  Nickel substitution at the cobalt site in NaxCoO2 induces a Metal Insulator
Transition (MIT) and the temperature at which this occurs (TMIT) increases with
Ni content. Low temperature far infrared measurements on polycrystalline
samples of Na0.75CoO2 and Na0.75Co0.95Ni0.05O2 (TMIT = ~ 150K) were carried out
to look for signatures of this transition. While both the samples show an
evident splitting of the high frequency Co-O mode on lowering the temperature,
dramatic changes are observed in the low frequency sodium mode in
Na0.75Co0.95Ni0.05O2 clearly pointing out to an ordering of the sodium ions in
the low temperature insulating state.
