Abstract
Calcium cobaltites, especially Ca3Co4O9 with a misfit layered structure, are promising thermoelectric materials due to their suitability for high temperature applications and low densities. The existence of low spin-state electronic configurations for both and species is one of the key parameter to explain the large thermopower values. Ruthenium oxide, with a layered structure, exhibits strong electron-electron correlation and the extended nature of their 4d electrons enhances orbital overlapping which is expected to influence the transport characteristics of CaCo1-xRuxOy (CCR) samples, by affecting the spin state of the 3d Co ions. The effect on thermopower and electrical resistivity due to partial substitution of Co by Ru ions, up to 0.33 moles, from 300 to 600 K was investigated. A sharp decline in resistivity and in thermopower was observed until a transition ion ratio (TIR), (Ru/(Ru + Co)), of 0.5 is reached, beyond which both the properties became less sensitive to TIR. These variations in the transport properties are explained by the presence of 4d Ru in close proximity to the Co, which could influence the spin and oxidation state of Co ions. The Co rich and Ru rich samples exhibit very distinct microstructures and phase assemblages.
Highlights
Ceramic materials comprising transition metal oxides exhibit a number of unusual and technologically important phenomena, such as high temperature superconductivity in cuprates, colossal magnetoresistance in manganites, and superior thermoelectric properties in layered cobaltites [1,2]
Comparing the resistivities of CCR-4 and -6, it was observed that replacement of Ca by Ru ions reduces resistivity, which can be explained by the extended nature of the Ru 4d electron orbitals and this reduction in resistivity is in line with the results found in literature for similar systems, for example calcium manganites [1,29]
Effective substitution of Co by Ru ions in the calciumcobalt-ruthenium oxide system (CCR), represented by the general formula CaCo1−xRuxOy, with x varying from 0.06 to 0.33, has been demonstrated without the formation of a substantial second phase. Both thermopower and resistivity of the CaCo1−xRuxOy samples decrease sharply until a transition ion ratio (TIR), (Ru/Co + Ru ratio), of 0.5 is reached, beyond which they are insensitive to the TIR
Summary
Ceramic materials comprising transition metal oxides exhibit a number of unusual and technologically important phenomena, such as high temperature superconductivity in cuprates, colossal magnetoresistance in manganites, and superior thermoelectric properties in layered cobaltites [1,2]. Ca3Co4O9 has a two-dimensional (2d) layered structure, which consists of alternating stacks of CaO-CoO-CaO rock salt type and CdI2-type hexagonal CoO2 layers along the c axis [1,4,5,6,7]. Both subsystems have identical a and c but different b parameters. The compound has two crystallographically distinct Co ion sites, electric conduction mainly occurs within the CoO2 sheets because the other Co ion site is situated in the highly oxygen-deficient Ca2CoO3.4 subsystem [5]
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