Complex structured "electron-poor" framework semiconductors with potential for thermoeletric application
Zusammenfassung der Projektergebnisse
A considerable number of new compounds with potential thermoelectrical properties has been synthesized and characterized. The physical property characterization of ZnSb and ZnAs showed an inherently low lattice thermal conductivity of both materials, which is comparable to the state-of-the-art thermoelectric material PbTe. A combination of high resolution X-ray diffraction studies and first principles calculations revealed peculiar vibrational properties for both materials. These properties manifest themselves in a multitude of localized low energy optical modes (which couple with the acoustic, heat carrying phonons) and a pronounced anharmonic thermal displacement behavior of the Zn atoms which are part of multicenter bonded structural Zb2X2 (X = Sb,As) entities. Our studies reveal that multicenter bonded structural entities appear to be a common feature of EPFS. The establishment of a direct correlation between bonding properties and the dynamic/vibrational behavior of EPFS might provide an elegant and unifying concept to control and manipulate the thermal conductivity behavior of EPFS. We therefore consider the establishment of this concept as milestone of the project. With regard to the sample processing aspects of the project the thermoelectric performance of ZnSb could be decisively increased in spark plasma sintered materials with Ag-doping. The property characterization of undoped ZnAs and Zn5Sb4In2 samples, however, disqualified these potential materials as state-of-the-art thermoelectrics. In case of ZnAs the rather high ionicity prevented high charge carrier mobility. In contrast, form open tetrahedral frameworks composed of B and Si in combination with Li electron precise Zintl phases, which due to their relatively large band gap remain semiconductors. Due to the extensive synthetic method, using high-pressure units doping could not be investigated within this project. In the case of clathrate compounds, electron-precise beside metallic compounds were obtained in dependency on the kind of doping of the tetrel element of the host framework. However no better element combination with respect to thermoelectrical performance than known Ba-Ga-Ge was reached. The successful synthesis of alpha-rhombohedral boron from Pt fluxes at high pressure represents another milestone of this project. Firstly, single crystals of extremely high quality could be obtained, which for the first time allow the determination of accurate experimental charge densities which even reveal the core shell contraction of the boron atoms due to covalent bond formation at sub-atomic resolution. Secondly, the p,T phase diagram of boron could be studied and extended without kinetic constraints. The p,T phase boundary between the ordered alphaboron and disordered beta boron was established. We also succeeded in establishing a bulk synthesis route for β-SiB3, which previously has only been reported as a byproduct in a complicated flux reaction. Finally, the Zintl-compound CaSi has been characterized via experimental charge density analyses employing high quality crystals. This study provides for the first time clear experimental evidence that cations in Zintl phases might be part of the covalently connected anionic substructure. To conclude: The establishment of a direct correlation between bonding properties and the dynamic/vibrational behavior of EPFS warrants further exploration to provide an elegant and unifying concept to control and manipulate the thermal conductivity behavior of EPFS.
Projektbezogene Publikationen (Auswahl)
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“Thermoelectric properties of Zn5Sb4In2-δ (δ = 0.15),” J. Appl. Phys., vol. 111, no. 12, pp. 123712–123712–7, 2012
Y. Wu, A. P. Litvinchuk, E. S. Toberer, G. J. Snyder, N. Newman, A. Fischer, E.-W. Scheidt, W. Scherer, and U. Häussermann
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“Transport properties of the II–V semiconductor ZnSb,” J. Mater. Chem. A, vol. 1, no. 4, pp. 1407–1414, 2012
D. Eklöf, A. Fischer, Y. Wu, E.-W. Scheidt, W. Scherer, and U. Häussermann
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“Structural Principles and Thermoelectric Properties of Polytypic Group 14 Clathrate-II Frameworks,” ChemPhysChem., vol.14, pp. 1807- 1817, 2013
A. Karttunen and T. F. Fässler
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“Synthesis of Large Single Crystals and Thermoelectrical Properties of the Type-I Clathrate K8Zn4Sn42,” Z. anorg. allg. Chem., vol. 639, no. 12–13, pp. 2125–2128, 2013
V. Baran, A. Fischer, W. Scherer, and T. F. Fässler
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„LiBSi2: A tetrahedral semiconductor framework from B and Si atoms bearing Li atoms in channels“ Angew. Chem. Int. Ed., vol. 52, no. 23, pp. 5978 –5982, 2013
M. Zeilinger, L. van Wüllen, D. Benson, V. F. Kranak, S. Konar, T. F. Fässler, U. Häussermann
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“A Combined Metal–Halide/Metal Flux Synthetic Route towards Type-I Clathrates: Crystal Structures and Thermoelectric Properties of A8Al8Si38 (A=K, Rb, and Cs),” Chem. Eur. J., vol. 20, no. 46, pp. 15077–15088, 2014
V. Baran, A. Senyshyn, A. J. Karttunen, A. Fischer, W. Scherer, G. Raudaschl- Sieber, and T. F. Fässler
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“Probing the Zintl–Klemm Concept: A Combined Experimental and Theoretical Charge Density Study of the Zintl Phase CaSi,” Angew. Chem. vol. 126, no. 11, pp. 3073–3077, 2014. Angew. Chem. Int. Ed., vol. 53, no. 11, pp. 3029–3032, 2014
I. M. Kurylyshyn, T. F. Fässler, A. Fischer, C. Hauf, G. Eickerling, M. Presnitz, and W. Scherer
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“Synthesis, Structure, and Properties of the Electron-Poor II–V Semiconductor ZnAs,” Inorg. Chem., vol. 53, no. 16, pp. 8691–8699, 2014
A. Fischer, D. Eklöf, D. E. Benson, Y. Wu, E.-W. Scheidt, W. Scherer, and U. Häussermann
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“On the nature of superconductivity in the anisotropic dichalcogenide NbSe2{CoCp2}x,” J. Phys.: Condens. Matter, vol. 27, no. 15, p. 155701, 2015
E.-W. Scheidt, M. Herzinger, A. Fischer, D. Schmitz, J. Reiners, F. Mayr, F. Loder, M. Baenitz, and W. Scherer
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“Thermal and Vibrational Properties of Thermoelectric ZnSb - Exploring the Origin of Low Thermal Conductivity,”, Phys. Rev. B., vol. 91, p. 224309, 2015
A. Fischer, E.-W. Scheidt, W. Scherer, D. Benson, Y. Wu, D. Eklöf, and U. Häussermann
