Thermoelectric properties of mixed-valence chalcogenides

本项目旨在寻找新的概念，将混合价过渡金属硫化物的热电性质与其晶体结构、真实结构和电子结构联系起来。这包括根据“声子-液体电子-晶体”的概念，动态无序的铜原子降低导热系数的额外好处。虽然热电材料可以可逆地相互转换热能和电能，但必须显著提高它们的效率才能将其用于废热收集等。我们专注于经验优化与预测热电性能的基本理解和理论模型的协同作用。广泛的衍射、光谱和显微(TEM)方法应为深入讨论提供可靠的基础。化合物的灵感来源于矿物，如具有扭曲的反氟矿结构的Cu5FeS4(斑铜矿)，具有纤锌矿相关结构的CuFe2S3(方铅矿)，链结构如KFe2S3(Rasvumite)，以及具有阳离子层结构的Cu5.5FeS6.5(NUKUNDAMITE)，这些化合物提供了将高温下铜原子的流动性与混合价态的影响(在某些情况下，在适当的取代之后)相结合的独特可能性。结构变化的补充是铜铋硫化物，具有类似于硫酸盐的复杂结构。廉价无毒的硫化物是一个很有吸引力的起点；然而，用Se或Te取代阴离子有望提高电导率，同时通过引入无序降低热导率。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而引起的自旋无序。在一种将结构和电子无序与可调载流子浓度和迁移率相结合的新方法中，我们的目标不仅是将热与电导解耦，而且还旨在独立地调谐Seebeck系数。通过在阳离子中心用Ag和/或Li取代铜，可以将铜的混价效应与铁的混价效应的影响分开。含铁化合物中的氧化态和化学键将通过Möber Bauer光谱进行探索，并辅之以EPR、XPS和磁化率测量，这些测量也适用于所涉及的其他元素。结合密度泛函理论计算，我们期望对铜-铜和铁-铜相互作用以及自旋无序有更深入的了解。这些结果的反馈对热电材料的优化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.