Unconventional Charge Ordering in Na0.70CoO2 below 300 K
Abstract
We present the results of measurements of the dc-magnetic susceptibility chi(T) and the 23Na-NMR response of Na0.70CoO2 at temperatures between 50 and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an effective configuration of Co3.4+. The 23Na-NMR response reveals pronounced anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is found in chi(T). Our data suggest the onset of a dramatic change in the Co 3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our results maybe interpreted as evidence for either a tendency to electron localization or an unconventional charge-density wave phenomenon within the cobalt oxide layer, CoO2, 3d electron system near room temperature.
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