Oxygen nonstoichiometry and the origin of Na ion ordering in P2 -Na _xCoO₂

The impact of oxygen deficiency on physical properties of Na_2/3 CoO_2-δ has been investigated. From the combined thermogravimetric, magnetic susceptibility, and synchrotron x-ray Laue diffraction studies, it is demonstrated that Na_2/3 CoO₂ shows no superlattice ordering due to Na ions; however Na_2/3 CoO _1.98, which has the same Co valence as that of Na_0.71 CoO₂, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of √ 12 a. It is proposed that the Na ion ordering found in Na_2/3 CoO_1.98 is identical to the ideal Na_0.71 CoO₂ of large √ 12 a×√ 12 a×3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.