Pressure-induced formation of hydrogen bonds in KNH2 studied by first principles

Abstract

Using particle swarm optimization technique implemented in the CALYPSO code, we have performed systematic research for the structures of KNH₂ at pressures up to 20 GPa. Here, phase transition from the ground-state α-KNH₂ (monoclinic P2₁/m) to β-KNH₂ (monoclinic P2₁) occurs at 4 GPa; then, with pressure increasing up to 6.8 GPa, β-KNH₂ transforms into γ-KNH₂ (monoclinic Pc). By analyzing the partial density of states and charge density, ionic bonding nature between K⁺ and [NH₂]⁻ is revealed and there exists a strong covalent bonding between N and H atoms in NH₂ groups in the three structures. Moreover, N–H⋯N hydrogen bonding between neighboring NH₂ groups is suggested in β- and γ-KNH₂ by investigating the structural details and charge density, which could be favorable for accelerating dehydrogenation behavior of complex metal hydrides.