Melting of the Na layers in solid Na0.8CoO2.

Data of $^{23}\mathrm{Na}$ NMR spectra and relaxation measurements are interpreted as suggesting that, upon increasing temperature, the Na layers in ${\mathrm{Na}}_{0.8}{\mathrm{CoO}}_{2}$ adopt a 2D liquid state at $T=291\text{ }\text{ }\mathrm{K}$. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K, the $^{23}\mathrm{Na}$ NMR response is similar to that previously observed in superionic conductors with planar Na layers.