Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

Abstract

Understanding the lattice dynamics and phonon transport from the perspective of chemical bonds is essential for improving and finding high‐efficiency thermoelectric materials and for many applications. Here, the coexistence of global and local weak chemical bonds is elucidated as the origin of the intrinsically low lattice thermal conductivity of non‐caged structure Nowotny–Juza compound, α‐MgAgSb, which is identified as a new type of promising thermoelectric material in the temperature range of 300–550 K. The global weak bonds of the compound lead to a low sound velocity. The unique three‐centered MgAgSb bonds in α‐MgAgSb vibrate locally and induce low‐frequency optical phonons, resulting in “rattling‐like” thermal damping to further reduce the lattice thermal conductivity. The hierarchical chemical bonds originate from the low valence electron count of α‐MgAgSb, with the feature shared by Nowotny–Juza compounds. Low lattice thermal conductivities are therefore highly possible in this series of compounds, which is verified by phonon and bulk modulus calculations on some of the compositions.

Elaborating the Crystal Structures of MgAgSb Thermoelectric Compound: Polymorphs and Atomic Disorders

“…Table 2 and Table S1 list reported that the weak bonds will result in a soft phonon mode and strong phonon vibrations during the heat conduction, which is suggested to be the origin of ultralow lattice thermal conductivity in SnSe. 34 Recently, the hierarchical chemical bonding were reported and suggested to be responsible for the intrinsically low lattice thermal conductivity of α-MgAgSb, 12 which is in good agreement with the present results. The ADPs of the atoms are refined with the absorption correction included, 35 where the ADPs represent the atomic thermal vibration plus potential contributions from atomic disorder.…”

Ultralow Lattice Thermal Conductivity of A0.5RhO2 (A = K, Rb, Cs) Induced by Interfacial Scattering and Resonant Scattering

“…To reduce uncertainty in the fittings, L is chosen to be half of the c axis lattice parameter determined by XRD, which refers to the condition of the strongest interfacial scattering, while B and C are fitting parameters. The fittings agree well with the experimental data reported previously, 10,36 which indicates the validity of our data analysis. The fitted B values for all of the samples are in the order of 10 −17 s K −1 , which has the same order as ion-filled skutterudites reported in the literature.…”

Vacancy-mediated anomalous phononic and electronic transport in defective half-Heusler ZrNiBi

“…Moreover, differing from ZrCoBi with a large acoustic-optical gap, Zr 0.88 NiBi features low-lying optical phonons (1.5−3 THz) that are strongly hybridized with acoustic branches (in the frequency range between 1.87 and 2.22 THz). We expect that these low-lying optical phonons originate from the compound’s local soft bonds, and an analogous phenomenon was also reported for α-MgAgSb 40 . Importantly, these soft optical modes with nearly vanishing group velocities show a non-propagating-like behavior while enabling additional scattering for heat-carrying acoustic phonons.…”