Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca2-x Bi x-δ MnO4-γ

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca2MnO4-gamma . The electrical conductivity sigma and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity kappa of doped Ca2MnO4-gamma is relatively low, 0.5 W/m K to 1.8 W/m K (27A degrees C to 827A degrees C). Consequently, the ZT value, ZT = sigma S (2) T/kappa, increases with Bi doping. The maximum ZT is 0.023 for Ca1.6Bi0.18MnO4-gamma at 877A degrees C, which is ten times higher than that of the end member, Ca2MnO4-gamma . The increase of ZT mainly results from the considerable increase of sigma, which can be explained in terms of structural change. Thea Mn-O(1) pound and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO3 layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.