Raman spectra study on nonstoichiometric compound NaxCoO2

Raman spectra of NaxCoO2 (x=0.7, 0.5, and 0.3) single crystals and x=0.7 polycrystals are carefully studied. Our results demonstrate that for Na0.7CoO2, only three Raman peaks, A(1g) at 576 cm(-1), E-2g at 460 cm(-1) from the ab surface, and E-1g at 497 cm(-1) from the ac surface, can be observed at room temperature. For x=0.5, an extra E-2g peak around 430 cm(-1) appears from the ab-plane scattering, and it disappears for x=0.3. Low temperature Raman spectra from the ab plane measured down to 80 K show that with decreasing temperature, E-2g peaks around 430 cm(-1) and 415 cm(-1) appear at a low temperature for x=0.7 and 0.5, respectively. These two E-2g modes are suggested to be closely correlated with the ordering of Na ions in NaxCoO2.