Phonon scattering and stability of Na_0.5CoO_2
  Raman scattering experiments have been performed on Na_xCoO_2 as function of temperature at the composition x=0.5 where a structural instability and a metal-insulator transition have been observed. Three additional phonon modes are observed compared to samples with larger x. An in-situ annealing study (T_max=550 K) with the initial presence of water vapor at ambient conditions reveals an irreversible structural instability of this composition. 
X.N. Zhang, P. Lemmens, V. Gnezdilov, K.Y. Choi, B. Keimer, D.P. Chen,   C.T. Lin, F.C. Chou