Low Lattice Thermal Conductivity-Driven Promising Thermoelectric Figure of Merit in NaSrSb and NaBaSb Zintl Phases

Zintl phases have excellent thermoelectric prospects to put the waste heat to good use. In the quest for the same, using first-principles methods combined with Boltzmann transport theory, we explored two recent phases, NaSrSb and NaBaSb. We found remarkably low lattice thermal conductivities of 0.7 (2.1) and 0.4 (1.4) W m–1 K–1 at 900 K (300 K) for NaSrSb and NaBaSb, respectively, which are of the same order as other potential Zintl phases such as Sr3AlSb3 and BaCuSb. We find that such low values could be attributed to the short phonon lifetimes and small phonon group velocities in the lattice. The calculated electrical transport properties reveal a large Seebeck coefficient for both materials. Furthermore, combining low lattice thermal conductivity with electrical coefficients, we obtained a high figure of merit of ZT ∼ 1.9 at 900 K for n-type NaSrSb. On the other hand, the figure of merit of NaBaSb reaches the unity. We are optimistic about our findings and believe that our work sets a basis for future experimental investigations.