Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co4+ Ions in the Ca3Co4-xRhxO9 System

The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4-xRhxO9 (0 <= x <= 0.4) samples have been investigated systematically. XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal-insulator transition (MIT) and an anomaly of the slope related to the transition from a Fermi liquid to an incoherent metal at low temperatures were observed in the resistivity curve for the undoped sample. As Rh ions were doped into the samples, an additional anomaly and MIT occurred in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition (SST) of Co ions. The low-temperature MIT temperature increased with increasing Rh-doping content, indicating that the spin-density-wave state became stable as a result of the enhanced random Coulomb potential in CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute at the Co3+ sites of CoO2 layers. The substitution induced a partial SST of Co4+ ions from the low-spin to the high-spin state, leading to the formation of a spin-state polaron. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4-xRhxO9. It should be noted that the thermopower of the system did not change obviously with Rh doping, but the thermal conductivity decreased significantly. As a result, the ZT value increased markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reached 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the thermoelectric performance of the Ca3Co4O9 system.