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.