Electronic texture of the thermoelectric oxide Na0.75CoO2

TL;DR

This study reveals how Na+ vacancy ordering influences the electronic states in Na0.75CoO2, showing a stable intermediate electronic configuration and its relation to Na+ mobility, which impacts thermoelectric properties.

Contribution

It provides new insights into the electronic texture and Na+ ordering in Na0.75CoO2, highlighting their role in thermoelectric behavior.

Findings

Na+ vacancy ordering affects cobalt electronic states

System adopts an intermediate electronic configuration

Na+ mobility correlates with electronic texture

Abstract

From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30 % of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.