Searching for stable Na-ordered phases in single-crystal samples of γ-NaxCoO2

We report on the preparation and characterization of single-crystal gamma phase NaxCoO2 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 similar to 0.75, 0.71, 0.50, 0.43, 0.33, and 0.25. The composition with x similar or equal to 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)similar to 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 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.