Comment on “Raman spectroscopy study of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ and superconducting ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2\bullet y{\mathrm{H}}_2\mathrm{O}$”

The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed and compared to experimental results from ${\mathrm{Co}}_3{\mathrm{O}}_4$ single crystals. We conclude that on NaCl-flux grown ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ crystals a surface layer of ${\mathrm{Co}}_3{\mathrm{O}}_4$ easily forms that leads to the observation of an intense phonon around $700{\mathrm{cm}}^{-1}$ [Y. G. Shi et al., Phys. Rev. B 70, 052502 (2004)]. The Raman spectra on freshly cleaved crystals from optical floating zone ovens do not show such effects and have a high-frequency phonon cutoff at approximately $600{\mathrm{cm}}^{-1}$ [P. Lemmens et al., Phys. Rev. Lett. 96, 167204 (2006)]. We discuss the relation of structural dimensionality, electronic correlations, and the high-frequency phonon cutoff of the thermolectric cobaltite.

Figure 1

(Color online) Raman scattering spectra of (a) NaCl-flux grown ${\mathrm{Na}}_{0.75}\mathrm{Co}{\mathrm{O}}_2$ at RT (Ref. 7), (b) ${\mathrm{Co}}_3{\mathrm{O}}_4$ at $T=200\mathrm{K}$, and (c) TFZ grown ${\mathrm{Na}}_{0.83}\mathrm{Co}{\mathrm{O}}_2$ at $T=90\mathrm{K}$ (Ref. 8).