Silicon Clathrates: Synthesis and Characterization
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SILICON CLATHRATES: SYNTHESIS AND CHARACTERIZATION GANESH K. RAMACHANDRAN *, JASON DIEFENBACHER **, OTI'O F. SANKEY ***, RENU SHARMA ****, ROBERT F. MARZKE ***, MICHAEL O'KEEFFE*, JAN GRYKO ***** AND PAUL F. MCMILLAN *"*** * Dept. of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, [email protected] "**Science and Engineering of Materials, Arizona State University, Tempe, AZ 85287 ***Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504 ****Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704 *****Dept. of Physical and Earth Sciences, Jacksonville State University, Jacksonville, AL 36235 ABSTRACT Two types of silicon clathrates, Na•Si 136(0•-< x < 24) and Na 8Si,6 and a mixed clathrate (Na,Ba)Si46 were synthesized and characterized by X-ray diffraction, 23Na NMR and high resolution TEM. Systematic changes in X-ray diffraction intensities enabled the sodium content and site occupancy in the Na.Si 136series to be followed and then refined by Rietveld profile analysis. 23Na NMR spectra of Na 8Si46 and NakSi,36 samples reveals two peaks with large paramagnetic shifts (1600-2000 ppm) for each phase, associated with the presence of Na atoms in two different environments. High resolution electron microscopy reveals the clathrate structures of these samples, and allows classes of defects to be characterized. INTRODUCTION Open framework compounds consisting of 3-D networks of silicon polyhedra were first synthesized and characterized by Cros et. al. in 1965 [1,2]. These materials were obtained as metastable phases during the thermal decomposition of the Zintl phase NaSi, which contains Na' and Si 44 ions. During decomposition and volatilization of sodium into the vacuum, the silicon framework reforms into one or more network structures termed "clathrates", by analogy with the isostructural ice clathrate structures, and because of the host-guest nature of the open silicon framework and the included sodium atoms. The first of these structures, NaSi46 (type I hydrate structure), contains two types of polyhedra, one with 20 silicon atoms (dodecahedral sites) and the other with 24 silicon atoms (tetrakaidecahedral sites). Two of the former and six of the latter constitute one unit cell. The positions of the centers of the polyhedra correspond to the vertices of the Cr.Si structure. In the mixed clathrate (Na,Ba)Si 46 the larger cages are occupied by barium atoms while sodium occupies the smaller cages. The NakSi 1 6 (0 < x -
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