Abnormally Low Lattice Thermal Conductivity in ABX Honeycomb Compounds

Planar structure and small atomic mass usually induce high lattice thermal conductivity in a crystalline solid. Here, we present a first-principles study on the lattice dynamics and phonon-transport properties of a series of ABX honeycomb compounds and show that replacing cations with lighter atoms may yield lower lattice thermal conductivity $({\ensuremath{\kappa}}_{l})$. The anomalous mass-trend ${\ensuremath{\kappa}}_{l}$, which is experimentally observed, is found to be inherent to the lattice vibration of the specific layered honeycomb structure, using A$\mathrm{Cu}\mathrm{Sb}$ (A = $\mathrm{Ca}$, $\mathrm{Sr}$, and $\mathrm{Ba}$) as prototypical compounds. We reveal that a lighter A atom bonds more weakly with the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Sb}$ honeycomb ring, giving rise to stronger vibrational anharmonicity, which induces ultralow ${\ensuremath{\kappa}}_{l}$ in $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ that is unexpected of light elements and a high-symmetry planar structure. Combined with the high degeneracy and strong anisotropy of the bottom conduction band, $\mathrm{Ca}\mathrm{Cu}\mathrm{Sb}$ exposes an ultrahigh n-type zT of about 2.2 at 700 K. These findings elucidate the mechanism governing anomalous phonon-transport behavior, which also offers guidance in discovering low-cost and low-mass-density materials for advanced thermoelectric applications.