Raman Phonons and Ageing-Related Surface Disorder in NaxCoO2

TL;DR

This study uses polarized Raman spectroscopy to investigate surface disorder and phonon modes in NaxCoO2 crystals, revealing rapid surface aging and disorder development over time.

Contribution

It provides detailed Raman spectra of NaxCoO2 surfaces and demonstrates how surface disorder evolves within hours to days after preparation.

Findings

Identification of three key Raman-active phonons at specific frequencies.

Significant surface disorder develops rapidly on ac surfaces.

Disorder extends from ac to ab surfaces over several days.

Abstract

The polarized Raman spectra from ab and ac surfaces of single crystal NaxCoO2 (x~0.7), parent compound of recently discovered superconductor NaxCoO2.yH2O, are reported and discussed. The crystals were hexagon platelets of typical size 3x3x0.1 mm. Three of the five (A1g+E1g+3E2g) Raman active phonons were unambiguously identified at 458 (E1g), 494(E2g) and 574 (A1g) cm-1. The spectra from ab and ac surfaces differ significantly and provide evidence that within hours after preparation the ac surface, unlike the ab one, is strongly disordered. Within several days the disorder extends over the ab surface too.