Crystal structure and transport properties of γ-Na_xCoO₂ (x=0.67 - 0.75)

Crystal structure and transport properties of γ-Na_xCoO₂ have been studied in the range of x=0.67 - 0.75. Single-phase samples were prepared by sintering mixture of raw materials, Na₂CO₃ (99.5%) and Co₃O₄ (99.9%). Na/Co composition ratio (x) was determined by inductively coupled plasma (ICP) analysis. The crystal structure parameters were refined by Rietveld analysis of powder neutron diffraction intensities, assuming P6₃/mmc type space symmetry. Little changes in the crystal structure are noticed, except for Na contents. Electric resistivity ρ and Seebeck coefficient S were measured at temperatures between 380 K and 1000 K by the standard four-probe method and temperature-gradient method, respectively. The ρ-T curves exhibit the thermal hysteresis in the initial measurement. But, in the following measurement, the thermal hysteresis is significantly suppressed. The S was slightly higher in the sample with x=0.75 than in the others at any temperatures. Power factor (S²/ρ) reaches its maximum value at x = 0.75 (5.3 × 10^-4 W/mK² at 980 K). Hot pressing technique is effective in removing the thermal hysteresis of ρ as well as increasing the power factor.