Electronic structure and lattice dynamics of Li_xCoO₂ single crystals

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li_xCoO₂ (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 Li_0.87CoO₂ 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 E_g and A_1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li_0.50CoO₂, large splitting of the E_g mode and a discontinuous change in the frequency of the A_1g mode were observed. These results highlight the importance of spin-phonon coupling in Li_0.50CoO₂.