Synthesis and Characterization of Inorganic Clathrate-II Materials
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Synthesis and Characterization of Inorganic Clathrate-II Materials Matt Beekman, and George S. Nolas Department of Physics, University of South Florida, 4204 East Fowler Ave., Tampa, FL, 33620 ABSTRACT Preliminary results from an investigation into the synthesis and characterization of silicon and germanium type II clathrates are reported. A series of NaxSi136 (0 < x < 24) clathrates was synthesized and characterized by powder X-ray diffraction and Rietveld analysis. The NaxSi136 lattice parameters are observed to first decrease, then increase with increasing Na content, indicating a non-monotonic structural response to Na filling. New type II Ge clathrate compositions Cs8Na16MyGe136-y (M = Cu, In) utilizing framework substitution are reported. Electrical transport measurements on a Cu substituted specimen indicate framework substitution modifies the transport properties of these materials. The potential type II clathrate phases possess for thermoelectric applications is discussed. INTRODUCTION Within the context of the search for novel, high efficiency thermoelectric materials, much attention is currently being paid to inorganic clathrate materials derived from elements of group IV [1-4]. Interest in these materials was initiated with the identification [5,6] of intermetallic phases crystallizing with the type I clathrate hydrate crystal structure as potential candidates for high temperature thermoelectric applications. Since then, relatively large thermoelectric figures of merit at high temperatures have been reported for type I [3,7] as well as other clathrate compounds [8]. The type II clathrate framework is formed by the space filling face sharing of two types of polyhedral units, shown in Figure 1: the dodecahedra E20 and hexacaidecahdra E28, that are present in the structure in the E20:E28 ratio of 2:1. There are three distinct crystallographic sites in the framework: 8a, 32e, and 96g. Guest atoms reside in the interiors of the E28 and E20 polyhedra, at the 8b and 16c sites, respectively. The resulting structure is face-centered cubic (space group Fd 3 m ), and the general chemical formula may be written as A8B16E136 (A = guest at the 8b site, B = guest at the 16c site, and E = Si, Ge, Sn, or substituents). One of the distinctive characteristics of group IV clathrate-II phases is the ability to continuously vary the guest concentration [4]. This offers the unique opportunity to study how the physical properties of inorganic clathrates evolve as the guest content is varied. In addition, this feature could prove useful for “tuning” the thermoelectric properties of these materials.
Figure 1. The E28 and E20 polyhedral cages that form the type II clathrate crystal structure. The independent crystallographic sites are labeled. Much effort has focused on understanding the physical properties of type I clathrate materials, and though new compositions continue to be reported, the knowledge of what compositions are possible is relatively well developed for this material system. A similar understanding i
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