Charge order superstructure with integer iron valence in Fe(2)OBO(3).

Solution-grown single crystals of Fe(2)OBO(3) were characterized by specific heat, Mössbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mössbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe(2)OBO(3) is the clearest example of ionic charge order so far.