Thermogravimetric study and high-temperature thermoelectric properties of [Ca₂(Co_1-xA_x)O₃] _0.62CoO₂

Systematic ion substitution has been performed on the Co site in the rock-salt (RS)-type [Ca₂CoO₃] subsystem of misfit-layered cobalt oxide [Ca₂(Co_1-xA_x)O _3-δ]_0.62CoO₂ (A = Nb, Ru, Ta, Pb, W, La and Zr). The oxygen deficiency δ and the decomposition temperature T _d of the substituted samples have been determined by means of the thermogravimetric analysis (TGA). Upon heating, the oxygen atoms are gradually released from the RS-type subsystem up to δ∼0.10, A small amount (x = 0.10) of the substitution remarkably increases T_d from 1173 K (x = 0) up to 1246 K (A = Nb). The Pb-substituted samples show substantial decrease in electrical resistivity ρ froth 19.2 mΩcm to 13.8 mΩcm (x = 0.10) at 950 K. The highest thermoelectric power, S² ρ^-1 = 2.1×10^-4 WK^-2m^-1, is realized for the Pb-substituted sample with x = 0.10 at 950 K.