Charge order in LuFe2O4: an unlikely route to ferroelectricity
  We present the refinement of the crystal structure of charge-ordered LuFe2O4, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast to the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The non-polar bilayers are inconsistent with charge order based ferroelectricity. 
J. de Groot, T. Mueller, R. A. Rosenberg, D. J. Keavney, Z. Islam,   J.-W. Kim, and M. Angst
cond-mat.str-el