Samples of the clathrate Na x Si 136 were saturated with hydrogen to 100 atm at 25°C in a Sievertstype apparatus and at pressures of 6 and 28 kbar in lentil-type high-pressure apparatuses at 100 and 250°C. X-ray powder diffraction analysis and Raman spectroscopy of the samples quenched after the saturation with hydrogen showed that the phase composition of the clathrates did not change. Heating of the quenched samples to room temperature in a thermal desorption setup produced not hydrogen, but hydrogen-containing gases, as we assumed, silanes. Heating to 650°C leads to decomposition of these compounds to form hydrogen.

Aminoborane NH3BH3 is proposed as an appropriate material to produce hydrogen in the high-pressure cells designed for the synthesis of hydrides in sizeable amounts at pressures of a few GPa and elevated temperatures. Aminoborane is a non-hydroscopic material and it does not noticeably react with air that permits assembling the high-pressure cells under ambient conditions without any precautions. If heated to 300 degrees C at any pressure from 0.6 to 9 GPa, aminoborane decomposes to H-2 gas and chemically inert amorphous BN and does not further absorb the liberated hydrogen. Experiments using NH3BH3 and AIH(3) alternatively as the internal hydrogen source gave coinciding isotherms of hydrogen solubility in rhodium at 600 degrees C and pressures up to 9 GPa therefore demonstrating that the partial pressure of impurities (if any) in the H-2 gas generated by NH3BH3 is well below the accuracy +/- 0.3 GPa of determination of the total gas pressure. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching H/Ta = 0.92(5) at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at P = 2.2 GPa. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-N-2 temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with a = 3.224(3) and c = 5.140(5) angstrom at T = 85 K. The hydrogen content determined by thermal desorption was H/Ta = 2.2(1).