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(Barely) Solid Li(NH3)4: The Electronics of an Expanded Metal

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journal contribution
posted on 16.03.2011, 00:00 by Eva Zurek, Xiao-Dong Wen, Roald Hoffmann
The highly expanded metal, lithium(0)tetraamine, and its electronic structure is as full of complexity and surprises as the lithium solutions in anhydrous ammonia from which it crystallizes at 90 K. Our theoretical studies of the Phase II, Z = 8, I4̅3d structure of this material reveal that the molecular building block is an almost ideal tetrahedron, in agreement with recent experiments. Close in enthalpy at P = 1 atm, and consistent with the low melting point, are bcc and Cs−IV configurations. Under pressure, the I4̅3d structure emerges as more stable than its alternatives. In this phase six relatively narrow bands, four of them occupied, separate from the conduction and valence bands. We trace these bands to pockets of electron density arising between sterically encumbered ammonias, six such pockets in the Z = 8 unit cell. The observed band structure can be explained by considering a Jortner−type model, where pseudoatoms are placed in these holes. The electride Li(NH3)4, while not a very good metal, is a unique material, by virtue of its low melting point.