  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.
