Melting of the Na layers in solid Na0.8CoO2

TL;DR

This study uses 23Na NMR to show that Na layers in Na0.8CoO2 transition to a 2D liquid state at 291 K, with increased Na ion mobility indicating a phase change resembling superionic conductors.

Contribution

It provides experimental evidence of a temperature-induced 2D liquid phase in Na0.8CoO2 using NMR techniques, revealing a first order phase transition.

Findings

Na layers become 2D liquid at 291 K

Na ion mobility increases rapidly above 200 K

NMR response similar to superionic conductors

Abstract

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.