Raman Spectroscopy Study of alpha-, beta-, gamma-NaxCoO2 and gamma-(Ca,Sr)xCoO2

TL;DR

This study uses Raman spectroscopy to analyze structural differences and interlayer interactions in various NaxCoO2 and Ca/Sr intercalated CoO2 phases, revealing how stacking and cation ordering influence lattice dynamics.

Contribution

It provides detailed Raman spectral data on different NaxCoO2 phases and shows how intercalated cation ordering affects spectral features and lattice interactions.

Findings

Different numbers of active phonons identified for each phase.

Intercalated Ca/Sr ordering alters Raman spectral structures.

Interlayer interactions significantly influence lattice dynamics.

Abstract

Raman spectroscopy measurements have been performed on alpha-, beta-, gamma-NaxCoO2 phases differing in their stacking of CoO6 octahedra along the c-axis direction. The results demonstrate that, in general, there are five active phonons for gamma-Na0.75CoO2, two Raman active phonons for alpha-NaCoO2, and four Raman active phonons for beta-NaCoO2. We have also performed Raman scattering measurements on several gamma-(Ca,Sr)xCoO2 (0.15 <= x <= 0.35) samples which show well-defined intercalated Ca/Sr-ordering. The experimental data show that the intercalated cation ordering could result in visible alterations on Raman spectral structures. The observations of the spectral changes along with the variation of the CoO6 stacking, as well as the intercalated Sr/Ca ordering suggest that the interlayer interaction plays an important role for understanding the lattice dynamics in this layered system.