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Synthesis and Characterization of Large Single Crystals of Silicon and Germanium Clathrate-II Compounds and a New Tin Compound with Clathrate Layers

Published online by Cambridge University Press:  01 February 2011

Svilen Bobev  and
Slavi C. Sevov
Svilen Bobev
Affiliation:
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556–5670 E-mail: ssevov@nd.edu
Slavi C. Sevov
Affiliation:
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556–5670 E-mail: ssevov@nd.edu
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Abstract

We have synthesized large single crystals of clathrate-II compounds with frameworks of silicon and germanium by employing mixed alkali metal countercations. The combinations of alkali metals are rationally selected in order to fit the different cages of the clathrate-II structure. This approach leads to the following stoichiometric and fully “stuffed” compounds: Cs8Na16Si136, Cs8Na16Ge136, Rb8Na16Si136 and Rb8Na16Ge136. The structures and the corresponding Si-Si and Ge-Ge distances are elucidated and established with high accuracy from extensive single crystal X-ray diffraction work. The compounds are stoichiometric, metallic, and are very stable at a variety of extreme conditions such as heat, concentrated acids, hydrothermal treatment etc. No evidence was found for vacancies in the silicon and germanium networks or partial occupancies of the alkali metal sites. The stoichiometry of these fully “stuffed” clathrates is consistent with the measured temperature independent Pauli paramagnetism, supported also by the conductivity measurements on single crystals and thermopower measurements on pellets. A new compound with novel clathrate-like structure forms when small and large cations are combined with tin. The new materials, A6Na18Sn46 (A = K, Rb, Cs), are made of clathrate layers and the interlayer space filled with Sn4-tetrahedra and alkali-metal cations. Its formula can be rationalized as A6Na6Sn34 + 3·Na4Sn4 (one clathrate layer and three tin tetrahedra). The compound is stable in air and is being currently tested at other conditions. Detailed measurements of its transport properties are under way.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 626: Symposium Z – Thermoelectric Materials 2000 - The Next Generation Materials for Small-Scale Refrigeration and Power Generation Applications , 2000 , Z13.5
Copyright
Copyright © Materials Research Society 2000

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References

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