Ab Initio Study of Thermoelectric Properties in Layered BaFMgBi with Topological Phonons

Layered materials, often realized as mixed-anion compounds, consist of anions coordinated by one or more cations to form periodically stacked structural units or building blocks. The present work investigates the electronic structure and the thermal and electronic transport properties of LaOAgS-type layered BaFMgBi using density functional theory. Weak interlayer coupling between the Ba2F2 and Mg2Bi2 layers hinders phonon transport across the layers, leading to reduced thermal conductivity along the out-of-plane direction. Three-phonon calculations estimate the ultralow lattice thermal conductivity (κl) of 1.05 (0.72) W/mK along the a (c) axis at a temperature of 900 K. In the case of BaFMgBi, the inclusion of spin–orbit coupling (SOC) significantly modifies the thermoelectric transport properties, reversing the carrier-type trend: while p-type (1.6) doping showed a higher thermoelectric figure of merit (ZT) than n-type (0.7) doping without SOC, the SOC-inclusive calculations predict a higher maximum ZT for n-type doping (0.7) than for p-type doping (0.48) at 900 K. At high temperatures (900 K) and low carrier concentrations, the p-type material shows bipolar effect, leading to a reduction in the overall Seebeck coefficient. Phonon dispersion calculations reveal nontrivial topological features, including nodal surfaces and nodal lines in specific modes. These features may introduce additional scattering channels in three-phonon processes, reducing the phonon mean free path and consequently lowering the lattice thermal conductivity.