Band Structure Engineering of Bi4O4SeCl2 for Thermoelectric Applications

Abstract

The mixed anion material Bi₄O₄SeCl₂ has an ultralow thermal conductivity of 0.1 W m^-1 K^-1 along its stacking axis (c axis) at room temperature, which makes it an ideal candidate for electronic band structure optimization via doping to improve its thermoelectric performance. Here, we design and realize an optimal doping strategy for Bi₄O₄SeCl₂ from first principles and predict an enhancement in the density of states at the Fermi level of the material upon Sn and Ge doping. Experimental work realizes the as-predicted behavior in Bi_4-x Sn _x O₄SeCl₂ (x = 0.01) through the precise control of composition. Careful consideration of multiple accessible dopant sites and charge states allows for the effective computational screening of dopants for thermoelectric properties in Bi₄O₄SeCl₂ and may be a suitable route for assessing other candidate materials.

Figure 1

(a) The unit cell of Bi₄O₄SeCl₂ with labels identifying the two bismuth sites and the ABCBA layered structure (left) and the 3 × 3 × 1 supercell of Bi₄O₄SeCl₂ containing a single dopant atom (shown in black) used for DFT calculations (right). (b) The electronic band structure of Bi₄O₄SeCl₂. (c) The predicted formation energies of Bi₄O₄SeCl₂ containing the candidate dopants as obtained from DFT calculations, where the element on the x axis is the dopant and the lattice site is defined by the legend.

Figure 2

Density of states of (a) Group I (Na, K), Group II (Mg, Ca, Sr, Ba), and Pb-doped Bi₄O₄SeCl₂ at the Bi (2) lattice site; (b) Si-doped Bi₄O₄SeCl₂ at Bi (1) and Bi (2) lattice sites and I-doped Bi₄O₄SeCl₂ at the Se lattice site; (c) Ge-doped Bi₄O₄SeCl₂ at Bi (1) and Bi (2) lattice sites; (d) Sn-doped Bi₄O₄SeCl₂ at Bi (1) and Bi (2) lattice sites.

Figure 3

Unfolded effective band structures of doped Bi₄O₄SeCl₂ using 3 × 3 × 1 supercells containing a single (a) Sn_Bi (1), (b) Sn_Bi (2), (c) Ge_Bi (1), or (d) Ge_Bi (2) subsitiution.

Figure 4

Partial density of states of Bi_4–xD_xO₄SeCl₂ with the dopant elements on different sites (D = Ge, Sn; x = 0.112). The doping sites are (a) Ge_Bi (1) and Ge_Bi (2), simultaneously; (b) Sn_Bi (1) and Sn_Bi (2), simultaneously; (c) two Ge atoms on only Bi (1) sites in the supercell; (d) two Sn atoms on only Bi (1) sites in the supercell; (e) two Ge atoms on only Bi (2) sites in the supercell; and (f) two Sn atoms on only Bi (2) sites in the supercell. For each panel, the top plot (black line) shows the combined DOS of the two dopant atoms, followed by the separate contribution of each of the two dopant atoms (pink and purple lines).

Figure 5

(a) The measured PXRD pattern (black line) and Pawley fit (red line) of single phase Bi_3.92Sn_0.08O₄SeCl₂. (b) The cell volumes of Bi_4–xSn_xO₄SeCl₂ (0 ≤ x ≤ 0.08) samples obtained from Pawley fitting of PXRD data measured with a LaB₆ internal standard plotted against the Sn content measured by SEM-WDX.

Figure 6

(a) The electrical conductivity, (b) Seebeck coefficient, and (c) power factors (σS²) of Bi_4–xSn_xO₄SeCl₂ (0.00 ≤ x ≤ 0.08) materials measured in the in-plane (ab) direction. (d) The power factors of Bi_4–xSn_xO₄SeCl₂ at 420 K. (e) The thermal conductivity (the orange line plots a model of the thermal conductivity up to 420 K so that the zT can be estimated at higher temperatures) and (f) zT of the best performing sample (Bi_3.99Sn_0.01O₄SeCl₂) measured in the in-plane (ab) direction.