  Raman spectroscopy measurements have been performed on alpha-, beta-,
gamma-NaxCoO2 phases differing in their stacking of CoO6 octahedra along the
c-axis direction. The results demonstrate that, in general, there are five
active phonons for gamma-Na0.75CoO2, two Raman active phonons for alpha-NaCoO2,
and four Raman active phonons for beta-NaCoO2. We have also performed Raman
scattering measurements on several gamma-(Ca,Sr)xCoO2 (0.15 <= x <= 0.35)
samples which show well-defined intercalated Ca/Sr-ordering. The experimental
data show that the intercalated cation ordering could result in visible
alterations on Raman spectral structures. The observations of the spectral
changes along with the variation of the CoO6 stacking, as well as the
intercalated Sr/Ca ordering suggest that the interlayer interaction plays an
important role for understanding the lattice dynamics in this layered system.
