Intrinsic Ultralow Lattice Thermal Conductivity in the Full-Heusler Compound Ba2AgSb

Full-Heusler thermoelectric materials have intrinsically low lattice thermal conductivity. Our first-principles calculations show that ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ is a semiconductor with an indirect band gap of 0.49 eV. The electronic band degeneracy and pockets near the Fermi level facilitate electron transport. The short phonon relaxation time, small group velocity (1.89 km ${\mathrm{s}}^{\ensuremath{-}1}$), and large phonon scattering space reflect the intense phonon-phonon scattering. The large Gr\"uneisen parameter (1.44) accounts for the strong phonon anharmonicity, thus the low lattice thermal conductivity of $0.5\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 800 K. The isotropic figure of merit with a maximum value of 4.7 at 750 K is comparable to that of reported materials. The distribution of phonon momentum uncovers the important role of $\mathrm{Ag}$ in resisting thermal transport. The analysis of symmetry-based phonon-phonon scattering routes reveals the significance of symmetry on phonon scattering. The crystal structure of ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ can be used to regulate chemical elements to build high-performance thermoelectric materials. Our calculations provide an effective way to design thermoelectric materials, stimulating the study of full-Heusler materials.