  We report a detailed study of de-intercalation of Na from the compound
NaxCoO2-d using an electrochemical technique. We find evidence for stable
phases with Na contents near the fractions ~1/3, 1/2, 5/8, 2/3, and 3/4.
Details regarding the floating-zone crystal growth of Na0.75CoO2 single
crystals are discussed as well as results from magnetic susceptibility
measurements. We observe the presence of significant oxygen deficiencies in
powder samples of Na0.75CoO2-d prepared in air, but not in single crystal
samples prepared in an oxygen atmosphere. The oxygen deficiencies in a
Na0.75CoO2-d sample with d ~ 0.08 remain even after electrochemically
de-intercalating to Na0.3CoO2-d.
