Cobalt charge states in sodium cobaltates at intermediate dopings

A likely crucial parameter in explaining the peculiar magnetism of Na(x)CoO(2) cobaltates in their paramagnetic state is their charge and spin states in the cobalt layers, responsible for the electronic properties. We examine here the situation for 0.50 <= x <= 0.62, for which it is shown that an increasing amount of Co(3+) ions appears among similar to 3.5+ ions for increasing x. Together with our previous results, this defines two regions in the phase diagram, with homogeneous Co states for x +/- 0.5 and heterogeneous states for x > 0.5. The links with the sodium structural ordering and the nature of magnetism are discussed. (C) 2008 Elsevier B.V. All rights reserved.