LAUSR

The electronic and thermoelectric properties of two phases of calcium cobaltite, a misfit layered compound, are investigated and compared using first-principles DFT calculations. The two phases considered here include the conventional bulk phase that consists of alternating layers of Ca2CoO3 and CoO2, and a new phase that consists of alternating layers of CaCoO2 and CoO2, which was recently discovered in nanotubes. Electronic structure calculations reveal that both phases are ferrimagnetic materials with one important difference: the bulk phase is metallic, whereas the nanotubular phase is semiconducting. The metal-to-semiconductor transition that accompanies the Ca2CoO3 to CaCoO2 structural transition is shown to arise from the depletion of free carriers from the donor Ca atoms. The implications of the difference in electronic structure for the thermoelectric performance of these two phases are further examined with Boltzmann transport calculations. Relative to the metallic phase, the semiconducting phase displays appreciably higher Seebeck coefficients at minimal doping levels; these increased Seebeck coefficients compensate for the reduced conductivity and result in large power factors. In conjunction with the fact that the semiconducting phase is peculiar to 1D nanotubes, it is expected that additional effects from quantum confinement could render these low-dimensional materials as promising thermoelectric materials.