Unconventional charge ordering in Na0.70CoO2 below 300 K?статья
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Аннотация:We report on measurements of the magnetic susceptibility chi(T) and Na-23 nuclear magnetic resonance (NMR) on polycrystalline samples of Na0.7CoO2. The chi(T) data suggest that for T > 75K, the Co ions adopt an effective configuration of Co3.4+. The temperature evolution of 2D exchange Na-23-NMR spectra and of the Na-23 spin-lattice relaxation time T-1 reveal significant anomalies between 230 and 300 K. These features provide evidence for a crossover phenomenon or a phase transition and suggest that, upon decreasing temperature, a dramatic change occurs in the Co 3d-electron spin dynamics, starting just below 300 K and being completed at 230 K. This phenomenom may involve a partial charge ordering within the Co subsystem. (c) 2005 Elsevier B.V. All rights reserved.