Thermoelectric properties of mixed-valence chalcogenides

本项目旨在探索混合价过渡金属硫族化合物的热电性质与其晶体结构、实结构和电子结构之间的联系。这包括根据“声子-液体电子-晶体”概念降低热导率的动态无序铜原子的额外好处。虽然热电材料可以可逆地相互转换热能和电能，但它们的效率必须显著提高，以便使用它们，例如废热收集。我们专注于经验优化与基本理解和理论模型的协同作用，以预测热电性能。广泛的衍射、光谱和微观（TEM）方法将为进一步的讨论提供可靠的基础。受矿物启发的化合物，如具有扭曲反铝矿型结构的Cu5FeS4（斑铜矿），具有纤锌矿相关结构的CuFe2S3 (cubanite)，具有链结构的KFe2S3 （rasvumite）和具有阳离子层结构的Cu5.5FeS6.5 (nukundamite)，提供了将cu原子在高温下的迁移率与混合价态（在某些情况下经过适当的取代）的影响结合起来的独特可能性。结构的多样性是补充铜铋硫化物具有复杂的结构类似于硫酸盐。廉价无毒的硫化物是一个有吸引力的起点；然而，用Se或Te取代阴离子有望提高电导率，同时通过引入无序性降低导热性。合成条件的变化和进一步的阳离子取代导致了不同长度尺度上的无序，包括由于磁性阳离子的混合价导致的自旋无序。在一种结合结构和电子无序与可调谐电荷载流子浓度和迁移率的新方法中，我们的目标不仅是热与电导率的解耦，而且是塞贝克系数的独立调谐。Ag和/或Li在阳离子位上取代Cu，可以将Cu的混合价效应与Fe的混合价效应分离开来。含铁化合物中的氧化态和化学键将通过Mößbauer光谱进行探测，并辅以EPR， XPS和敏感性测量，这些测量也适用于其他涉及的元素。结合DFT计算，我们期望对Cu-Cu和Fe-Cu相互作用以及自旋无序有更深入的了解。这些结果对热电材料优化的反馈产生了基础研究和材料科学的有趣相互作用。

This project aims at new concepts to correlate the thermoelectric properties of mixed-valence transition-metal chalcogenides with their crystal structures, real structures and electronic structures. This includes the additional benefit of dynamically disordered copper atoms that reduce the thermal conductivity according to the "phonon-liquid electron-crystal" concept. Although thermoelectric materials can reversibly interconvert heat and electric energy, their efficiency must be significantly increased in order to use them e.g. for waste-heat harvesting. We focus on the synergism of empirical optimization with fundamental understanding and theoretical models for the prediction of thermoelectric properties. A broad range of diffraction, spectroscopic and microscopic (TEM) methods shall provide a reliable basis for advanced discussion. Compounds inspired by minerals such as Cu5FeS4 (bornite) with distorted antiflourite-type structure, CuFe2S3 (cubanite) with a wurtzite-related structure, chain structure like KFe2S3 (rasvumite) and structures with cation layers like Cu5.5FeS6.5 (nukundamite) offer unique possibilities of combining Cu-atom mobility at high temperatures with the effects of mixed valence states (in some cases after suitable substitutions). The structural variety is supplemented by copper bismuth sulfides with complex structures analogous to sulfosalts. The cheap and nontoxic sulfides are an attractive starting point; however, anion substitution by Se or Te is expected to enhance the electrical conductivity and to simultaneously reduce the thermal conductivity by introducing disorder. The variation of the synthesis conditions and further cation substitution leads to disorder on various length scales, including spin disorder due to the mixed valence of magnetic cations. In a new approach to combine structural and electronic disorder with tuning charge-carrier concentration and mobility, we aim at decoupling not only thermal from electrical conductivity, but also at an independent tuning of the Seebeck coefficient. The substitution of Cu by Ag and/or Li on cation sites allows separating the influence of mixed-valence effects of Cu from those of Fe. Oxidation states and chemical bonding in iron-containing compounds will be probed by Mößbauer spectroscopy, supplemented by EPR, XPS and susceptibility measurements which are also suitable for the other elements involved. In combination with DFT calculations, we expect a deep understanding of Cu-Cu and Fe-Cu interactions as well as spin disorder. The feedback of such results on the optimization of thermoelectric materials yields an intriguing interplay of fundamental research and materials science.