LixCoO2單晶樣品之電化學製備與磁性研究
Abstract
本論文利用溶劑生長法來成長LixCoO2 單晶樣品，透過電化學法製備出不同鋰離子含量的LixCoO2 單晶樣品，並使用單晶X 光繞射儀及超導量子干涉儀檢視LixCoO2 單晶樣品的結構變化與磁性特性。
在磁性部份，當鋰離子含量介於0.71 – 0.87 及0.33 – 0.38 的區間內，LixCoO2單晶樣品表現出Curie-Weiss 順磁性行為；但當鋰離子含量介於0.38 – 0.71 的範圍內時，LixCoO2 單晶樣品表現出Pauli 自由電子順磁行為。在LixCoO2 單晶樣品的Co4+皆處於低自旋態而非高自旋態。
鋰離子含量為0.87 的LixCoO2 單晶樣品，當外加磁場平行晶體的c 軸方向，絕對溫度達9 K 時，磁化率出現一個寬峰，推測此行為可能與鈉離子含量為0.82的NaxCoO2 的A 型反鐵磁有序類似。鋰離子含量為0.53 的LixCoO2 單晶樣品，當絕度溫度達66 K，磁化率強度出現一遞減的趨勢，推測為自旋狀態變化所導致此一行為。鋰離子含量為0.50 的LixCoO2 單晶樣品，當絕度溫度達200 K 時，磁化率出現一個小峰，推測為反鐵磁有序的相轉變。鋰離子含量為0.47 的LixCoO2
單晶樣品，當絕度溫度到達175 K 時，會出現磁化率熱滯現象，此現象於LixCoO2粉末樣品中也有發現，此磁性異常行為可能是自旋狀態的相轉變或是電荷分佈不均勻，或表面磁性所致。這些磁性異常現象可能與LixCoO2 單晶樣品的自旋自由度複雜性有關。不同鋰離子濃度的LixCoO2 的磁性相圖已經完整透過單晶樣品製作完成。
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]
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]
Description
Keywords
單晶, 電化學, LixCoO2