Structural Phase Transitions and Sodium Ordering in Na0.5CoO2: a Combined Electron Diffraction and Raman Spectroscopy Study

摘要

The nonstoichiometric NaxCoO2 system exhibits extraordinary physicalproperties that correlate with temperature and Na concentration in its layeredlattice without evident long-range structure modification when conventionalcrystallographic techniques are applied. For instance, Na0.7CoO2, athermodynamically stable phase, shows large thermoelectric power;water-intercalated Na0.33CoO2.1.3H2O is a newly discovered superconductor withTc ~ 4K, and Na0.5CoO2 exhibits an unexpected charge ordering transition ataround Tco ~ 55 K. Recent studies suggest that the transport and magneticproperties in the NaxCoO2 system strongly depend on the charge carrier densityand local structural properties. Here we report a combined variable temperaturetransmission electron microscopy and Raman scattering investigation onstructural transformations in Na0.5CoO2 single crystals. A series of structuralphase transitions in the temperature range from 80 K to 1000 K are directlyidentified and the observed superstructures and modulated phases can beinterpreted by Na-ordering. The Raman scattering measurements reveal phaseseparation and a systematic evolution of active modes along with phasetransitions. Our work demonstrates that the high mobility and ordering ofsodium cations among the CoO2 layers are a key factor for the presence ofcomplex structural properties in NaxCoO2 materials, and also demonstrate thatthe combination of electron diffraction and Raman spectroscopy measurements isan efficient way for studying the cation ordering and phase transitions inrelated systems.