Electronic structure and lattice dynamics of LixCoO2 single crystals

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal LixCoO2 (x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33–0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of first-principles calculations, the observed optical excitations were appropriately assigned. The charge-transfer absorption bands shift to higher energies in Li0.87CoO2 because of symmetry-breaking-induced distortions of the hybridized Co-O orbitals with shifted oxygen 2p states. Furthermore, two Raman-active phonon modes, which display Eg and A1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li0.50CoO2, large splitting of the Eg mode and a discontinuous change in the frequency of the A1g mode were observed. These results highlight the importance of spin-phonon coupling in Li0.50CoO2.