Optical conductivity and charge ordering in NaxCoO2

The infrared conductivity ?(?) of NaxCoO2 is studied as a function of doping and temperature for 0.5<=x<=1. A far-infrared peak (FIP) in ?(?), which coexists with a small Drude contribution, indicates charge localization in the CoO2 layers. Long-range ordering at x=0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At low T and high incommensurate x values, in correspondence with the reported formation of a spin-density wave, the FIP abruptly shifts to higher energy, indicating a deepening of the localizing potential. An analysis of the in-plane E1u phonon lifetime shows that Na+ ions lattice is "frozen in" at any T<295K for commensurate x and at T150K for incommensurate x. A comparison with the behavior of the FIP suggests that the Na+ "freezing" induces carrier localization only for low charge density and high Na+ concentration.