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Polarized Raman scattering measurements have been performed on Na0.5CoO2single crystal from 8 to 305 K. Both the A1g and E1g phonon modes show asoftening below Tc1 ~ 83 K. Additionally, the A1g phonon mode, which isforbidden in the scattering geometry of cross polarization for the triangularCoO2 layers, appears below Tc1. In contrast, the metal-insulator transition atTc2 ~ 46 K has only secondary effect on the Raman spectra. The phonon softeningand the ``forbidden'' Raman intensity follow closely magnetic order parameterand the gap function at the Fermi surface, indicating that the distortion ofCoO6 octahedra at Tc1, instead of the Na ordering at ~350 K, is the relevantstructural component of the 83 K phase transition.

Phonon softening and "forbidden" mode observed by Raman scattering in Na0.5CoO2