Calcium cobaltites, especially Ca 3 Co 4 O 9 with a misfit layered structure, are promising thermoelectric materials due to their suitability for high temperature applications and low densities. T he 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 4 d electrons enhances orbital overlapping which is expected to influence the transport characteristics of CaCo 1- x Ru x O y (CCR) samples, by affecting the spin state of the 3 d 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 4 d 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.
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