Low thermal conductivity and promising thermoelectric performance induced by resonant bonding in a two-dimensional lead-tin-selenide ordered alloy

Resonant bonding, arising from an unsaturated electronic occupation configuration, has been demonstrated as a crucial signature for intrinsically low lattice thermal conductivity (${\ensuremath{\kappa}}_{l}$), which offers a promising avenue for achieving high-performance thermoelectric materials. Using density-functional theory combined with Boltzmann transport equation calculations, we demonstrate the significant impact of resonant bonding on the thermal transport properties of a two-dimensional lead-tin-selenide ordered alloy, namely $({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5})\mathrm{Se}$. Our findings demonstrate that resonant bonding induces interactions that extend beyond the typical covalent bonding range, i.e., resonant interaction, as evidenced by the perturbed electron density distribution, the nonvanishing trace of interaction force constants over long distances, and the large convergent cutoff radius observed in thermal conductivity calculations. Consequently, the resonant interactions lead to enhanced phonon-scattering events and strong anharmonicity, resulting in an ultralow ${\ensuremath{\kappa}}_{l}$ around $1\phantom{\rule{0.2em}{0ex}}{\mathrm{Wm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 K. More interestingly, an excellent thermoelectric performance is observed in two-dimensional $({\mathrm{Pb}}_{0.5}{\mathrm{Sn}}_{0.5})\mathrm{Se}$, with a remarkably high ZT value around 3 at 800 K. This study highlights the underlying relation between the electronic bonding and thermal transport property, providing valuable insights for the design of high-efficiency thermoelectric materials.