Violation of the Relationship in the Lattice Thermal Conductivity of MgSb with Locally Asymmetric Vibrations.

Most crystalline materials follow the guidelines of temperature-dependent lattice thermal conductivity ( ) at elevated temperatures. Here, we observe a weak temperature dependence of in MgSb, from theory and from measurements, based on a comprehensive study combining molecular dynamics calculations and experimental measurements on single crystal MgSb. These results can be understood in terms of the so-called "phonon renormalization" effects due to the strong temperature dependence of the interatomic force constants (IFCs). The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport, and more importantly, the phonon-phonon interactions are weakened. In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb tetrahedron. With increasing temperature, these Mg atoms tend to locate at the areas with relatively low force in the force profile, leading to reduced effective 3-order IFCs. The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization. The present work sheds light on the fundamental origins of anomalous temperature dependence of in thermoelectrics.