Hydrogen has been considered as the most promising alternative source of energy as the world seeks a replacement for the conventional fossil fuel energy source. However, the storage of hydrogen still poses as a big challenge. As a result, development and investigation of hydrogen storage materials have received increasing attention. Here we report studies of two promising hydrogen storage materials, NaNH2 and NH3BH3, under high pressure by Raman and IR spectroscopy.
First, sodium amide (NaNH2) was investigated at room temperature and pressures up to 15 GPa. Starting with an orthorhombic crystal structure at ambient pressure, sodium amide was found to transform to two new phases upon compression as evidenced by changes of characteristic Raman and IR modes as well as by examining the pressure dependences of these modes. Raman and IR measurements on NaNH2 collectively provided consistent information about the structural evolution of NaNH2 under compression. Upon decompression, all Raman and IR modes were completely recovered indicating the reversibility of the pressure-induced transformations in the entire pressure region. The Raman and IR spectroscopic data together allowed for the analysis of possible structures of the new high-pressure phases of NaNH2.
Another potential hydrogen storage material, ammonia borane (NH3BH3), was investigated at simultaneous high pressures (up to 15 GPa) in a diamond anvil cell and low temperatures (down to 80 K) using a cryostat by Raman spectroscopy in situ. Upon cooling from room temperature to 220 K at near ambient pressure, an expected phase transformation from 74TM to Pmn2\ was observed. Then the sample was compressed to 15 GPa isothermally at 180 K. Three pressure induced structural transformations were observed as evidenced by the change of Raman profile as well as the pressure dependen ce of the major Raman modes. The decompression and warming-up experiments suggest these P-T-induced transformations are reversible. These observations, together with fact or group analysis, allowed us to examine the possible structures of the new high pressure phases and the nature of phase transitions. Raman measurements from multiple runs covering various P-T paths, when combined with previously established room-tempera ture and high-pressure data, enabled the update of the P-T phase diagram of ammonia borane in the pressure region of 0-15 GPa and the temperature region of 80-350 K
