Ab initio study of sodium ordering in Na0.75CoO2 and its relation to Co3+/Co4+ charge ordering -: art. no. 172103

Charge ordering and Na-vacancy ordering in Na0.75CoO2 are investigated using first-principles electronic structure methods within the generalized gradient approximation (GGA) and GGA+U approximations. We find that Na ordering is determined not only by a competition between different Na site energies and Na+-Na+ repulsion, but also by the Co4+-Na+ repulsion when charge localization occurs. We predict a ground-state structure for Na0.75CoO2 in good agreement with experimental transmission electron microscopy and neutron-diffraction data and find that Co3+/Co4+ charge ordering is strongly coupled to the Na-vacancy ordering.