Modulated and unmodulated structures, and the transport mechanisms in the triangular lattice system NaxCoO2 with x ≃ 0.48, 0.58 and 0.65

The crystal structures of the triangular lattice system. gamma-phase NaxCoO2 with compositions of x similar or equal to 0.48, 0.58 and 0.65 have been determined by means of x-ray four-circle diffraction. Na0.58CoO2 has lattice constants of a(h) = 2.8180(5) and c(h) = 11.005(9) angstrom with space group P6(3)/mmc and does not exhibit structural modulation. On the other hand, Na0.65CoO2 has a P6(3)/mmc-type superlattice with a doubled unit cell of ash = 5.6527(5) and C-sh = 10.9356(10) angstrom, and Na0.48CoO2 indicates a Pmmn-type superlattice with a(so) = 5.6261(5), b(so) = 11.1406(10) and c(so) = 4.8723(5) angstrom. The structural modulation for x similar or equal to 0.65 is attributed to the incomplete order of Na ions without an order for Co valence, while that for x similar or equal to 0.48 results in almost complete Na order and partial valence order for Co. For x similar or equal to 0.48, the metal-poor metal transition occurs at about 50 K. The electrical resistivities for x similar or equal to 0.58 and 0.65 follow a T-3/2 dependence below 200 and 80 K, respectively, which may be due to antiferromagnetic spin fluctuations in three dimensions. At higher temperatures, the resistivities for the CoO2 plane have a T-1 dependence, likely due to an enhancement of two dimensionality. For x similar or equal to 0.65, the resistivity maximum appears at about 250 K, which may be attributed to the renormalization effect of the Fermi surface by the fluctuations.