Grain boundary (GB) engineering is one of the most common strategies to reduce the lattice thermal conductivity and improve thermoelectric materials. However, in the case of the promising thermoelectric material Mg(SbBi), an abnormal dependence of lattice thermal conductivity on grain size was observed at the concentration of = 0.25, where smaller grain polycrystalline materials exhibited higher lattice thermal conductivity compared to larger grain materials. The proposed theory about the overestimation of lattice thermal conductivity in inhomogeneous materials with GBs cannot clarify the concentration dependence of this anomaly. Here we elucidate the interplay between segregation, concentration, and lattice thermal conductivity in Mg(SbBi) alloys through atomistic simulations with a highly accurate machine learning interatomic potential. We find the largest segregation of Bi atoms to GBs in Mg(SbBi) at the concentration of = 0.25 for both twist and tilt GB structures due to the combination effects of site spectrality and solute interactions. Our molecular dynamic simulations demonstrate that the pronounced segregation of heavier Bi atoms, particularly at = 0.25, leads to a substantial increase in the lattice thermal resistance at GBs, thus contributing to the degree of inhomogeneity. The concentration dependent segregation reveals the atomic origin of the observed unphysical inverse relationship between grain size and lattice thermal conductivity at the specific concentration of = 0.25. These results highlight the need to design the alloy concentration to tune the atomic segregation and tailor the thermal properties of thermoelectric materials with GBs.

中文翻译：

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Mg3(Sb, Bi)2 中晶格热导率的非物理晶粒尺寸依赖性：浓度依赖性偏析效应的原子观

晶界（GB）工程是降低晶格导热系数和改善热电材料的最常见策略之一。然而，在有前景的热电材料Mg(SbBi)的情况下，在浓度= 0.25时观察到晶格热导率对晶粒尺寸的异常依赖性，其中较小晶粒的多晶材料与较大晶粒的材料相比表现出更高的晶格热导率。所提出的关于具有晶界的非均质材料中晶格热导率的高估的理论无法澄清这种异常的浓度依赖性。在这里，我们通过具有高精度机器学习原子间势的原子模拟阐明了 Mg(SbBi) 合金中的偏析、浓度和晶格热导率之间的相互作用。我们发现，由于位点光谱和溶质相互作用的综合效应，对于扭曲和倾斜晶界结构，Mg(SbBi) 中 Bi 原子与晶界的最大偏聚在浓度 = 0.25 时。我们的分子动力学模拟表明，较重的 Bi 原子的明显偏析，特别是在 = 0.25 时，导致 GB 处的晶格热阻大幅增加，从而导致不均匀程度。浓度依赖性偏析揭示了在特定浓度 = 0.25 时观察到的晶粒尺寸与晶格热导率之间的非物理反比关系的原子起源。这些结果强调需要设计合金浓度来调整原子偏析并调整具有GB的热电材料的热性能。