Structural, electronic, and thermoelectric properties of La2CuBiS5

The basic physical properties of La2CuBiS5 are studied by the first-principle calculations and the semiclassical Boltzmann theory. Charge density difference calculations show that electrons accumulate between Bi–S atoms, indicating considerable covalent bonding of Bi and S atoms. A similar charge density difference indicates that the Cu–S bonds also exhibit covalent character. The calculated minimum thermal conductivity of La2CuBiS5 is low, which is conducive to its use as a thermoelectric material. Owing to a bipolar effect, induced by thermal excitation, the material’s Seebeck coefficient decreases sharply at T = 800 K. For the n-type and p-type doping conditions, the largest values of S 2 σ / τ were calculated as −1.71×1011 and 1.837×1011 W K−2 ms−1, respectively. The combination of a large dispersion and a high band degeneracy along the Γ - Y direction in the band structure simultaneously induces the highest S y value and a high σ / τ y value. Thus, the thermoelectric performance of La2CuBiS5 is anisotropic and most favorable along the y direction.