  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.
