Bi4V2O11 polymorph crystal structures related to their electrical properties

Combining X-ray single crystal and neutron powder diffraction data, the crystal structures of the three polymorphs of Bi4V2O11 were refined. These polymorphs can be described from a common orthorhombic mean cell with approximate parameters: am≈5.5, bm≈5.6 and cm≈15.3 Å. They are all built upon well-defined Bi2O22+ layers spaced by perovskite-like VO3.5 slabs. A large disorder of the oxygen located in these slabs was observed for the γ phase in which the diffusion is very fast. The decrease in temperature leads to an ordering of the oxygen vacancies in the β and α-polymorphs. The oxygen packing in the β-polymorph remains close to that of a distorted perovskite with a vacancy located on one site. This perovskite lattice is not maintained in the α-polymorph. The β to α transition is reconstructive with the formation of typical trigonal bipyramids leading to a 6am superstructure. Because of the high parameter to data ratio, the structure of this α-polymorph was modelled in the smaller 3am superstructure. This superstructure can be explained by an ordering of some oxygen vacancies along [010]. However, this ordering could be more complex and some disorder may remain in this area, which would explain the low activation energy observed for this α-polymorph. The specific structural characteristics of the three polymorphs of Bi4V2O11 are correlated with their corresponding conductivity and activation energy.