Electronic structure, bonding character, and thermoelectric properties of semiconducting rhenium silicide with doping

The electronic structure and thermoelectric properties of ${\text{ReSi}}_{1.75}$ and its doped systems are studied by the first-principles calculation using the full-potential linearized augmented plane-wave method in the local density approximation with self-interaction correction and the semiclassical Boltzmann theory. ${\text{ReSi}}_{1.75}$ shows narrow gap semiconductor behavior with an indirect gap of 0.12 eV and a direct gap of 0.36 eV. The Fermi levels of Al- and Mo-doped systems move into the valence band. The Al-doped compound remains a semiconductor while the gap of Mo-doped compounds becomes small and nearly disappears. The relations between the electronic structure and thermoelectric properties of doped ${\text{ReSi}}_{1.75}$ are analyzed and explained in detail. Our calculations strongly suggest that an excellent thermoelectric performance can be obtained for $p$-doped ${\text{ReSi}}_{1.75}$ along [100] and for $n$-doped ${\text{ReSi}}_{1.75}$ along [001] compared with the undoped compound.

Figure 1

Band structure (a) and DOS (b) of ${\text{ReSi}}_{1.75}$.

Figure 2

Band structures of (a) ${\text{ReSi}}_{1.625}{\text{Al}}_{0.125}$ and (b) ${\text{Re}}_{0.875}{\text{Mo}}_{0.125}{\text{Si}}_{1.75}$.

Figure 3

Valence bonding charge densities for (a) ${\text{ReSi}}_{1.75}$ and (b) ${\text{Re}}_{0.875}{\text{Mo}}_{0.125}{\text{Si}}_{1.75}$ on the (110) plane. The contours range from $-0.2$ to $0.2\text{e}/{\text{Å}}^3$ and increase by $0.2\text{e}/{\text{Å}}^3$. The solid (dotted) contours represent contours of increased (decreased) charge.

Figure 4

Thermoelectric properties along different directions calculated from the band structure of ${\text{ReSi}}_{1.75}$ at 600 K as a function of carrier concentration. (a) Seebeck coefficient, (b) electrical conductivities relative to relaxation time $\sigma /\tau$, and (c) power factor relative to relaxation time $S^2\sigma /\tau$ as a function of doping.