LixCoO2 單晶樣品之光譜性質研究

Abstract

我們研究摻雜不同鋰離子濃度 LixCoO2 單晶樣品之橢圓偏光與拉曼散射光譜。Li0.33CoO2 室溫吸收光譜顯示 3 個吸收峰，其頻率位置分別約為 1.61 eV、3.35 eV 及 5.21 eV。前二者對應 Co3+ 電子由 t2g 躍遷到 eg 軌域，而後者則與電子由 O 2p 躍遷到 Co 3d 軌域有關。隨著鋰離子濃度增加，吸收峰頻率位置大致維持不變，但Li0.87CoO2吸收峰能量轉變為 3.08 eV、4.55 eV 及 5.76 eV，代表其電子結構產生變化。 其次，Li0.33CoO2 室溫拉曼散射光譜具有 2 個拉曼活性振動模，其頻率位置約為 468 cm-1 ( Eg 對稱性 ) 和 568 cm-1 ( A1g 對稱性 )，分別對應氧原子沿著 ab 平面與 c 軸伸張振動。隨著鋰離子濃度增加，因 c 軸縮短，鍵能增加，故 A1g 拉曼峰展現藍移變化。我們另發現 Eg 拉曼峰的半高寬隨著鋰離子濃度增加而變大，這意味著晶格的無序度擴增。有趣地是，Li0.87CoO2 拉曼散射光譜展現兩個六方晶系結構相的共存，分別為第一六方晶系的 490 cm-1 ( Eg ) 和 598 cm-1 ( A1g ) 拉曼峰與第二六方晶系的 480 cm-1 ( Eg ) 和 570 cm-1 ( A1g ) 拉曼峰，此與之前文獻[J. Raman Spectrosc. 28, 613 (1997).] 的發表結果亦相符合。 最後，當樣品降溫至 200 K 時，Li0.50CoO2 的 Eg 與 A1g 拉曼峰呈現異常藍移，此現象為反鐵磁相轉變所致，A1g 拉曼峰在 200 K 至 120 K 溫度區間卻轉變為紅移，其與 Co3+ 和 Co4+ 的電荷有序性排列有關。Li0.53CoO2 的 A1g 拉曼峰在 66 K 附近產生偏離藍移現象，推測亦與其反鐵磁相轉變有關。Li0.50CoO2 與Li0.53CoO2的高頻雙磁振子拉曼峰之半高寬在尼爾溫度附近變窄，暗指其反鐵磁有序性的相干長度變長。We present spectroscopic ellipsometry and Raman-scattering studies of LixCoO2 single crystals. Room-temperature optical absorption spectrum of Li0.33CoO2 shows three absorption peaks at about 1.61 eV, 3.35 eV, and 5.21 eV. The first two optical excitations near 1.61 eV and 3.35 eV are assigned as electronic transitions between the Co3+ t2g and eg orbitals. The last one near 5.21 eV is associated with charge-transfer transitions between the O 2p and Co 3d states. With increasing Li concentration, the positions of these three absorption peaks remain unchanged. However, the absorption peaks of Li0.87CoO2 shift to 3.08 eV, 4.55 eV, and 5.76 eV, indicating changes of its electronic structures.

Room-temperature Raman-scattering spectrum of Li0.33CoO2 exhibits two phonon modes at about 468 and 568 cm-1, displaying symmetries of Eg and A1g that can be associated with Co-O stretching vibrations along the ab-plane and c axis, respectively. As the concentration of the Li ions increases, the Co-O bond energy strengthens which is caused by the contraction of c axis. As a result, the A1g phonon mode shows a blueshift. Furthermore, the linewidth of Eg phonon mode becomes broader, reflecting an increase of lattice disorder. Interestingly, Raman-scattering spectrum of Li0.87CoO2 shows the coexistence of two hexagonal phases. The phonon modes associated with the first hexagonal structure are observed at about 490 cm-1 (Eg) and 598 cm-1 (A1g), while the phonon modes related to the second hexagonal structure appears at about 480 cm-1 (Eg) and 570 cm-1 (A1g). These results are similar with the previous studies published in J. Raman Spectrosc. 28, 613 (1997). When the sample is cooled from 300 to 200 K, the Eg and A1g phonon modes of Li0.50CoO2 are found to exhibit anomalous hardening related to antiferromagnetic ordering. With further lowering temperature down to 120 K, the A1g phonon mode shows softening affected by charge ordering of Co3+ and Co4+ ions. The A1g phonon mode of Li0.53CoO2 also shows noticeable hardening with antiferromagnetic ordering at 66 K. Finally, the two-magnon excitation observed in Li0.50CoO2 and Li0.53CoO2 shows a narrowing of the resonance linewidth near the Neel temperature, indicating an increased antiferromagnetic correlation length.