Raman Scattering Study of Hydrogen Storage Material LiNH2

Abstract

Raman scattering spectra of LiNH 2 have been measured from 3.4 to 673 K. Precise polarization dependence of the single crystalline LiNH 2 and first principles calculation have successfully assigned all observed peaks. Li vibration with the lowest energy at 133 cm -1 shows the anomaly, where its energy decreases with decreasing temperature. This anomaly shows that the Li vibration is highly anharmonic with large amplitude. In addition, the energy of 133 cm -1 gives the very small force constant of 0.05 mdyn/Å between Li and NH 2 in the diatomic model. This weak interaction suggests that LiNH 2 easily decomposes to Li and NH 2 . Below 100 K, we have found new peaks in the energy range from 100 to 700 cm -1 . No additional degrees of freedom for vibrations conclude that rotational motion of NH 2 molecule freezes below 100 K. With increasing temperature, the bond angle of H–N–H and bond length N–H in NH 2 become narrow and long, respectively. At the reaction temperature region, the correlation between the integrated intensity of N–H vibration in Li 2 NH and back ground slope has been observed and the background change is originated from the desorption of NH 3 molecules. Final materials due to chemical reactions are different between single crystal and powder samples. In the single crystal, the spectrum similar with that at room temperature appears at 673 K and this suggests that the reaction finishes only at the thin surface layer, not entire volume. However, the powder sample of LiNH 2 decomposes to Li 2 NH and finally change to the Li 3 N for the sake of NH 3 desorption.