Significant anharmonic scattering in single-crystalline Mg_4.8Ag_1.4Sb₄ with site-occupation disorders

The exploration and development of novel materials with intrinsically low thermal conductivity hold significant scientific and technological implications for thermoelectrics and thermal barrier coatings. In this study, a single crystal of Mg_4.8Ag_1.4Sb₄ has been successfully synthesized, demonstrating an ultralow thermal conductivity of approximately 0.6 W m⁻¹ K⁻¹ at room temperature. Structure determination based on single-crystal x-ray diffraction analysis identifies the F m 3 ¯ m space group and the mixed occupancy of Mg atoms (∼60 at. %), Ag atoms (∼17.5 at. %), and vacancies (22.5 at. %) at the 8c site. These crystallographic site-occupation disorders lead to substantial phonon scattering, which is the primary cause of the observed low thermal conductivity. First-principles calculations reveal high anharmonic scattering rates and short mean free paths in Mg_4.8Ag_1.4Sb₄. Complementary molecular dynamics simulations demonstrate the anomalous atomic vibrations of Ag atoms with large amplitudes at 300 K, which induces a low-lying acoustic phonon branch and enhanced phonon density of states at 0.6 THz. This phenomenon accounts for the enhanced anharmonic scattering rates at elevated temperatures, inducing a further reduction in the lattice thermal conductivity. Our work can shed new light on the discovery of novel materials with low thermal conductivity.