Synthesis and Characterization of Large Single Crystals of Silicon and Germanium Clathrate-II Compounds and a New Tin Co
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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 Svilen Bobev and Slavi C. Sevov* Department of Chemistry and Biochemistry University of Notre Dame Notre Dame, IN 46556-5670 E-mail: [email protected]
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.
INTRODUCTION
The clathrates are inclusion compounds with open three-dimensional frameworks, made of tetrahedrally coordinated group 14 element with voids that are occupied by alkali metals. Their structures are isotypical with the structures of the gas hydrates, reported as early as 1811. Until now, two types of such compounds are known, clathrates of types I and II, with prototypes the gas hydrates (G8(H20)46), where G = Kr, Xe, Cl2, CH4, etc., and liquid or double hydrates (G24(H2O)136 or G16G8(H2O)136), where G = H2S, CHCl3, CH3NO2, C3H8, etc., respectively. An analogy can be also drawn between the zeolites and the clathrates - the latter being often referred to as "zeolites without oxygen" or "oxygen-free zeolites". Due to the many ways to link SiO4/2 tetrahedra in the zeolites via the rather flexible Si–O–Si angle, there are a great
Z13.5.1 Downloaded from https://www.cambridge.org/core. University of California, Santa Cruz, on 22 Dec 2019 at 01:23:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-626-Z13.5
variety of zeolite struct
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