Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

Pauli-like regime to the high-x Curie-Weiss regime. For 0.5x 0.62, the presence of a maximum T in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for xx. This contradicts the commonly assumed uncorrelated Pauli behavior in this x range and is at odds with the observed ferromagnetic correlations for xx. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T. From 59 Co NMR data, it is shown that moving toward higher x away from x = 0.5 results in the progressive appearance of nonmagnetic Co 3+ sites, breaking the homogeneity of Co states encountered for x 0.5. The main features of the NMR-detected 59 Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co 3+ sites. In light of this ensemble of experimental observations, a phase diagram taking into account the systematic presence of correlations and their x dependence is proposed.