Lithium Amide (LiNH2) Under Pressure

Abstract

Static high pressure lithium amide (LiNH₂) crystal structures are predicted using evolutionary structure search methodologies and intuitive approaches. In the process, we explore the relationship of the structure and properties of solid LiNH₂ to its molecular monomer and dimer, as well as its valence-isoelectronic crystalline phases of methane, water, and ammonia all under pressure. A NaNH₂ (Fddd) structure type is found to be competitive for the ground state of LiNH₂ above 6 GPa with the P = 1 atm I4̅ phase. Three novel phases emerge at 11 (P4̅2₁m), 13 (P4₂/ncm), and 46 GPa (P2₁2₁2₁), still containing molecular amide anions, which begin to form N–H···N hydrogen bonds. The P2₁2₁2₁ phase remains stable over a wide pressure range. This phase and another Pmc2₁ structure found at 280 GPa have infinite ···(H)N···H···N(H)···H polymeric zigzag chains comprising symmetric N···H···N hydrogen bonds with one NH bond kept out of the chain, an interesting general feature found in many of our high pressure (>280 GPa) LiNH₂ structures, with analogies in high pressure H₂O-ices. All the predicted low enthalpy LiNH₂ phases are calculated to be enthalpically stable with respect to their elements but resist metallization with increasing pressure up to several TPa. The possibility of Li sublattice melting in the intermediate pressure range structures is raised.