New phase transition in Na$_{2}$Ti$_{2}$As$_{2}$O revealed by Raman scattering

TL;DR

This study reveals a new phase transition in Na$_{2}$Ti$_{2}$As$_{2}$O characterized by a symmetry-breaking structural change and charge instability, identified through Raman scattering and supported by first-principles calculations.

Contribution

The paper uncovers a novel phase transition involving lattice distortion and symmetry breaking in Na$_{2}$Ti$_{2}$As$_{2}$O, combining experimental Raman data with theoretical calculations.

Findings

Symmetry breaking structural transition at ~150 K.

Detection of new Raman peaks below transition.

Evidence of charge-driven lattice distortion.

Abstract

We performed a Raman scattering study of Na$_2$Ti$_2$As$_2$O. We identified a symmetry breaking structural transition at around $T_s = 150$ K, which matches a large bump in the electrical resistivity. Several new peaks are detected below that transition. Combined with first-principles calculations, our polarization-dependent measurements suggest a charge instability driven lattice distortion along one of the Ti-O bonds that breaks the 4-fold symmetry and more than doubles the unit cell.