Doping-Induced Metal-Insulator Transition and the Thermal Transport Properties in Ca3-xYxCo4O9

We report the electrical, thermal, magnetic, and thermoelectric properties of Y-doped Ca3Co4O9 from 300 down to 5 K. The results indicate that with Y doping, the increase of resistivity originates from the decreases of carrier concentration and mobility, while the increase of Seebeck coefficient is caused by the reduction of carrier concentration together with the enhanced electronic correlation. Point-defect scattering, is the dominant thermal transport mechanism in this system. Due to the considerable difference in mass between Y3+ and Ca2+, thermal conductivity is observably suppressed by doping. The substitution of Y also disturbs the interlayer ferrimagnetic coupling. The ground state of this System converts front ferrimagnetism to paramagnetism gradually. The alteration of transport properties of Ca3-xYxCo4O9 reveals two Crossovers: the transition from Fermi-liquid-like metal to thermally activated semiconductor occuring at x approximate to 0.25, and the transition from thermally activated semiconductor to two-dimensional variable range hopping semiconductor occurring at x approximate to 0.5. The optimal thermoelectric response In Ca3-xYxCo4O9 is found to exist only at the critical state after which the doping-induced metal-insulator transition takes place. Oil the basis of these experimental results, a possible phase diagram for Ca3-xYxCo4O9 is proposed.