Raman spectra of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ ($x=0.7$, 0.5, and 0.3) single crystals and $x=0.7$ polycrystals are carefully studied. Our results demonstrate that for ${\mathrm{Na}}_{0.7}\mathrm{Co}{\mathrm{O}}_2$, only three Raman peaks, $A_{1g}$ at $576{\mathrm{cm}}^{-1}$, $E_{2g}$ at $460{\mathrm{cm}}^{-1}$ from the $ab$ surface, and $E_{1g}$ at $497{\mathrm{cm}}^{-1}$ from the $ac$ surface, can be observed at room temperature. For $x=0.5$, an extra $E_{2g}$ peak around $430{\mathrm{cm}}^{-1}$ appears from the $ab$-plane scattering, and it disappears for $x=0.3$. Low temperature Raman spectra from the $ab$ plane measured down to $80\mathrm{K}$ show that with decreasing temperature, $E_{2g}$ peaks around $430{\mathrm{cm}}^{-1}$ and $415{\mathrm{cm}}^{-1}$ 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 ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$.

• Received 11 December 2005

DOI:https://doi.org/10.1103/PhysRevB.73.092518