Prediction of Thermoelectric Performance in Topological Phonon Compounds MgSnP ₂ and MgSnAs ₂

Thermoelectric materials face scalability challenges due to the reliance on rare or toxic elements. Here, using first-principles calculations, we identify two magnesium-based semiconductors, MgSnP₂ and MgSnAs₂, as promising eco-friendly alternatives that exhibit intrinsically low lattice thermal conductivity (<i>κ_L</i>) and exceptionally high power factor (<i>PF</i>). Four-phonon calculations predict <i>κ_L</i> values of 10.8 (2.8) for MgSnP₂ and 7.1 (2.1) for MgSnAs₂ at 300 (800) K along the <i>a</i>-axis. Symmetry analysis identifies the presence of symmetry-protected nodal lines and a doubly degenerate Weyl point in the two compounds, inducing linearly crossing phonon branches that substantially enhance group velocities. However, the increased phonon scattering ultimately offsets the positive contribution to <i>κ_L</i>. Remarkably, <i>p</i>-type doping achieves <i>PF</i> values of 1.96 × 10^-5 W K^-2 cm^-1 for MgSnP₂ and 6.65 × 10^-5 W K^-2 cm^-1 for MgSnAs₂ at 800 K, yielding <i>ZT</i> values of 0.41 and 1.31. These findings highlight their potential as eco-friendly, high-performance alternatives and advance the design of Mg-based thermoelectric materials.