Hierarchical Bonding Enables Dual-Channel Thermal Transport and Ultralow Lattice Thermal Conductivity in Zintl X2Te2 (X = Cs, Rb)

Zintl compounds are promising thermoelectric materials owing to their intrinsically low lattice thermal conductivity (κL), yet the microscopic origin of their thermal transport remains incompletely understood. Here, first-principles calculations combined with finite-temperature lattice dynamics, four-phonon scattering, and the unified thermal transport theory are used to investigate the lattice dynamics, thermal transport, and thermoelectric properties of X2Te2 (X = Cs, Rb). The results reveal a distinct hierarchical bonding network consisting of rigid Te–Te covalent dimers and a flexible X–Te weakly polar coordination environment. This bonding hierarchy simultaneously leads to reduced phonon group velocities, enhanced anharmonic phonon scattering, and substantial coherent heat transport, resulting in intrinsically ultralow κL. At 300 K, the κL values along the a direction are only 0.24 and 0.23 W m–1 K–1 for Cs2Te2 and Rb2Te2, respectively. Electronic-structure calculations further demonstrate favorable band-edge characteristics, and both Cs2Te2 and Rb2Te2 achieve a predicted maximum ZT of approximately 1.73 under p-type doping at 1000 K. These findings establish a microscopic connection between hierarchical bonding, lattice dynamics, and dual-channel thermal transport, providing deep insights into the design of high-performance Zintl thermoelectric materials.