Anisotropic Electron and Phonon Transport Properties in Pnictogen Chalcohalides: PnSI (Pn = Sb, Bi)

Semiconducting heavy pnictogen-containing chalcohalides are gaining wide attention in thermoelectrics, ascribed to their inherently low thermal conductivity. The present work uses first-principles computations to report the excellent thermoelectric performance of weak van der Waals 1D PnSI (Pn = Sb, Bi) crystals. Considering the substantial anisotropy in the crystal structure, the direction-dependent electron and phonon transport properties are studied. At 500 K, SbSI and BiSI exhibit ultralow lattice thermal conductivities (kL) of 0.260 W m–1 K–1 (0.428 W m–1 K–1) and 0.182 W m–1 K–1 (0.311 W m–1 K–1) along the x-direction (y-direction). Simultaneously, the moderate power factors of 4.03 (12.19) and 4.43 (7.91) mW m–1 K–2 are realized at specific hole concentrations under similar conditions. Eventually, the combination of ultralow kL and moderate power factor in the y-direction turned into excellent figure of merit, zT of 1.97 (1.78) and 2.71 (1.71) at 500 K with 7.0 × 1019 (2.7 × 1019) and 4.4 × 1019 (2.0 × 1019) cm–3 carriers for p-type (n-type) SbSI and BiSI, respectively. Therefore, both chalcohalides can rival the existing mid-temperature thermoelectric materials.