Rearrangement of sodium ordering and its effect on physical properties in theNaxCoO2system

Abstract

We systematically study Raman spectroscopy of cleaved ${\text{Na}}_{x}{\text{CoO}}_{2}$ single crystals with $0.37\ensuremath{\le}x\ensuremath{\le}0.80$. The Raman shift of ${A}_{1g}$ mode is found to be linearly dependent on Na content, while the Raman shift of ${E}_{1g}$ mode has an abnormal shift to high frequency around $x=0.5$. The abnormal shift is ascribed to the occurrence of Na rearrangement in O1 structure. Temperature-dependent Raman spectrum for $x=0.56$ sample shows that Na rearrangement transition from O1 structure to H1 structure occurs around 240 K. Electronic transport and susceptibility for the sample with $x=0.56$ show a response to the Na rearrangement transition from O1 to H1 structure and that different Na ordering pattern causes distinct physical properties. These results give a direct evidence to prove Na ordering effect on physical properties of Co-O plane.