The Rattler-Induced\nFlat Phonon Modes in KCaBi Driven\nLower Thermal Conductivity Compared to KCaSb

Low lattice thermal conductivity is a desirable property\nfor enhancing\nthe figure of merit (ZT) of thermoelectric materials, as it is decoupled\nfrom electronic transport. Here, we investigated the lattice thermal\nconductivity and thermoelectric properties of the compounds KCaX (X\n= Bi, Sb) through first-principles calculations. Our finding indicates\nthat KCaBi and KCaSb exhibit notably low lattice thermal conductivity\nof 0.95 and 2.07 W/mK at 300 K, respectively. The reduction in lattice\nthermal conductivity originates from the anharmonic rattling effects\nof the K atoms and their weak bonding with adjacent Bi/Sb atoms, which\ninduces a coupling between acoustic and low-frequency optical phonon\nbranches. This coupling enhances phonon–phonon scattering,\nsignificantly reducing phonon group velocities and increasing Grüneisen\nparameters. Remarkably, the lattice’s enhanced anharmonicity,\nparticularly in KCaBi, is linked to its softer structure and lower\nthermal conductivity. The antibonding properties of the highest occupied\nvalence band also have an important effect on the lattice thermal\nconductivity. Furthermore, both materials demonstrate semiconducting\nbehavior with indirect band gaps and display increased electrical\nconductivity under n-type doping. Notably, KCaBi achieves superior\nthermoelectric performance with a predicted maximum ZT value of 0.67\nat 800 K. Our work demonstrates that integrating rattling and antibonding\nstates in materials design is an effective strategy for developing\nhigh-performance thermoelectric materials.