Influence of Y3+ doping on the high-temperature transport mechanism and thermoelectric response of misfit-layered Ca3Co4O9

The high-temperature transport and thermoelectric characteristics of Ca3-x Y (x) Co4O9 (x=0-0.75) series were studied up to 1000 K. The results reveal that the substitution of Y3+ for Ca2+ not only increases resistivity but also gradually alters the transport mechanism. The localization of carriers narrows bandwidth, which induces the evolution of the system from metal to variable-range hopping semiconductor and then to thermally activated semiconductor. The increasing thermopower with doping originates from the reduction of carrier concentration along with enhanced electronic correlations. Thermoelectric figure of merit ZT of Ca3Co4O9 system is improved by Y doping. However, the optimal thermoelectric performance is found to only exist at the critical doping level where doping-induced metal-semiconductor transition occurs. This result suggests an intrinsic correlation between transport mechanism and thermoelectric response in this system.