| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 98 === We have prepared a series of high quality and Li content well controlled LixCoO2 single crystals using electrochemical de-intercalation method. Li content x is determined by means of inductively coupled plasma-mass pectrometer (ICP-MS).We used the flux method to obtain pristine Li0.87CoO2 single crystal. X-ray diffraction and magnetic susceptibility measurements have been performed on single crystal samples of 0.33 ≤ x ≤ 0.87.
In 0.71 ≤ x ≤ 0.87 and 0.33 ≤ x ≤ 0.38 region, the LixCoO2 crystals show pure Curie-Weiss paramagnetic behavior. But for 0.38 < x < 0.71, LixCoO2 crystals exhibit
Pauli paramagnetic behavior. Under the assumption of all spins are localized to follow Curie-Weiss law, we propose that Co4+ in the LixCoO2 (0.33 ≤ x ≤ 0.87) is low-spin state instead of high-spin state, in contrast to that proposed by Hertz et al.[1] for 0.94 ≤ x ≤ 1.00. In the case of Li0.87CoO2, there is a broad peak observed in the magnetic susceptibility near 9 K for H // c only, which indicates the occurrence of a possible A-type antiferromagnetic ordering similar to that found in Na0.82CoO2.[2] Additionally,a small reduction of magnetic susceptibility near 66 K is seen for Li0.53CoO2, which suggests the existence of a spin state transition. Li0.50CoO2 shows a cusp near 200 K when H // ab direction, which is suggested to be an antiferromagnetic transition similar to that found in Na0.50CoO2 near 88 K. Furthermore, magnetic anomaly near 175 K have been observed for Li0.47CoO2, similar anomalies have also been found in Li0.5CoO2, Li0.67CoO2, and Li0.7CoO2 [1,3] powder samples in the past. The nature of these magnetic anomalies is unknown for now, but is possibly related to the spin state transition, charge disproportionation or surface magnetism. All of these observations suggest a complex nature of spin degrees of freedom in these materials. A detailed magnetic phase diagram is constructed and compared with Mukai et al.[4]
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