Weakening Local Chemical Bonds for Enhanced Thermoelectric Performance in YbMg 2 Sb 2 Zintl Compounds

Abstract

ABSTRACT Alloying is a well‐established strategy for reducing lattice thermal conductivity ( κ L ) and enhancing thermoelectric performance of materials, typically explained by the Klemens model, which describes the point‐defect phonon scattering via mass and size contrasts. Yet, in certain isovalent alloyed thermoelectrics, we observe an anomalously low κ L that falls below Klemens model predictions. Here, using YbMg 2 Sb 2 as a model system, we show that alloying Mg with isovalent Zn or Cd induces a substantial reduction in κ L beyond that attributable to mass and size effects of the dopants alone. Density‐functional‐theory calculations demonstrate that this stems from weakened local chemical bonds, which soften and slow both acoustic and optical phonons. Reduced interatomic force constants also lead to avoided crossing, further suppressing acoustic phonon propagation. Deformation electron density analyses reveal that the weak Zn‐Sb and Cd‐Sb bonds arise from delocalized electrons within Zn 2 Sb 2 and Cd 2 Sb 2 rhomboid rings. Benefiting from this pronounced κ L reduction and an enhanced density‐of‐states effective mass by band alignment, Cd‐alloyed YbMg 2 Sb 2 achieves a record‐high ZT of 1.4 at 873 K. These findings highlight the critical role of chemical bonds in reducing κ L and provide a design principle for enabling materials with ultralow thermal conductivity through local bond tailoring.