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

Abstract

The nonstoichiometric NaxCoO2 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, Na0.7CoO2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na0.33CoO2.1.3H2O is a newly discovered superconductor with Tc ~ 4K, and Na0.5CoO2 exhibits an unexpected charge ordering transition at around Tco ~ 55 K. Recent studies suggest that the transport and magnetic properties in the NaxCoO2 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 Na0.5CoO2 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 CoO2 layers are a key factor for the presence of complex structural properties in NaxCoO2 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.

22 pages, 5 figures

References in corpus (10)

• Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)
• Low temperature phase transitions and crystal structure of Na0.5CoO2
• Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}
• Crystal Structure of the Sodium Cobaltate Deuterate Superconductor NaxCoO2o4xD2O (x=1/3)
• Unconventional Charge Ordering in Na0.70CoO2 below 300 K
• Infrared spectroscopy of the charge ordering transition in Na${}_{0.5}$CoO${}_2$
• Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2.yH2O
• Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O
• Na-site substitution effects of the thermoelectric properties of NaCo_2O_4
• Phase separation, effects of magnetic field and high pressure on charge ordering in y-Na0.5CoO2

Cited by in corpus (6)

• Raman Spectroscopy Study of alpha-, beta-, gamma-NaxCoO2 and gamma-(Ca,Sr)xCoO2
• Charge and Spin Ordering in Insulating Na${}_{0.5}$CoO${}_2$: Effects of Correlation and Symmetry
• Structural properties and cation ordering in layered hexagonal CaxCoO2
• Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
• Sodium Trimer Ordering on NaxCoO2 Surface
• Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53