Searching for stable Na-ordered phases in single-crystal samples of γ- Nax CoO2
Journal
Physical Review B - Condensed Matter and Materials Physics
Journal Volume
76
Journal Issue
18
Date Issued
2007
Author(s)
Abstract
We report on the preparation and characterization of single-crystal γ phase Nax CoO2 with 0.25≤x≤0.84 using a nonaqueous 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 xc ∼0.53, where the diffusion process above xc has a diffusion coefficient about five times larger than that below xc. 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 and 88 K. The resistivity of the Curie-Weiss metallic Na0.71 CoO2 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. © 2007 The American Physical Society.
Type
journal article