Large enhancement of the thermopower in Na(x)CoO2 at high Na doping.

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate Na(x)CoO2 exhibits several interesting electronic phases as the Na content x is varied, including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, Na(x)CoO2 has moderately large thermopower S and conductivity sigma. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.6<x<0.7). Here we report that, in the poorly explored high-doping region x>0.75, S undergoes an even steeper enhancement. At the critical doping x(p) approximately 0.85, Z (at 80 K) reaches values approximately 40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.