Phase segregation in NaxCoO2 for large Na contents

TL;DR

This study uses Nuclear Quadrupole Resonance to analyze phase stability and sodium content in NaxCoO2, revealing phase transformations and the most stable composition at x=2/3.

Contribution

It identifies four stable phases and an intermediate phase in NaxCoO2, and demonstrates how environmental exposure affects phase purity and sodium content.

Findings

Four stable phases and one intermediate phase identified

Room temperature humid air causes phase degradation

x=2/3 is the most stable sodium composition

Abstract

We have investigated a set of sodium cobaltates (NaxCoO2) samples with various sodium content (0.67 \le x \le 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x=2/3 composition which appears to be the most stable phase in this part of phase diagram.