Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2

TL;DR

This study identifies six stable Na-ordered phases in gamma-NaxCoO2 single crystals, revealing how Na concentration influences magnetic behavior, ion diffusion, and structural properties, with implications for understanding phase stability and ion ordering.

Contribution

The paper reports the discovery of six distinct stable Na-ordered phases in gamma-NaxCoO2 and analyzes their magnetic, diffusion, and structural properties using advanced electrochemical and characterization techniques.

Findings

Six stable Na-ordered phases identified at specific x values.

Na ion diffusion coefficient varies significantly across x_c ~ 0.53.

Na ordering influences magnetic and transport properties.

Abstract

We report on the preparation and characterization of single crystal gamma phase NaxCoO2 with 0.25 < x < 0.84 using a non-aqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x < 0.84), we find six distinct stable phases with Na levels corresponding to x ~ 0.75, 0.71, 0.50, 0.43, 0.33 and 0.25. The composition with x ~0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x_c ~ 0.53, where the diffusion process above x_c has a diffusion coefficient about five times larger than that below x_c. The series of crystals were studied with X-ray diffraction, susceptibility, and transport measurements. The crystal with x = 0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T = 51 K and 88 K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.