Rearrangement of sodium ordering and its effect on physical properties in the NaxCoO2 system

We systematically study Raman spectroscopy of cleaved NaxCoO2 single crystals with 0.37 <= x <= 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.