Extended “orbital molecules” and magnetic phase separation in Bi0.68Ca0.32MnO3

The low-temperature structure of Bi0.68Ca0.32MnO3 has been solved from electron and neutron diffraction data. The quantitative simultaneous refinement indicates an ordering of the Mn cations in a “stripe/chess”-like pattern. The ordering is accompanied by the formation of short Mn—Mn distances and the rearrangement of the Mn—O bonds indicating the development of complex extended “orbital molecules.” The primary order parameter breaks inversion symmetry and allows the generation of a spontaneous electrical polarization as the secondary order parameter. The neutron data at low temperature indicate the coexistence of a pseudo-CE long-range-ordered structure with a strongly reduced moment and short-range ferromagnetic correlations. These results indicate an intricate competition between the charge, orbital, and magnetic degrees of freedom and the Bi3+ stereoactivity in this manganite system.