strong resonance signals were observed at room temperature as the energy was tuned through the Mn K edge for several superstructures of the CO phase. The energy, polarization and azimuth angle dependences agree with a checkerboard ordering in the ab plane of manganese atoms of two types, in terms of their different local geometrical structures. One of the sites is anisotropic-a tetragonal distorted oxygen octahedron-and the other is isotropic-a nearly undistorted one, as observed for Bi0.5Sr0.5MnO3 and other half-doped manganites. This result indicates that the checkerboard pattern is strongly stable and extends to doping concentrations x<0.5. No superstructures corresponding to the doubling of the c axis were detected. Intermediate valence states lower than 3.5, according to the fractional charge segregation, were deduced for the two non-equivalent Mn atoms, i.e. Mn3.30+ and Mn3.44+