The infrared conductivity $\sigma \left(\omega \right)$ of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ is studied as a function of doping and temperature for $0.5⩽x⩽1$. A far-infrared peak (FIP) in $\sigma \left(\omega \right)$, which coexists with a small Drude contribution, indicates charge localization in the $\mathrm{Co}{\mathrm{O}}_2$ 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 $E_{1u}$ phonon lifetime shows that ${\mathrm{Na}}^{+}$ ions lattice is “frozen in” at any $T<295\mathrm{K}$ for commensurate $x$ and at $T\lesssim 150\mathrm{K}$ for incommensurate $x$. A comparison with the behavior of the FIP suggests that the ${\mathrm{Na}}^{+}$ “freezing” induces carrier localization only for low charge density and high ${\mathrm{Na}}^{+}$ concentration.

• Received 25 January 2005

DOI:https://doi.org/10.1103/PhysRevB.72.024550