Na order and Co charge disproportionation in NaxCoO2

Abstract

We have synthesized and characterized different stable phases of sodium cobaltates NaxCoO2 with sodium content 0.65<x<0.80. We demonstrate that 23Na NMR allows us to determine the difference in the susceptibility of the phases and reveals the presence of Na order in each phase. 59Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction (≈25%) of cobalt are in a non-magnetic Co3+ charge state whereas electrons delocalize on the other cobalt. The magnetic and charge properties of the different Co sites are highly correlated with each other as their magnetic shift KZZ scales linearly with their quadrupolar frequency νQ. This reflects the fact that the hole content on the Co orbitals varies from site to site. The unusual charge differentiation found in this system calls for better theoretical understanding of the incidence of the Na atomic order on the electronic structures of these compounds.