The material possesses a symmetry element D6h and Co, O
andNa atoms possess D3d, C3v
d andD3h 1 site symmetries respectively [15].
Out of these, the symmetry elements corresponding to O and Na atoms
result in five Raman active modes of vibration, namely A1g+E1g+3 E2g
[15]. Table 2 summarizes different Raman active vibration modes of
Na0.7CoO2 prepared by different techniques. It is observed that, all the
positions of Raman active vibrations are in good agreement with those
reported forNaxCoO2 compositions [15]. TheE1gmode is attributed to inplane
vibration of oxygen with diagonal displacement, whereas A1g
mode is attributed to out of-plane vibration of oxygen [14,15]. Further,
the presence of the A1gmode strongly depends onNa site occupancy and
Na occupation in the layerwhich divides the octahedral of oxygen in the
c-axis direction. The E2g modes are attributed to combinations of
vibration of both Na and O atoms [14,15]. Careful analysis of Raman
spectra of ball milled and sol gel prepared samples has revealed slight