Na atomic order, Co charge disproportionation and magnetism in Na x CoO 2 for large Na contents

We have synthesized and characterized four different stable phases of Na ordered Na x CoO 2 , for 0.65< x <0.8. Above 100 K they display similar Curie-Weiss susceptibilities as well as ferromagnetic q =0 spin fluctuations in the CoO 2 planes revealed by 23 Na NMR data. In all phases from 59 Co NMR data we display evidences that the Co disproportionate already above 300 K into non-magnetic Co 3+ and magnetic ≈Co 3.5+ sites on which holes delocalize. This allows us to understand that metallic magnetism is favored for these large Na contents. Below 100 K the phases differentiate, and a magnetic order sets in only for x ≳0.75 at T N =22 K. We suggest that the charge order also governs the low- T energy scales and transverse couplings.