The Change in Physical Properties of Bi2Ca2Co2O8 Thermoelectric Materials Induced by Pb and Rare-Earth Dopings

We report in this paper the change in the physical properties due to dopant effect in Bi2Sr2Co2O8 parent compound system. The doped samples consists of Pb-doped samples (Bi2Sr2Co2O8, x = 0, 0.5, 1.0, 1.5, and 2.0) and RE-doped samples (Bi2-xPbxCa1.9RE0.1Co2O8, RE = Y, La, Pr, Sm, Eu, Gd, and Ho). The structure of this layered oxide system consist of an alternate stack of CoO2 layer and Bi2Sr2O4 block along the c-axis with misfit structure along b-direction. From the analysis of XRD data it is found that the c-axis parameter reduced significantly in the RE-doped samples compared with those of Pb-doped samples, while the value of misfit degree almost the same. The changes in these lattice structures are accompanied by variation in the spin-state of Co-ions and electrical conductivity. We argue that these structural changes and its effect on the electrical transport properties as well as control of the magnetism plays a pivotal role in determining the thermopower in this class of magnetic thermoelectric materials through the entropy of the charge carriers. This, in turn, will determine the thermoelectric performance for application.