Raman signatures of lattice dynamics across inversion symmetry breaking phase transition in quasi-1D compound, (TaSe$_4$)$_3$I

TL;DR

This study uses Raman spectroscopy to investigate a temperature-induced inversion symmetry-breaking phase transition in the quasi-1D compound (TaSe$_4$)$_3$I, revealing new vibrational modes, strong electron-phonon coupling, and confirming a displacive-first-order transition.

Contribution

It provides the first detailed Raman spectroscopic analysis of the inversion symmetry-breaking transition in (TaSe$_4$)$_3$I, highlighting the role of electron-phonon coupling and low-dimensional effects.

Findings

Three additional Raman modes in low-temperature phase

Fano-shaped asymmetry indicating strong electron-phonon coupling

Confirmation of displacive-first-order transition nature

Abstract

Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics reveals three additional Raman active modes in the low-temperature non-centrosymmetric structure. Two vibrational modes become Raman active due to the absence of an inversion center, while the third mode is a soft phonon mode resulting from the vibration of Ta atoms along the \{-Ta-Ta-\} chains. Furthermore, the most intense Raman mode display Fano-shaped asymmetry, inferred as the signature of strong electron-phonon coupling. The group theory and symmetry analysis of Raman spectra confirm the displacive-first-order nature of the structural transition. Therefore, our results establish (TaSe$_4)_3$I as a model system with broken inversion symmetry and strong electron-phonon coupling in the quasi-1D regime.