Structural phase transitions and sodium ordering in Na 0.5CoO 2: a combined electron diffraction and Raman spectroscopy study

Abstract

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 0.7CoO 2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na 0.33CoO 2·1.3H 2O is a newly discovered superconductor with T c∼4 K, and Na 0.5CoO 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 0.5CoO 2 single crystals. A series of structural phase transitions in the temperature range from 80 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.