Phonon properties of intrinsic insulating phase of the cobalt oxide superconductor NaCoO2

We report a systematic and detailed lattice dynamical investigation on the intrinsic insulating phase of the cobalt oxide superconductor NaCoO2. The phonon dispersion curves, partial and total phonon densities of states are obtained. The calculated Raman, infrared phonon modes and generalized phonon density of states agree reasonably well with the experimental spectra. These studies have enabled an atomic level understanding of the phonon density of states. The effect of Na site occupancy on the phonon spectra are discussed and found significant differences. The lattice specific heat at constant volume and Debye temperature are also reported. The estimated Debye temperature reveals the importance of phonons in the transport properties in the intrinsic insulating phase of the NaCoO2 compound at room temperature. The temperature dependence of the vibrational amplitudes has also been discussed.