Crystal chemistry in phase change thermoelectrics
Synthesis and Properties of Functional Materials
Final Report Abstract
At the beginning of this project, the structural details and the potential of chemically influencing the phase transition temperature, i.e., the stability window of a polymorph, were investigated in AgBiSe2. It was shown that the 6s2 lone electron pair of Bi, respectively the 5s2 lone electron pair of Sb in the substitution series AgBi1- xSbxSe2, leads to a local structural distortion, which was investigated utilizing pair distribution function analyses. This local structural feature was proposed as one reason for the astonishingly low lattice thermal conductivities that are frequently reported in the I-V-VI2 material class. Moreover, the proposed goal of lowering the phase transition temperature by substitution was achieved as part of this work, with a reduction of the phase transition by 40 K going from AgBiSe2 to AgBi0.85Sb0.15Se2. Based on this work, further research questions where developed, e.g., which other substitutions can influence the phase transition temperature and are local structural distortions present in all polymorphs of the material class. Here, the second question was answered in a collaboration with the research group Prof. Dr. Kanishka Biswas as part of this project, showing that local distortions exist in the high temperature phase of the closely related AgBiS2. Unfortunately, the exploration of more substitutions and their influence on the phase transition temperature was already conducted by other research groups, so that continuing the work on AgBiSe2 was deemed not promising. Therefore, the project was continued by investigating the second proposed material system MgAgSb. This work was done as part of a M.Sc. project, which revealed that solid state synthesis of MgAgSb is challenging, leading to the necessity of extensive synthesis optimization. While the synthesis was successfully optimized for MgAgSb, it was quickly realized that any substitution series requires similar, month-long, optimizations. Facing these adversities, it was decided that it is more fruitful to extend the ideas and concepts of this project to a new material system, the Ag argyrodites. This decision proved successful, as it led to another publication in context of this project. In this study, it was shown that the Ag argyrodites conduct thermal energy like a glass, by so-called diffusons, questioning the validity of historically established transport models. From this, it the transport mechanism revealed for the argyrodites for the first time in this work, is likely prevalent in other material classes, motivating further research. Moreover, it was shown that the low thermal transport is not macroscopically related to the fast ionic conduction of this material glass, which opens possibilities to reduce the ionic conductivity while keeping the beneficial (for thermoelectric application) thermal transport properties. This is especially relevant, since high ionic conductivities cause stability issues, which in the past lead to the discontinuation of efforts to use these materials in thermoelectric generators.
Publications
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„Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe2” Inorg. Chem. 2019, 58, 9236-9245
Bernges T., Peilstöcker J., Dutta M., Ohno S., Culver S.P., Biswas K., Zeier W.G.
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„Origin of Ultralow Thermal Conductivity in n-Type Cubic Bulk AgBiS2: Soft Ag Vibrations and Local Structural Distortion Induced by the Bi 6s2 Lone Pair” Chem. Mater. 2019, 31, 2106-2113
Rathore E., Juneja R., Culver S.P., Minafra N., Singh A.K., Zeier W.G., Biswas K.
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„Considering the Role of Ion Transport in Diffuson Dominated Thermal Conductivity” Adv. Energy Mater. 2022, 12, 2200717
Bernges T., Hanus R., Wankmiller B., Imasato K., Lin S., Ghidiu M., Gerlitz M., Peterlechner M., Graham S., Hautier G., Pei Y., Hansen M.R., Wilde G., Snyder G.J., George J., Agne M.T., Zeier W.G.
