Theoretical Studies of High-Pressure Phases, Electronic Structure, and Vibrational Properties of NaNH2

Thermodynamically stable phases of sodium amide (NaNH2) at pressures up to 20 GPa have been determined using the ab initio evolutionary structure prediction. We find that the ground-state phase alpha-NaNH2 (orthorhombic, Fddd) first transforms into beta-NaNH2 (orthorhombic, P2(1)2(1)2) at 2.2 GPa; then, gamma-NaNH2 (monoclinic, C2/c) becomes stable at 9.4 GPa. In addition to strong ionic bonding between Na+ and [NH2](-) ions and covalent bonding between H and N in NH2 groups, the N- H center dot center dot center dot N hydrogen bonding between neighboring NH2 groups could not be ignored anymore in the high-pressure beta-NaNH2, as suggested by the analysis of charge density distribution and structural and vibrational properties. The covalent N H bonds in the high-pressure phase of NaNH2 are weakened by additional hydrogen bonding, which could be favorable for the hydrogen desorption,