  Data of 23Na NMR spectra- and relaxation measurements are interpreted as
suggesting that, upon increasing temperature the Na layers in Na0.8CoO2 adopt a
2D liquid state at T=291 K. The corresponding first order phase transition is
preceded by a rapidly increasing mobility and diffusion of Na ions above 200K.
Above 291 K the 23Na NMR response is similar to that previously observed in
superionic conductors with planar Na layers.
