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26 April 2024 |
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Mechanism and kinetics of phase formation of cobalt oxyhydrates (Na,K)x(H2O)yCoO2-delta synthesized using aqueous permanganate solution route | | Chia-Jyi Liu
; Wen-Chin Hung
; Chia-Yuan Liao
; Jung-Sheng Wang
; Hwo-Shuenn Sheu
; | | Date: |
27 Mar 2006 | | Subject: | Superconductivity; Materials Science | | Abstract: | We have synthesized the potassium sodium cobalt oxyhydrates (Na,K)x(H2O)yCoO2-delta using the aqueous KMnO4 solution. According to the chemical analyses, it is found that a partial or even almost complete replacement of K+ for Na+ in the alkaline layers. Direct formation of the c = 13.9 angstrom phase is apparently associated with the larger size of K+ (~ 1.4 angstrom) as compared to Na+ (~ 1.0 angstrom). It was thought that the Br2/CH3CN route involves an oxidative de-intercalation process. However, we have found that de-intercalation of Na+ can be simply achieved by immersing gamma-Na0.7CoO2 in the tap water, which indicates that de-intercalation of Na+ is not necessarily brought about by oxidation, instead it should be associated with the escape tendency of the Na+ of the 2b site due to its stronger Na-Co Coulomb repulsion. Formation of the (Na,K)x(H2O)yCoO2-delta not only involves the de-intercalation, oxidation, and hydration process, but also an ion exchange reaction. The hydration reaction is a slow process based on a systematic study on the phase formation of (Na,K)x(H2O)yCoO2, particularly when using aqueous KMnO4 solution with low molar ratio of KMnO4/Na. According to the Co K-edge XANES spectra, the oxidation number of the Co for Na0.33K0.02(H2O)1.33CoO1.95 (c = 19.6 angstrom) is higher than that of Na0.07K0.21(H2O)0.73CoO1.81 (c = 13.9 angstrom) with a chemical shift of 4.1 eV, which is consistent with the iodometric titration analyses with Co3.56+ for the former as compared to Co3.34+ for the latter. The oxygen deficiency delta = 0.19 for Na0.07K0.21(H2O)0.73CoO1.81 might be responsible for being unable to form the c = 19.6 angstrom phase due to being unable to stabilize the hydrated structure of bilayers of water molecule in the lattice. | | Source: | arXiv, cond-mat/0603701 | | Services: | Forum | Review | PDF | Favorites | |
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