Thermoelectric Properties of Ca₃Co2-xMnxO₆ (x = 0.05, 0.2, 0.5, 0.75, and 1).

High-temperature instability of the Ca₃Co4-yO9+δ and CaMnO3-δ direct p-n junction causing the formation of Ca₃Co2-xMnxO₆ has motivated the investigation of the thermoelectric performance of this intermediate phase. Here, the thermoelectric properties comprising Seebeck coefficient, electrical conductivity, and thermal conductivity of Ca₃Co2-xMnxO₆ with x = 0.05, 0.2, 0.5, 0.75, and 1 are reported. Powders of the materials were synthesized by the solid-state method, followed by conventional sintering. The material Ca₃CoMnO₆ (x = 1) demonstrated a large positive Seebeck coefficient of 668 μV/K at 900 °C, but very low electrical conductivity. Materials with compositions with x < 1 had lower Seebeck coefficients and higher electrical conductivity, consistent with small polaron hopping with an activation energy for mobility of 44 ± 6 kJ/mol and where both the concentration and mobility of hole charge carriers were proportional to 1-x. The conductivity reached about 11 S·cm-1 at 900 °C for x = 0.05. The material Ca₃Co1.8Mn0.2O₆ (x = 0.2) yielded a maximum zT of 0.021 at 900 °C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca₃Co2-xMnxO₆ layers between Ca₃Co4-yO9+δ and CaMnO3-δ open for new geometries and designs of oxide-based thermoelectric generators.