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.

• Received 8 January 2016
• Revised 9 March 2016

DOI:https://doi.org/10.1103/PhysRevB.93.140501