Doping-induced metal-insulator transition and the thermal transport properties in Ca_3-xY_xCo₄O₉

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 from ferrimagnetism to paramagnetism gradually. The alteration of transport properties of Ca_3-xY_xCo₄O₉ reveals two crossovers: the transition from Fermi-liquid-like metal to thermally activated semiconductor occurring at x φ 0.25, and the transition from thermally activated semiconductor to two-dimensional variable range hopping semiconductor occurring at x φ 0.5. The optimal thermoelectric response in Ca_3-xY_xCo₄O₉ is found to exist only at the critical state after which the doping-induced metal-insulator transition takes place. On the basis of these experimental results, a possible phase diagram for Ca_3-xY_xCo₄O₉ is proposed.