Low energy structural dynamics and constrained libration of Li(NH3)4, the lowest melting point metal

The lattice and molecular dynamics for the solid phases of the lowest melting-point metal, Li(NH3)4, are determined by incoherent inelastic neutron scattering. Measurements of internal molecular displacements and distortions of the Li(NH3)4 units have been modelled and assigned using density functional theory calculations for the solid and molecular system. Inelastic neutron scattering measurement allows for the first determination of NH3 vibrational transitions.

Few metal–insulator transitions (MITs) can be deemed as remarkable as that found in the system of Li-metal dissolved in anhydrous NH3. The deep blue solutions of the dilute system undergo a phase change to the golden metallic liquid at 8 mole-percent metal (MPM) with the well documented phase separation occurring around 4 MPM, whereby the metallic phase floats above the semi- conducting liquid phase. The electronic and structural properties for this system have been recently reviewed and revisited. The most concentrated system at 20 MPM, Li(NH3)4, forms a eutectic and is currently the metal with the lowest melting point known, solidifying only at 88.8 K. Just as remarkable is the existence of several structural phase changes that occur in the solid system as it is cooled further. The highest temperature phase is cubic, conventionally termed phase I, and persists in the short range 80 o T o 88.8 K. This phase is not witnessed in Li(ND3)4,5 and has been determined by quasi-elastic neutron scattering measurements to be a plastic phase.6 Below 80 K the system crystallizes into the orthorhombic phase II, I4%3d, before finally crystallizing in a further cubic phase with possible anti- ferromagnetic ordering, phase III, P213. Tetrahedral geometry around Li is essentially retained in all phases, with only minor displacements of Li(NH3)4 units differentiating them. Studies on the vibrational dynamics of Li(NH3)4 are limited to molecular systems, including inelastic X-ray scattering7 (IXS) in the liquid phase, theoretical studies, and determination of the vibrational structure of the excited electronic state in the gas phase.

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