Hydrogen encapsulation in a silicon clathrate type I structure: Na 5.5(H2)2.15Si46: Synthesis and characterization

Doinita Neiner, Norihiko L. Okamoto, Cathie L. Condron, Quentin M. Ramasse, Ping Yu, Nigel D. Browning, Susan M. Kauzlarich

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56 Citations (Scopus)


A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na 5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300°C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H 2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23Na, 1H, and 29Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23Na NMR shows that sodium completely fills the small cage and is deficient in the larger cage. The 1H NMR spectrum shows a pattern consistent with mobile hydrogen in the large cage. 29Si NMR spectrum is consistent with phase pure type I clathrate framework. Elemental analysis is consistent with the stoichiometry Na5.5(H2.15)2Si46. The sodium occupancy was also examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the Na occupancy of the large cage, 6d sites, is less than 2/3, consistent with the NMR and elemental analysis.

Original languageEnglish
Pages (from-to)13857-13862
Number of pages6
JournalJournal of the American Chemical Society
Issue number45
Publication statusPublished - 2007 Nov 14


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