| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 100 === 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.
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