New phase transition in Na${}_2$Ti${}_2$As${}_2$O revealed by Raman scattering

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.

5 pages, 3 figures, 1 table