Anharmonicity of soft optical phonons as the origin of low lattice thermal conductivity in SrAgSb

Developing materials with low thermal conductivity requires an understanding of phonon transport, and it is of significance for practical applications such as thermoelectrics. Traditionally, weak chemical bonding, heavier atomic masses, and complex structure are the most common features in high-performance thermoelectric materials with low thermal conductivity. In this work, we applied the ab initio phonon Boltzmann transport method to investigate the abnormally low lattice thermal conductivity observed in the Zintl-phase compound, SrAgSb, among BaCuSb, SrCuSb, and the isomorphism heavier BaAgSb. The optical-acoustic branches coupling is considered the main reason for the diminishing thermal conductivity of SrAgSb, whereas one low-energy optical branch with low and negative Gr\"uneisen parameters. The hybridization between optical and acoustic branches plays an essential role as the scattering center in reciprocal space, like defects in the crystal lattice. Our analysis illustrates that the soft and anharmonic lowest-lying optical mode contributes to the lower thermal conductivity of SrAgSb and suggests more potential chances for achieving high thermoelectric performance with intrinsic low ${\ensuremath{\kappa}}_{\mathrm{lat}}$ among abundant Zintl-phase compounds.