Phonon softening and "forbidden" mode observed by Raman scattering in Na0.5CoO2

TL;DR

This study uses Raman scattering to observe phonon softening and forbidden mode appearance in Na0.5CoO2, revealing a structural transition linked to magnetic ordering at 83 K, distinct from Na ordering.

Contribution

It reports the first observation of phonon softening and forbidden mode activation associated with a phase transition in Na0.5CoO2 using Raman spectroscopy.

Findings

Phonon modes soften below 83 K

Forbidden A1g mode appears below 83 K

Transition at 83 K linked to CoO6 distortion

Abstract

Polarized Raman scattering measurements have been performed on Na0.5CoO2 single crystal from 8 to 305 K. Both the A1g and E1g phonon modes show a softening below Tc1 ~ 83 K. Additionally, the A1g phonon mode, which is forbidden in the scattering geometry of cross polarization for the triangular CoO2 layers, appears below Tc1. In contrast, the metal-insulator transition at Tc2 ~ 46 K has only secondary effect on the Raman spectra. The phonon softening and the ``forbidden'' Raman intensity follow closely magnetic order parameter and the gap function at the Fermi surface, indicating that the distortion of CoO6 octahedra at Tc1, instead of the Na ordering at ~350 K, is the relevant structural component of the 83 K phase transition.