The nonstoichiometric Na x CoO 2 system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na 0.7 CoO 2 , a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na 0.33 CoO 2 ·1.3H 2 O is a newly discovered superconductor with T c ~ 4K, and Na 0.5 CoO 2 exhibits an unexpected charge ordering transition at around T co ~ 55 K. Recent studies suggest that the transport and magnetic properties in the Na x CoO 2 system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na 0.5 CoO 2 single crystals. A series of structural phase transitions in the temperature range from 80 K to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO 2 layers are a key factor for the presence of complex structural properties in Na x CoO 2 materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems. 64.70.Rh.; 64.75.+g. PACS:

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