Zintl-phase materials exhibit great potential in thermoelectric applications owing to their high electrical conductivity and low thermal conductivity induced by unique structural and electronic characteristics. In this work, we investigate the electrical and thermal transport properties and evaluate the thermoelectric performance of Zintl-phase tellurides $XY{\mathrm{Te}}_2$ $(X=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs}; Y=\mathrm{Al}, \mathrm{Ga}, \mathrm{In})$ through systematic analysis based on density functional theory, the Boltzmann transport equation, and unified transport theory. Results show that these tellurides can exhibit ultralow lattice thermal conductivity ${\kappa }_L;$ in particular, ${\mathrm{NaInTe}}_2$ shows a value as low as 0.33 W/mK, which is mainly attributed to strong anharmonicity introduced by bonding heterogeneity and the rattling vibrations of the weakly bonded X-site cations. The multiband degeneracy and flat band characteristic near the valence band maximum $(\mathrm{\Gamma }\to M,$ $\mathrm{Z}\to P)$ render into a high power factor of $1.5-2.3\mathrm{mW}/\mathrm{m}{\mathrm{K}}^2$ ($p$ type). By optimizing carrier concentration, exceptional thermoelectric performance with ZT values up to 4.0 ($p$ type) is achieved at 900 K. In contrast, ${\mathrm{NaGaTe}}_2$ shows inferior thermoelectric performance due to the narrow band gap induced bipolar effect that reduces the Seebeck coefficient significantly. In addition, by considering wavelike tunneling effects and higher-order phonon scattering, the tellurides $XY{\mathrm{Te}}_2$ maintain favorable thermoelectric performance with ZT values around 3.3. This work showcases the importance of bond heterogeneity and rattling mode induced ultralow lattice thermal conductivity for enhancing thermoelectric performance in Zintl-phase tellurides.

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• Anharmonic lattice dynamics
• Electron-phonon coupling
• Phonons
• Seebeck effect
• Thermal conductivity
• Thermoelectrics
• 3-dimensional systems
• Ab initio molecular dynamics
• Band structure methods
• Boltzmann theory
• Density functional theory