The Na xCoO 2-δ system - a host lattice for superb thermoelectrics and parent of the watery CoO 2-layer superconductor - is found to be oxygen nonstoichiometric for x≤0.7, such that with decreasing x, the concentration of oxygen vacancies increases. Accordingly, upon Na deintercalation, the Co valence first increases but does not exceed the value of 3.5. To reveal this, the actual valence of Co was systematically determined by means of wet-chemical redox analysis for a series of Na xCoO 2-δ samples with the Na content x ranging from 0.78 down to 0.12. The low-Na-content samples were obtained from Na-rich ones (0.70≤x≤0.78) through chimie douce extraction of Na utilizing I 2, Br 2, or NO 2BF 4 as an oxidizing agent. For the lowest-Na-content sample of x=0.12, the oxygen nonstoichiometry parameter δ was 0.23. The δ-x relation presently established for a wide x range of Na xCoO 2-δ is indispensable for proper interpretations of physical properties of this system.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 1 Mar 2005|
|MoE publication type||A1 Journal article-refereed|