Probing the Na atomic order in Na$_{x}$CoO$_{2}$, x=0.67 and 0.71 by NMR spectroscopy*

The sodium cobaltate Na$_{x}$CoO$_{2}$ has a layered structure, consisting of alternating triangular CoO$_{2}$ and Na planes. Evidences of Na atomic ordering have been reported at certain Na contents by different diffraction experiments. The Co magnetism, strongly influenced by the Na ordering, gives a unique phase diagram in Na$_{x}$CoO$_{2}$. In order to investigate the Na ordering and the Co magnetism, we conducted $^{23}$Na and $^{59}$Co NMR experiments in single crystals Na$_{x}$CoO$_{2}$ for x=0.67 and 0.71. We found that Na$_{0.67}$CoO$_{2}$ does not have well-defined Na structural order. However, the oxygen slightly-deficient sample Na$_{0.67}$CoO$_{1.98}$ shows a superstructure, as evidenced by the narrow and well-resolved NMR spectrum. As for Na$_{0.71}$CoO$_{2}$, Na ordering is also observed. We have tried to solve the Na ordering pattern from our NMR spectra. The results will be discussed and be compared with the existing structural models.