Pressure-Induced Phase Transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at ∼12 GPa is complete at ∼14 GPa from ambient-pressure α-LiNH2 (tetragonal, I4̄) to a high-pressure phase denoted here as β-LiNH2. This phase transition is reversible upon decompression with the recovery of the α-LiNH2 phase at ∼8 GPa. The N−H internal stretching modes (ν([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure β-LiNH2 phase appears at ∼12.5 GPa. Beyond ∼14 GPa, the N−H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above ∼14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure β-LiNH2 phase.