Electronic structure and lattice dynamics of LixCoO2 single crystals

Abstract

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li _x CoO _2 ( 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 _0.87 CoO _2 because of symmetry-breaking-induced distortions of the hybridized Co-O orbitals with shifted oxygen 2 p states. Furthermore, two Raman-active phonon modes, which display E _g and A _1 _g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li _0.50 CoO _2 , large splitting of the E _g mode and a discontinuous change in the frequency of the A _1 _g mode were observed. These results highlight the importance of spin-phonon coupling in Li _0.50 CoO _2 .

Keywords

• strongly correlated electron system
• optical properties
• Raman and optical spectroscopy
• 71.27.+a
• 71.35.Cc
• 74.25.nd