X-ray diffraction studies of solutions of lithium in ammonia: The structure of the metal–nonmetal transition

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Abstract
The structure of solutions of lithium in ammonia has been studied at 0, 2, 8, and 22 mol % metal (MPM) and 200 K by wide-angle x-ray diffraction. The principal diffraction peak shifts from 2.14(2) Å−1 at 0 MPM to 1.93(3) Å−1 at 22 MPM, reflecting the 30% decrease in overall density as the solution expands to accommodate the excess electrons. We find that the solvent is significantly perturbed over both the short- and intermediate-length scales. The nearest neighbor (N–N) coordination number decreases from 11.8(10) at 0 MPM to 7.6(10) at 22 MPM. In addition, electrostriction around the fourfold coordinated lithium ions causes N–N correlations to become progressively shorter as concentration is increased. At 22 MPM a strong diffraction prepeak is located at 1.05(3) Å−1. Upon dilution to 2 MPM, our experiments find that this feature shifts to 1.29(5) Å−1. We conclude that the prepeak observed in our experiments is a signature of polaronic solvent cavities of approximate radius 2.6 Å. The first solvation shell of an excess electron then contains about 7 ammonia molecules, the second shell about 30 ammonia molecules. This picture is in excellent agreement with interpretation of magnetic resonance data.