First principles lattice dynamics of NaCoO$_2$

TL;DR

This study uses first principles calculations to analyze phonon properties of NaCoO$_2$, revealing how Na site occupancy affects vibrational modes and providing insights into dielectric and infrared characteristics.

Contribution

It presents the first comprehensive first-principles analysis of phonon modes in NaCoO$_2$, including effects of Na site occupancy and detailed dielectric properties.

Findings

Most phonon modes are insensitive to Na site occupancy.

Two out-of-plane Na-dominated modes show significant frequency variation with geometry.

Calculated Raman-active mode frequencies align with experimental data.

Abstract

We report first principles linear response calculations on NaCoO$_2$. Phonon frequencies and eigenvectors are obtained throughout the Brillouin zone for two geometries with different Na site occupancies. While most of the phonon modes are found to be unsensitive to the Na site occupancy, there are two modes dominated by out-of-plane vibrations of Na giving very different frequencies for different geometries. One of these two modes, the A$_{2u}$ mode, is infrared-active, and can be used as a suitable sensor of Na distribution/ordering. The longitudinal-transverse splitting of the zone-center optical-mode frequencies, Born effective charges and the dielectric constants are also reported, showing considerable anisotropy. The calculated frequencies of Raman-active modes generally agree with the experimental values of corresponding Na de-intercalated and/or hydrated compounds, while it requires better experimental data to clarify the infrared-active mode frequencies.