We performed a Raman scattering study of Na2Ti2As2O. We identified a symmetry breaking structural transition at around Ts = 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.PACS numbers: 74.25. Kc, 74.25.nd, 71.45.Lr Because their layered structure resembles both the copper oxide and the iron-based superconductors, special attention is devoted to the ATi 2 P n 2 O (A = Na 2 , Ba, P n = As, Sb, Bi) system [1], in particular regarding anomalies in their transport properties that may be caused by orders competing with superconductivity. Although the maximum T c reported among these compounds is only 6.1 K in Ba 1−x K x Ti 2 Sb 2 O (x = 0.12), their phase diagram points towards a competition between densitywave states and superconductivity [2,3]. However, the origin of the density-wave remains controversial. While some calculations attribute the anomalies of their transport properties to a spin-density-wave (SDW) transition [4][5][6], most experiments rather favor a charge-densitywave (CDW) scenario [7,8]. Moreover, nuclear quadruple resonance (NQR) experiments show that the in-plane 4-fold symmetry at the Sb site is broken below the transition temperature in BaTi 2 Sb 2 O [9]. Recently, neutron diffraction experiments suggest an "intra-unit-cell nematic charge order" in BaTi 2 As 2 O [10], which makes the titanium oxypnictides more intriguing.In this Rapid Communication we focus on Na 2 Ti 2 As 2 O, which has the highest CDW transition temperature among this series [11]. Previous optical spectroscopy measurements indicate two CDW transitions at 42 K and 320 K in this material [12], while there is only little known about the lattice dynamics. Our Raman scattering study of Na 2 Ti 2 As 2 O is supported by first-principles calculations of the vibration modes. We observe a total of 4 single-phonon modes predicted by our first-principles calculations in the high-temperature phase and 1 additional feature assigned to a doublephonon excitation. By doing temperature-dependent Raman measurements, we reveal a structural transition breaking the 4-fold symmetry at around T s = 150 K, which is consistent with a yet not understood anomaly in the electrical resistivity. Combining the results of * p.richard@iphy.ac.cn † dingh@iphy.ac.cn our experiments with calculations, we suggest that this structural transition does not rotate the high-symmetry axes but more than doubles the primitive cell.The Na 2 Ti 2 As 2 O single crystals used in our measurements were grown by the flux method [13]. Freshly grown plate-like samples with typical size of 1 × 2 × 0.06 mm 3 were prepared and electrodes were glued on them in a glove box for avoiding air contamination. The in-plane resistivity was measured using a commercial physical properties ...