Crystal structure of the monoclinic and cubic polymorphs of BiMn7O12

The complex perovskite BiMn7O12 occurs with two polymorphic structures, cubic and monoclinic. Currently their crystal structures are investigated with high-resolution synchrotron powder X-ray diffraction at room temperature. Rietveld analysis reveals unusual behavior for, respectively, the oxygen and bismuth atoms in the monoclinic and cubic phases. Bond valence calculations indicate that all the Mn atoms in both the phases are in trivalent state. Possible roles of the 6s2 lone-pair electrons of Bi3+ in BiMn7O12 are discussed in comparison with the LaMn7O12 phase that is isomorphic to monoclinic BiMn7O12. Multiple roles of the lone-pair electrons are revealed, causing (i) A-site cation deficiency, (ii) octahedral tilting, (iii) A-site cation displacement, and (iv) Mn3+ Jahn–Teller (JT) distortion. Relationships between the monoclinic and cubic phases are discussed with emphasis on the MnO2 and MnO6 local structural aspects. All Mn atoms in the monoclinic polymorph have distorted coordination consistent with JT-active Mn(III) high spin, whereas for the cubic polymorph, the B-site Mn atoms show regular octahedral coordination.