水素貯蔵物質LiNH2のラマン散乱による研究 <学位論文要旨>

LiNH2 is a light-weight hydrogen storage material, and emits hydrogen above 200°C reacting with LiH. Polarization dependence of Raman scattering spectra for the single crystalline LiNH2 gives us the assignments of all observed phonon modes successfully. We also measured temperature dependence of Raman spectra from 3.4K to 673K(400°C). The energy of Li vibration, of which is the lowest, anomalously decreases with decreasing temperature. This anomaly designates that Li vibration is highly anharmonic with large amplitude. The Li vibration energy of 133cm–1 reveals that the force constant between Li and NH2 is very small. This weak interaction suggests that LiNH2 easily decomposes to Li and NH2. Below 100K, we observed new peaks in the energy range from 100 to 700cm–1. Since new peaks are originated in the dynamical properties of NH2, we conclude that rotational motion of NH2 molecule freezes below 100K. By the temperature dependence of the NH2 vibration energy, we conclude that N-H bond length becomes long and H-N-H bond angle becomes narrow with increasing temperature. Since the lattice vibrations with the E symmetry splits into 2 peaks at 200°C, the crystal structure changes. Above 300°C, we observed the similar spectra at room temperature at the energy range from 100 to 700cm–1 for the single crystal. This suggests that the reaction finishes at the thin surface layer in the single crystal. We have measured Raman scattering of powder LiNH2 above room temperature. We have found that the reaction starts by about 50°C lower temperature than that of the single crystal. The powder LiNH2 decomposes to the Li2NH and finally becomes to Li3N.