Acoustic phonon softening enhances phonon scattering in Zintl-phase II-I-V compounds

The thermoelectric properties of nine Zintl-phase semiconductors II-I-V (II = Ca, Sr, Ba, I = Cu, Ag, Au, and V = As, Sb, Bi) are studied by using first-principles calculations. The electronic and thermal transport properties are calculated to elucidate the thermoelectric performance. The electron localization functions and crystal orbital Hamilton population show regular and anisotropic bonding in II-I-V, which makes anisotropic thermal and electronic transport properties. The phonon dispersion curve also shows element dependent distributions. We suggest that the regularity of phonon and electron distribution makes the adjusting of thermoelectric performance in $P{6}_{\text{3}}/mmc$ type Zintl-phase compounds possible. The mix of ionic and weak covalent bonding leads to the coexistence of soft phonon modes and favorable electronic properties and thus a high figure of merit (0.41--0.94). We also investigate the three phonon scattering properties. The importance of acoustic phonon softening in lowering thermal conductivity is observed. The symmetry-based three-phonon scattering pathways demonstrate the possible intense phonon-phonon scattering. These data provide a deep understanding of the thermoelectric properties in Zintl-phase compounds.