Significant anharmonic scattering in single-crystalline Mg4.8Ag1.4Sb4 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 Mg4.8Ag1.4Sb4 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 Fm3¯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 Mg4.8Ag1.4Sb4. 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.