Infrared Conductivity and Charge Ordering in NaCoO2
  The infrared conductivity of NaxCoO2 is studied as a function of doping and temperature for x between 0.5 and 1. Charge localization in CoO2 layers shows up through a far-infrared peak (FIP) in the infrared conductivity which coexists with a small Drude contribution. Long-range ordering at x = 0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At high x, the formation of a Spin-Density Wave reported below 22 K dramatically shifts the FIP to higher energy when x is incommensurate with the lattice, indicating an abrupt deepening of the localizing potential. The in-plane E1u phonon lifetime is shown to be sensitive to both "freezing" and ordering of the mobile Na+ ions. A comparison with the behavior of the FIP shows that such "freezing" is not the only origin of charge localization in the CoO layers. 
S.Lupi, M. Ortolani, L. Baldassarre, P. Calvani, D.Prabhakaran, A. T.   Boothroyd