The ever‐increasing concerns on energy crisis and sustainable resources have spurred a worldwide action in developing alternative energy conversion technologies. Thermoelectric materials, which provide a promising solution for the conversion of heat into electricity and vice versa, have attracted increasing attention in the fields of solid‐state physics, chemistry and materials science. From the perspective of historical development, the past two decades have witnessed surged advances in thermoelectrics, featured by the deeper understanding of thermoelectric physics, establishment of novel optimization strategies, discovery of novel thermoelectric materials, success of high‐efficiency thermoelectric devices, and practical applications of thermoelectric technologies in multiple areas of public life.

To promote the wide applications of thermoelectric technologies, one fundamental issue lies in the improvement of the conversion efficiency of thermoelectric devices, which is directly tied to the dimensionless figure of merit zT of thermoelectric materials. There are two main research directions in the development of thermoelectric materials: one is to deeply understand the transport mechanism of existing good thermoelectric materials and thereby further enhance the zT; the other is to explore new thermoelectric candidates using the established thermoelectric guidelines. In reality, the progress of both directions is strongly related to the advances in solid‐state physics. Namely, the discovery of new electronic and phononic structures and the insightful understanding of electron and phonon transport mechanisms in the solid‐state materials will bring new opportunities for the thermoelectric field.

This special issue of the Annalen der Physik on “Thermoelectric Materials” contains eight original articles and one review article on the dynamic of various thermoelectric studies. The topics range from the understanding of thermoelectric band engineering and scattering mechanism, and the interplay between spintronics and thermoelectrics, to the discovery of new thermoelectric semiconductors.

AgSbTe₂, as an interesting ternary thermoelectric material, attracts considerable attention with the focus on the origin of its intrinsically low thermal conductivity and high thermoelectric performance. In the first original article, Liu et al.^[1] report that AgSbTe₂ is thermodynamically unstable and would partially decompose into Ag₂Te and Sb₂Te₃ during thermal cycling. Instead, they find that the compound Ag_0.366Sb_0.558Te exhibits a single‐phase. With the Sn substitution at Sb sites, the electrical transport properties of Ag_0.366Sb_0.558Te are optimized. A peak zT of ∼1.3 and an average zT of ∼0.9 are achieved, highlighting the potential of Ag_0.366(Sb_1‐xSn_x)_0.558Te in thermoelectric application.

Layer‐structured semiconductors have recently attracted increasing attention in the field of thermoelectrics as they generally display low thermal conductivity along the out‐of‐plane direction. One distinct feature of layer‐structured semiconductors is that they usually display strong anisotropy in the transport properties. Yin et al. report a feasible strategy to improve thermoelectric performance by anisotropic tuning in misfit‐layered chalcogenide.^[2] This strategy, based on in‐plane covalent bond and out‐of‐plane van der Waals bond, induces a higher in‐plane electrical transport performance and a lower out‐of‐plane lattice thermal conductivity, deriving from the natural intercalated structure where the out‐of‐plane phonon is strongly scattered without influencing the in‐plane mobility. The authors show how the in‐plane lattice thermal conductivity is reduced by introducing point defects, while the out‐of‐plane mobility is maintained, thereby leading to a synergistic optimization of anisotropic thermoelectric performance. The present finding opens up a new opportunity for of manipulating thermoelectric performance via anisotropy engineering.

Band convergence is one of the most popular band engineering strategies in the past ten years’ thermoelectric research. Tan et al. report a theoretical study on the origin of band convergence in the representative Mg₂Si_1‐xSn_x solid solution using the Wannier function analysis. They find that the convergence of conduction bands in Mg₂Si_1‐xSn_x is simply driven by the variation of lattice constant, resulting from the different dependence on the bonding length in the heavy and light conduction valleys.^[3] Moreover, they predict that Mg_2‐xSr_xSi could be a new material system with band convergence, awaiting experimental confirmation.

The interplay of different fields could generate new research directions. The dynamic development in the interdisciplinary fields of thermoelectrics and spintronics is one typical example. In the review article, Hu et al. introduce the fundamental physical concepts that are important to spin‐dependent thermoelectric research.^[4] Particularly, they highlight some exceptional latest experiments on ferromagnetic and half‐metallic Heusler compounds. The potential of using the anomalous Nernst effect to convert heat into electricity is also discussed. This interdisciplinary field may offer new opportunities for discovering novel thermoelectrics.

Li et al. investigate the thermoelectric transport properties of 19‐electron half‐Heusler compound VCoSb,^[5] They find that the nominal VCoSb is actually a composite of an off‐stoichiometric V_0.955CoSb single phase with impurities. Using Ti substitution at the V site, they simultaneously optimize the electrical properties and suppress the thermal conductivity of V_0.955CoSb. Consequently, a peak zT of 0.7 at 973 K is reported for V_0.855Ti_0.1CoSb. This work demonstrates the potential of 19‐electron VCoSb‐based half‐Heusler compounds as thermoelectric materials.

Lattice thermal conductivity can be significantly reduced by introducing multiple‐scale scattering sources, such as atomic‐scale point defects, microscale grain boundaries, and nanoscale precipitates. In the original article, Fang et al. investigate the effect of electron‐phonon interaction on phonon transport by taking P‐doped single‐crystal Si as a case study.^[6] They find the rapid reduction in the lattice thermal conductivity in P‐doped single‐crystal Si can be well explained by considering the roles of both point defect scattering and electron‐phonon interaction. This work demonstrates the important role of electron‐phonon interaction in reducing the lattice thermal conductivity of heavily doped thermoelectric semiconductors.

PbTe is regarded as one of the most promising intermediate‐temperature thermoelectric materials. However, the difference between conduction and valence bands leads to a large performance mismatch in p‐ and n‐type PbTe. To match p‐type counterpart, Wang et al. report an interesting strategy^[7] that can synergistically improve the power factor and reduce the lattice thermal conductivity of n‐type PbTe. It is found that the amphoteric Indium exhibits mixed valences (In⁺ and In³⁺), which can effectively form defect level to dynamically optimize the power factor in the entire working temperature range. To further reduce its lattice thermal conductivity, Sulfur is introduced to intensify phonon scattering by forming point defects. With the combined roles of Indium doping and Sulfur alloying, both power factor and lattice thermal conductivity are optimized, which finally contributes to a high zT of 1.4 at 773 K. This work indicates that the combination of dynamic doping and point defect scattering is one promising strategy to improve the thermoelectric performance of n‐type lead chalcogenides.

In the original article by Du et al.^[8], they report the thermoelectric transport properties of Se substituted pseudo‐binary Ge₂Sb₂Te_5‐xSe_x. They reveal this substitution strategy can optimize the hole concentration and enhance the hole effective mass with the help of the band‐structure calculations. Meanwhile, the alloying scattering can reduce the lattice thermal conductivity. Thus, the zT at 703 K increases from 0.24 for Ge₂Sb₂Te₅ to 0.41 for Ge₂Sb₂Te_3.5Se_1.5. This work provides an effective way to improve the thermoelectric properties of Ge₂Sb₂Te₅.

As a kind of the most promising mediate‐temperature oxide thermoelectric materials, oxyselenides (BiCuSeO and Bi₂O₂Se) have been widely researched in the thermoelectric community. In the last original article, Zhang et al. report a new oxyselenide thermoelectric material Bi₆Cu₂Se₄O₆,^[9] which can be considered as a composition of 2 BiCuSeO and 2 Bi₂O₂Se. Bi₆Cu₂Se₄O₆ is found to be one n‐type thermoelectric oxide. With halogen (Br, Cl) doping at Se site, a maximum zT of 0.15 is reached at 823 K for Bi₆Cu₂Se_3.2Br_0.8O₆, indicating the potential of Bi₆Cu₂Se₄O₆ as an n‐type oxide thermoelectric material.

Finally, we are very thankful to all the contributors and the editors and staff of Annalen der Physik who make this exciting special issue possible.

对能源危机和可持续资源的日益关注促使世界范围内采取行动开发替代能源转换技术。热电材料为固态热能的转化提供了一种有希望的解决方案，反之亦然，在固态物理，化学和材料科学领域已引起越来越多的关注。从历史发展的角度来看，过去二十年来，热电学取得了突飞猛进的发展，其特征是对热电物理有了更深入的了解，建立了新的优化策略，发现了新颖的热电材料，高效热电设备的成功应用和实际应用。热电技术在公共生活的多个领域的应用。

为了促进热电技术的广泛应用，一个基本问题在于提高热电设备的转换效率，这直接与热电材料的无量纲品质因数zT挂钩。热电材料的发展主要有两个研究方向：一是深入了解现有优质热电材料的传输机理，从而进一步提高zT。; 另一种是使用已建立的热电准则来探索新的热电候选物。实际上，这两个方向的进步都与固态物理学的进步密切相关。即，新的电子和声子结构的发现以及对固态材料中电子和声子传输机理的深刻理解将为热电场带来新的机遇。

Annalen der Physik的特刊“热电材料”包含8篇原创文章和1篇有关各种热电研究动态的评论文章。主题包括对热电带工程和散射机制的理解，以及自旋电子学与热电学之间的相互作用，以及新的热电半导体的发现。

AgSbTe ₂作为一种有趣的三​​元热电材料，由于其固有的低热导率和高热电性能而引起人们的广泛关注。在第一篇原始文章中，Liu等人。^{[ 1 ]}报告说AgSbTe ₂在热力学上不稳定，在热循环过程中会部分分解为Ag ₂ Te和Sb ₂ Te ₃。相反，他们发现化合物Ag _0.366 Sb _0.558 Te呈现单相。在Sb位点上进行Sn取代后，Ag _0.366 Sb _0.558的电传输性能Te已优化。zT的峰值约为1.3，zT的平均值约为0.9，突出了在热电应用中Ag _0.366（Sb _{1- x} Sn _x）_0.558 Te的电势。

层状结构的半导体近来在热电领域引起了越来越多的关注，因为它们通常沿平面外方向显示出低的热导率。层状半导体的一个显着特征是，它们通常在传输特性上表现出强烈的各向异性。Yin等。报告了通过各向异性调整层错配硫族化物来改善热电性能的可行策略。^{[ 2 ]}这种策略基于平面内共价键和平面外范德华键，可产生较高的平面内电传输性能和较低的平面外晶格热导率，这是由于天然的插层结构平面外声子在不影响平面内迁移率的情况下被强烈散射。作者展示了如何通过引入点缺陷来降低面内晶格热导率，同时保持面外迁移率，从而导致各向异性热电性能的协同优化。本发现为通过各向异性工程操纵热电性能开辟了新的机会。

频带收敛是过去十年热电研究中最受欢迎的频带工程策略之一。Tan等。报告了使用Wannier函数分析对代表性Mg ₂ Si _{1 x} Sn _x固溶体中能带会聚起源的理论研究。他们发现，Mg ₂ Si _{1– x} Sn _x中导带的收敛仅受晶格常数变化的驱动，这是由于在重和轻导谷中对键长的不同依赖性所致。^{[ 3 ]}而且，他们预测Mg _{2- x} Sr _xSi可能是具有能带收敛性的新材料系统，等待实验证实。

不同领域的相互作用可能会产生新的研究方向。热电和自旋电子学的交叉学科领域的动态发展就是一个典型的例子。在评论文章中，Hu等人。介绍对依赖自旋的热电研究非常重要的基本物理概念。^{[ 4 ]}特别是，它们着重介绍了有关铁磁和半金属赫斯勒化合物的一些杰出的最新实验。还讨论了使用异常能斯特效应将热量转化为电能的可能性。这个跨学科领域可能为发现新型热电学提供新的机会。

Li等。研究了19电子半霍斯勒化合物VCoSb的热电输运特性，^{[ 5 ]}他们发现标称VCoSb实际上是化学计量比为V _0.955 CoSb单相与杂质的复合物。通过在V位置使用Ti替代，它们可以同时优化电性能并抑制V _0.955 CoSb的热导率。因此，据报道V _0.855 Ti _0.1 CoSb在973 K处的峰值zT为0.7 。这项工作证明了19电子基于VCoSb的半霍斯勒化合物作为热电材料的潜力。

通过引入多尺度的散射源，例如原子尺度的点缺陷，微观尺度的晶界和纳米尺度的沉淀物，可以大大降低晶格的热导率。在原始文章中，Fang等人。以P掺杂单晶硅为例，研究电子-声子相互作用对声子传输的影响。^{[ 6 ]}他们发现，通过考虑点缺陷散射和电子-声子相互作用的作用，可以很好地解释P掺杂单晶硅中晶格热导率的迅速下降。这项工作证明了电子-声子相互作用在降低重掺杂热电半导体的晶格热导率中的重要作用。

PbTe被认为是最有前途的中温热电材料之一。但是，导带和价带之间的差异导致p型和n型PbTe的性能严重失配。为了匹配p型对应物，Wang等。报告了一种有趣的策略^{[ 7 ]}，可以协同提高功率因数并降低n型PbTe的晶格热导率。发现两性铟表现出混合价（In ⁺和In ³⁺），可以有效地形成缺陷水平，从而在整个工作温度范围内动态优化功率因数。为了进一步降低其晶格热导率，引入了硫以通过形成点缺陷来增强声子散射。结合铟掺杂和硫合金的作用，功率因数和晶格热导率均得到优化，最终有助于在773 K下实现高zT 1.4。这项工作表明，动态掺杂和点缺陷散射相结合是一种很有前途的技术改善n型硫属铅化物的热电性能的策略。

在Du等人的原始文章中。^{[ 8 ]}，他们报告了Se取代的伪二元Ge ₂ Sb ₂ Te _{5- x} Se _x的热电输运性质。他们揭示了这种替代策略可以借助能带结构计算来优化空穴浓度并提高空穴有效质量。同时，合金化散射会降低晶格热导率。因此，在703 K处的zT从Ge ₂ Sb ₂ Te _5的0.24增加到Ge ₂ Sb ₂ Te _3.5 Se _1.5的0.41。这项工作为改善Ge ₂ Sb ₂ Te ₅的热电性能提供了有效的途径。

作为最有前途的中温氧化物热电材料，氧硒化物（BiCuSeO和Bi ₂ O ₂ Se）已在热电领域得到了广泛的研究。在上一篇原始文章中，Zhang等人。报道了一种新的氧硒化物热电材料Bi ₆ Cu ₂ Se ₄ O ₆，^{[ 9 ]}可以认为是由2 BiCuSeO和2 Bi ₂ O ₂ Se组成。发现Bi ₆ Cu ₂ Se ₄ O ₆是一种n型热电氧化物。在Se位置掺杂卤素（Br，Cl），最大zTBi ₆ Cu ₂ Se _3.2 Br _0.8 O ₆在823 K时达到0.15的最大值，表明Bi ₆ Cu ₂ Se ₄ O ₆作为n型氧化物热电材料的潜力。

最后，我们非常感谢Annalen der Physik的所有撰稿人以及编辑和工作人员，使这一激动人心的特刊得以实现。