Anomalous phonon transport and thermoelectric properties in honeycomb compounds ACuTe (A = Na, K, Rb)†

Developing materials with low thermal conductivity is crucial for practical applications such as thermoelectrics, and this requires a deep understanding of phonon transport. Typically, weak chemical bonding, heavy atomic masses, and complex structures are common features of high-performance thermoelectric materials with low lattice thermal conductivity (κ_L). In this work, we used first-principles calculations to investigate the lattice dynamics and phonon transport properties of a series of ABX honeycomb compounds, finding that substituting the cation with a lighter atom can reduce the κ_L. We studied ACuTe (A = Na, K, Rb) and found that κ_L behaves contrary to the expected trend based on conventional mass considerations: substituting Na with heavier Rb or K leads to an increase in κ_L in ACuTe. This anomaly is attributed to the larger atomic displacements and stronger quartic anharmonicity in NaCuTe. Our research shows that lighter A atoms form weaker bonds with the Cu–Te honeycomb ring, enhancing vibrational anharmonicity and resulting in the lowest κ_L for NaCuTe. Visualization of the vibrational modes in RbCuTe reveals that optical phonons contribute over 40% to the anomalous thermal conductivity in ACuTe, primarily due to the out-of-phase vibrations of the A atoms. Considering spin–orbit coupling, we predict a ZT value of 1.72 for p-type KCuTe at 800 K. These findings elucidate the mechanisms controlling anomalous phonon transport behavior and provide guidance for discovering low-cost and low-density materials.