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 $\approx$ 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.

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

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

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Na atomic order, Co charge disproportionation and magnetism in Naxx​CoO22​ for large Na contents

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Na atomic order, Co charge disproportionation and magnetism in Naxx​CoO22​ for large Na contents

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

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

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