Enhancement of the thermopower in Na_x CoO₂ in the large-x regime (x ≥ 0.75)

We observe two distinct phases of Na_x CoO₂ in the large Na-doping region (x ≥ 0.75), a pure phase H₂, and a mixed phase H₂ + H₃. The H₂ phase is characterized by a steep increase of the Peltier conductivity α defined as J = α (- ∇ T), where J the thermoelectric current driven by the temperature gradient - ∇ T. The mixed-phase state H₂ + H₃ exhibits an unusual layered phase separation with conducting layers existing in parallel with insulating ones. At the critical doping x_p ∼ 0.85, the figure-of-merit Z at 80 K is 40 times larger than that of the as-grown sample (x ∼ 0.7). The Hall coefficient and magnetoresistance are also reported.