Improved Thermoelectric Performance of (Fe,Co)Sb₃-Type Skutterudites from First-Principles

Skutterudite materials have been considered as promising thermoelectric candidates due to intrinsically good electrical conductivity and tailorable thermal conductivity. Options for improving thermal-to-electrical conversion efficiency include identifying novel materials, adding filler atoms, and substitutional dopants. Incorporating filler or substitutional dopant atoms in the skutterudite compounds can enhance phonon scattering, resulting in reduction of thermal conductivity, as well as improving electrical conductivity. The structures, electronic properties, and thermal properties of double-filled Ca 0.5 Ce 0.5 Fe 4 Sb 12 and Co 4 Sb 12À2x Te x Ge x compounds (x ¼ 0, 0.5, 1, 2, 3, and 6) have been studied using density functional theory-based calculations. Both Ca/Ce filler atoms in FeSb 3 and Te/Ge substitution in CoSb 3 cause a decrease in lattice constant for the compounds. As Te/Ge substitution concentration increases, lattice constant decreases and structural distortion of pnictogen rings in the compounds occurs. This indicates a break in cubic symmetry of the structure. The presence of fillers and substitutions cause an increase in electrical conductivity and a gradual decrease in electronic band gap. A transition from direct to indirect band-gap semiconducting behavior is found at x ¼ 3. Phonon density of states for both compounds indicate phonon band broadening by the incorporation of fillers and substitutional atoms. Both systems are also assumed to have acoustic-mode-dominated lattice thermal conductivity. For the Co 4 Sb 12À2x Te x Ge x compounds, x ¼ 3 has the lowest phonon dispersion gradient and lattice thermal conductivity, agreeing well with experimental measurements. Our results exhibit the improvement of thermoelectric properties of skutterudite compounds through fillers and substitutional doping. V C 2016 AIP Publishing LLC.