Soft Phonon Mode-Driven High Thermoelectric Figure of Merit in Two-Dimensional MX₃ (M = Ca, Ba, and X = P, As)

In recent years, thermoelectric (TE) materials have attracted attention due to their ability to convert waste heat into electricity. TE materials can be used to generate electricity from temperature gradients (Seebeck effect) and cooling applications (Peltier effect). Through first-principles calculations and using the Boltzmann transport equation, we explore the TE properties of low-dimensional MX3 (M = Ca, Ba and X = P, As) type compounds. Due to quantum confinement effects, these low-dimensional materials show exceptionally high figures of merit (ZT). The lattice thermal conductivities of these materials are low, whereas their electrical conductivities and Seebeck coefficients (S) are high, which makes them suitable candidates for applications in TE materials. We also observe that the soft phonon modes in materials can lower the lattice thermal conductivity (κl) and thereby increase their TE performance. Our results show the anisotropic figure of merit (ZT) for CaP3 and the unexpectedly high ZT for BaP3 and CaAs3 at room temperature. On the other hand, BaAs3, being electron-rich, shows high ZT at higher carrier concentrations and temperatures. This work provides insightful results and a mechanism for obtaining highly efficient TE materials.