New phase transition in Na$_{2}$Ti$_{2}$As$_{2}$O revealed by Raman   scattering
  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. 
D. Chen, T. T. Zhang, Z.-D. Song, H. Li, W.-L. Zhang, T. Qian, J.-L.   Luo, Y.-G. Shi, Z. Fang, P. Richard and H. Ding
cond-mat.supr-con cond-mat.str-el