Spin-orbit coupling driven topological transition in p-type thermoelectric KMg4Sb3-xBix: a DFT study

We report a theoretical study of the KMg₄Sb_3-xBi_x (0 ≤ x ≤ 3) Zintl compounds, focusing on how increasing Bi content tunes the band topology and transport properties. Density-functional theory (DFT) band structure calculations show that Bi substitution systematically narrows the band gap and sharpens the valence-band edge. In particular, KMg₄Sb₃ is a wide-gap semiconductor, whereas KMg₄Bi₃ has a closed gap that undergoes a topological transition. Artificially scaling the spin-orbit coupling (SOC) strength in KMg₄Sb₃ (0–300 %) reproduces the Bi induced evolution of band structures, confirming that SOC is the driving force. Phonon and Boltzmann-transport calculations show that the sample of x = 1.5 maximally suppresses lattice thermal conductivity, while enhancing carrier mobility. Consequently, the intermediate alloy KMg₄Sb_1.5Bi_1.5 achieves an ultrahigh ZT∼0.5 at 300 K and ∼2.7 at 800 K under proper p-type doping. Our work establishes “SOC-tuned topological transition” as a general design strategy, uniting phonon-glass architectures with topological band engineering to achieve high-efficiency thermoelectric materials.