Hydrogen encapsulation in a silicon clathrate type I structure: Na _5.5(H₂)_2.15Si₄₆: Synthesis and characterization

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 Na_5.5(H_2.15)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.