Thermoelectric properties of mixed-valence chalcogenides

该项目旨在提出新概念，将混合价过渡金属硫属化物的热电性质与其晶体结构、真实的结构和电子结构联系起来。这包括动态无序铜原子的额外益处，其根据“声子-液体电子-晶体”概念降低热导率。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，以便将它们用于例如废热收集。我们专注于经验优化的协同作用与基本的理解和理论模型预测热电性能。广泛的衍射、光谱和显微镜（TEM）方法应为高级讨论提供可靠的基础。受矿物启发的化合物，如具有扭曲的反铁锰矿型结构的Cu 5 FeS 4（铁锰矿），具有纤锌矿相关结构的CuFe 2S 3（立方铁矿），如KFe 2S 3（斜方铁矿）的链结构和如Cu5.5FeS6.5（nukundamite）的阳离子层结构，提供了将高温下铜原子迁移率与混合价态效应（在某些情况下经过适当取代）相结合的独特可能性。结构多样性的补充是铜铋硫化物与复杂的结构类似的磺酸盐。廉价且无毒的硫化物是一个有吸引力的起点;然而，被Se或Te取代的阴离子预计会提高电导率，同时通过引入无序来降低热导率。合成条件的变化和进一步的阳离子取代导致在各种长度尺度上的无序，包括由于磁性阳离子的混合价的自旋无序。在一种新的方法，联合收割机结构和电子的无序与调谐电荷载流子浓度和迁移率，我们的目标是去耦不仅从电导率的热，而且在一个独立的调谐塞贝克系数。由Ag和/或Li在阳离子位点上的Cu的取代允许将Cu的混合价效应的影响与Fe的混合价效应的影响分开。含铁化合物中的氧化态和化学键将通过穆斯堡尔谱进行探测，并辅以EPR，XPS和磁化率测量，这些测量也适用于其他相关元素。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之间有趣的相互作用。

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.