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

Using ²³Na NMR measurements on sodium cobaltates at intermediate dopings (0.44≤x≤0.62), we establish the qualitative change in behavior of the local magnetic susceptibility at x^=0.63--0.65, from a low-x Pauli-like regime to the high-x Curie-Weiss regime. For 0.5≤x≤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 x<x^. This contradicts the commonly assumed uncorrelated Pauli behavior in this x range and is at odds with the observed ferromagnetic correlations for x>x^. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T^. From ⁵⁹Co NMR data, it is shown that moving toward higher x away from $x=0.5$ results in the progressive appearance of nonmagnetic Co³⁺ sites, breaking the homogeneity of Co states encountered for x≤0.5. The main features of the NMR-detected ⁵⁹Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co³⁺ 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.