New phase transition inNa2Ti2As2Orevealed by Raman scattering

We performed a Raman scattering study of ${\mathrm{Na}}₂{\mathrm{Ti}}₂{\mathrm{As}}₂\mathrm{O}$. 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 fourfold symmetry and more than doubles the unit cell.