Strengthening of Thermoelectric Performance via Ir Doping in Layered Ca3Co4O9 System

The effects of Ir doping on thermoelectric (TE) as well as transport and magnetic properties of Ca3Co4-xIrxO9 (0x0.4) series samples have been investigated systematically. Based on the analysis of X-ray photoelectron spectroscopy data, the valence state of the doped Ir ions is suggested to be +4. As Ir ions are doped into system, both the resistivity and the metal-insulator transition temperature increase till to x=0.3, indicating the more stable spin-density-wave state in these Ir-doped samples. The thermopower increases monotonously with increasing x. For x=0.4 sample, its room-temperature thermopower S-300K reaches 166.2V/K, which is 36% larger than that of the undoped sample. The results show that a proper Ir doping may be an effective route to strengthen the TE performance of Ca3Co4O9 system, which is suggested to originate from the variations of carrier concentration and lattice disharmony induced by Ir doping. At the same time, Ir doping suppresses the low-temperature ferrimagnetic state and also introduces a spin-glass behavior at low temperatures.