Anomalous suppression of thermal conductivity in SnSe induced by aliovalent doping

As a universally employed strategy for suppressing phonon propagation, element doping plays a critical role in reducing lattice thermal conductivity ${\ensuremath{\kappa}}_L$ of thermoelectric materials. Traditionally, heavy-element doping with a substantial atomic mass/radius difference leads to a pronounced ${\ensuremath{\kappa}}_L$ reduction through the phonon velocity attenuation and phonon scattering enhancement. In this study, using accurate neural network potentials combined with the phonon Boltzmann transport equation, we systematically investigate the comparative impacts of isoelectronic (Ge, Pb) and aliovalent (Ag) doping on phonon transport in SnSe. Counterintuitively, despite the fact that Pb has a heavier atomic mass and exhibits a greater mismatch in both mass and radius compared to Sn, aliovalent Ag doping achieves superior ${\ensuremath{\kappa}}_L$ suppression than isoelectronic Pb doping. While the softening of acoustic and optical phonons, resulting, respectively, from weakened bond strength and enhanced electron screening, contributes to some reduction in phonon group velocity of Ag-doped SnSe, it is not the main mechanism. The anomalous suppression of ${\ensuremath{\kappa}}_L$ is primarily attributed to the enhanced coupling between the acoustic and optical phonon branches. Further results reveal that the p-d antibonding interaction between $\mathrm{Se}\text{\ensuremath{-}}4p$ and $\mathrm{Ag}\text{\ensuremath{-}}4d$ orbitals induces the weakening of bond strength, thereby causing the downward shift of low-frequency optical branches and thus enhancing the coupling effect between acoustic and optical branches. This work deepens our understanding of the phonon transport mechanism in aliovalent-doped materials and provides theoretical guidance for experimentally reducing the ${\ensuremath{\kappa}}_L$ through doping strategies.