Intrinsic ultra-low lattice thermal conductivity in orthorhombic BiSI: An excellent thermoelectric material

Eco-friendly chalcohalide compounds composed by heavy elements are the interesting candidates for thermoelectric energy conversion applications due to their intrinsically low lattice thermal conductivity, abundance and semiconducting behavior. This piqued the present study to investigate BiSI chacohalide for its feasibility in thermoelectric conversion through first principles computations. The deep insights into mechanical and lattice dynamical properties exposes the stability of BiSI. The transport properties of BiSI as a function of carrier concentration builds noticeable power factor of 1.3 mW m ⁻¹ K ⁻² (300 K) and 1.9 mW m ⁻¹ K ⁻² (500 K). The calculated lattice thermal conductivity of BiSI is ∼0.2 W m ⁻¹ K ⁻¹ at 300 K. This low k _L can be resulted from phonon-phonon scattering process promoted by strong coupling between acoustic-optical branches. Moreover, the optimal figure of merit of 1.1–2.5 is observed at 300–500 K for both p- and n-type BiSI. The current study provides evidence for orthorhombic BiSI as a potentially beneficial thermoelectric material for room-to-mid temperature applications and it also accomplishes a gravitation towards pertinent experimental investigations.