Synthesis and characterization of K(8-x)(H2)ySi46

A hydrogen-containing inorganic clathrate with the nominal composition, K(7)(H(2))(3)Si(46), has been prepared in 98% yield by the reaction of K(4)Si(4) with NH(4)Br. Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. Elemental analysis and (1)H MAS NMR confirmed the presence of hydrogen in this material. Type I clathrate structure is built up from a Si framework with two types of cages where the guest species, in this case K and H(2), can reside: a large cage composed of 24 Si, in which the guest resides in the 6d position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position (cubic space group Pm3n). Potassium occupancy was 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 K is deficient in both the 2a and the 6d sites. (1)H and (29)Si MAS NMR are consistent with the presence of H(2) in a restricted environment and the clathrate I structure, respectively. FTIR and (29)Si{(1)H} CP MAS NMR results show no evidence for a Si-H bond, suggesting that hydrogen is present as H(2) in interstitial sites. Thermal gravimetry (TG) mass spectrometry (MS) provide additional confirmation of H(2) with hydrogen loss at approximately 400 degrees C.