Pudding-mold-type band as an origin of the large seebeck coefficient coexisting with metallic conductivity in carrier-doped FeAs₂ and PtSe₂

We theoretically study the thermoelectric properties of electron-doped FeAs₂ and hole-doped PtSe₂ from a band-structure point of view using first-principles band calculations. The band structure of both materials has a peculiar band shape with a flat portion at the top (bottom) of the band, namely the pudding-mold-type band as found in Na _x CoO ₂. The pudding-mold-type band has a quasi-one-dimensional nature in FeAs₂ and a quasi-two-dimensional nature in PtSe₂. We study the origins of the pudding-mold-type band and find that the dz² orbital in FeAs₂, and not only the p _z but also the p _x and the p _y orbitals in PtSe₂ play an important role in making the pudding-mold-type band. We calculate the Seebeck coefficients by the Boltzmann equation approach using a tight-binding model constructed from first-principles band calculations, finding values close to experimental observations. The present study shows the general efficiency of the pudding-mold-type band. We suggest that an efficient route towards obtaining good thermoelectric materials is to realize ideal pudding-mold-type bands by modification of lattice structures.