Ab initio study of NaSrSb and NaBaSb as potential thermoelectric prospects

Zintl phases are excellent thermoelectric prospects to put the waste heat to good use. In the quest of the same, using first-principles methods combined with Boltzmann transport theory, we explored two recent phases NaSrSb and NaBaSb. We found low lattice thermal conductivity of 1.9 and 1.3 W m$^{-1}$ K$^{-1}$ at 300~K for NaSrSb and NaBaSb, respectively, which are of the same order as other potential Zintl phases such as Sr$_3$AlSb$_3$ and BaCuSb. We account for such low values to short phonon lifetimes, small phonon group velocities, and lattice anharmonicity in the crystal structure. The calculated electrical transport parameters based on acoustic deformation potential, ionized impurity, and polar optical phonon scattering mechanisms reveal large Seebeck coefficients for both materials. Further, we obtain a high figure of merit of ZT$\sim$2.0 at 900~K for \textit{n}-type NaSrSb. On the other hand, the figure of merit of \textit{n}-type NaBaSb surpasses the unity. We are optimistic about our findings and believe our work would set a basis for future experimental investigations.