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A3: High temperature materials

High-Temperature Thermoelectric Properties of Eu9Cd4-xCM2+x-y◊ySb9: Stuffed Ca9Mn4Bi9 type structure with Coinage Metals (Cu, Ag, Au)

Nasrin Kazem1, Saneyuki Ohno2, Antonio Hurtado1, G. Jeffrey Snyder 2, Susan M. Kauzlarich1
1 — Department of Chemistry, University of California, One Shields Ave., Davis, CA 95616, USA2 — Materials Science, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA

Complex crystal structures are good candidates to search for improved materials for thermoelectric (TE) application as they can provide low thermal conductivity an essential for high efficient TE materials. These compounds may also provide separate routes for the electrons and the phonons leading to Phonon-Glass Electron-Crystal (PGEC) materials which are ideal for thermoelectric applications. New intermetallic compounds of Eu9Cd4-xCM2+x-y◊ySb9 CM = Coinage Metals (Au, Ag and Cu) were synthesized through the flux reaction and their structures determined by single-crystal X-ray diffraction shows a lot of disorder in these compounds. All compounds are isostructural and crystallize in the centrosymmetric orthorhombic space group Pbam (no. 55, Z = 2) with unit cells ranging a = 12.9086‒12.9406 Å , b = 22.8276‒23.0141 Å, and c = 4.718‒4.7603 Å.  The structure can be considered as the imaginary Eu9Cd4Sb9 structure in which the polyanionic network requires 19 electrons to satisfy octet rule while the cations only provide 18 electrons. This electron deficiency can be alleviated by filling the interstitial sites by transitional metals. To provide one electron to this structure to reach the optimal bonding condition, the interstitial position (a 4g Wyckoff site) needs to be occupied by either 25% of a divalent or 50% of a monovalent transitional metal to give Eu9Cd4MSb9 composition (M = monovalent transitional metal). Crystal structure refinements on Eu9Cd4-xCM2+x-y◊ySb9 CM = Au, Ag and Cu show that the so-called interstitial position (TM3 site) gets occupied > 50% and make these compounds valence imprecise compounds leading to metallic conducting materials. In this presentation, I will discuss the thermoelectric and magnetic properties of these new intermetallic phases and I will compare these materials with the state of the art thermoelectrics and present how these materials are superior to other intermetallics in providing a PGEC compound that scatters phonons without disrupting the electrons.