Effect of cation stoichiometry on the transport properties of calcium ruthenium oxide ceramics

Seebeck coefficient and electrical resistivity have been determined on a series of compounds having the general composition Ca_nRu_1−nO_y. From property and structural considerations, these compounds have been divided into two categories: one group resembling Ca₂RuO₄ and the second category exhibiting properties similar to CaRuO₃. While the former is an antiferromagnetic insulator, the latter is a paramagnetic metal. The Seebeck coefficient remains relatively unchanged within the members of the two groups, irrespective of the molar ratios of the cations. Even between the two groups of compounds, the Seebeck coefficient exhibits minimal difference even as the cation ratio (Ru/Ca) of the compounds is varied from 0.35 (n = 0.74) to 1.94 (n = 0.34). The resistivity, however, varies by nine orders of magnitude as the cation ratio is varied in the same range. Consequently, the resulting power factor (s²σ, where s is the Seebeck coefficient and σ is the electrical conductivity), which is a measure of the usefulness or thermopower conversion efficiency of a thermoelectric material, is found to vary by nine orders of magnitude.