  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.
