Oxygen nonstoichiometry and the origin of Na ion ordering in P2-Na[subscript x]CoO[subscript 2]

Abstract

The impact of oxygen deficiency on physical properties of Na[subscript 2/3]CoO[subscript 2−delta] has been investigated. From the combined thermogravimetric, magnetic susceptibility, and synchrotron x-ray Laue diffraction studies, it is demonstrated that Na[subscript 2/3]CoO[subscript 2] shows no superlattice ordering due to Na ions; however Na[subscript 2/3]CoO[subscript 1.98], which has the same Co valence as that of Na0.71CoO[subscript 2], shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of [sqrt]12a. It is proposed that the Na ion ordering found in Na[subscript 2/3]CoO[subscript 1.98] is identical to the ideal Na[subscript 0.71]CoO[subscript 2] of large [sqrt]12a×[sqrt]12a×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.