  The crystal structure of Fe3O4 below the 122 K Verwey transition has been
refined using high-resolution X-ray and neutron powder diffraction data. The
refinements give direct evidence for charge ordering (CO) over four independent
octahedral Fe sites, two with a charge of +2.4 and the other two of +2.6. CO
schemes consistent with our model do not meet the widely-accepted Anderson
condition of minimum electrostatic repulsion. Instead we propose that CO is
driven primarily by a [001] electronic instability, which opens a gap at the
transition through a charge density wave mechanism.
