Comment on "Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2 yH2O"

TL;DR

This paper examines how surface degradation affects Raman spectra of NaxCoO2, highlighting differences between degraded and fresh samples and discussing implications for structural and electronic properties.

Contribution

It provides a detailed comparison of Raman spectra from degraded and fresh NaxCoO2 crystals, clarifying the impact of surface layers like Co3O4 on spectral features.

Findings

Surface degradation leads to a prominent phonon at 700 cm-1.

Freshly cleaved crystals show a phonon cutoff at ~600 cm-1.

Surface layers like Co3O4 influence Raman spectral observations.

Abstract

The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed and compared to experimental results from Co3O4 single crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer of Co3O4 easily forms that leads to the observation of an intense phonon around 700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved crystals from optical floating zone ovens do not show such effects and have a high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96, 167204 (2006)]. We discuss the relation of structural dimensionality, electronic correlations and the high frequency phonon cut-off of the thermolectric cobaltite.