Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co⁴⁺ Ions in the Ca₃Co_4–xRh_xO₉ System

Abstract

The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4–x Rh x O9 (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–x Rh x O9. 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.

Thermoelectric transport in the layered Ca3Co4–xRhxO9 single crystals

“…Below room temperature, the Seebeck coefficient slightly but systematically changes with Rh content; In particular, below 100 K where the Seebeck coefficient shows a metallic behavior, it is considerably enhanced by Rh substitution. The present results are distinct from the previous report using polycrystalline samples which shows that Rh substitution does not affect the Seebeck coefficient [31], probably attributed to an extrinsic effect of averaging large transport anisotropy in the polycrystalline samples. Above room temperature, the enhancement of the Seebeck coefficient seen in the parent compound is significantly suppressed with Rh substitution.…”

“…3(a). This behavior is also reported in the previous study on the polycrystalline samples [31], although the absolute value is quite different because of the single-crystalline nature in the present study. Since A can be scaled to m * 2 , where m * is the effective mass [33], this result indicates that the effective mass becomes small with Rh substitutions, as is naturally understood from a weak correlation effect of Rh 4d electrons compared with Co 3d electrons.…”

“…As claimed in the previous polycrystalline study [31], in the low-temperature Fermi-liquid regime, the A value in the resistivity is reduced by Rh substitution, indicating that an electron-electron correlation effect is weakened by Rh 4d electrons with broad orbitals. We find that, in contrast to such a weak correlation effect, the low-temperature Seebeck coefficient is unexpectedly enhanced in the Rh-substituted single crystals, which may attribute to an enlargement of the pseudogap in the Rhsubstituted samples associated with the short-range order of SDW.…”

“…It is interesting to enlarge the power factor by decreasing effective mass in contrast to the usual approach to the high thermoelectric performance [45]. Moreover, previous report using the polycrystalline samples shows that the Rh substitutions strongly suppress the thermal conductivity in this system [31]. Thus the thermal conductivity of the single-crystalline samples should be investigated as a future issue.…”

“…The physical properties of polycrystalline samples of isovalent Rh-substituted Ca 3 Co 4 O 9 have been recently examined for this purpose [31], however, the polycrystalline samples have an unavoidable extrinsic effect on the charge transport such as the boundary scattering. Also note that the anisotropy of the resistivity in the parent compound is significantly large [7].…”