The Structure of Metal-Amine Solutions and the Metal-Nonmetal Transition

Jonathan Wasse, Cecilia Gejke, Helen Thompson, Chris Howard, Tom Weller, Arthur Lovell, Neal Skipper, Shusaku Hayama & Jennifer Walters

We are currently using neutron and X-ray scattering to study the solvation of electrons and the metal-nonmetal transition in metal-amine solutions. This research is carried out at UCL, Institut Laue Langevin (figure below), ESRF (figure left), and ISIS Facility.

Alkali metals dissolve readily in amines, without chemical reaction. Since their discovery by Sir Humphrey Davy, in 1808, these solutions have been the subject of intense experimental and theoretical study.

The phase diagram for lithium in ammonia is shown opposite. At low concentrations (I blue), below about 1 mole per cent metal (MPM), the solutions have an intense blue colour, due to the presence of solvated electrons. These can exist as either polarons or bipolarons. Around 4MPM the solutions take on a wonderful bronze colour, and become metallic (II gold). Below the consolute temperature (TC) the metallic and nonmetallic liquids do not mix, and there is phase separation, in which the metal floats on the nonmetal (III red). Regions VI and V are solid ammonia and lithium respectively. Note also that there is a deep pseudo-eutectic at saturation, which extends to 88K and gives the lowest temperature liquid metal (top inset). The expansion of the solution due to the presence of excess electrons is shown in the two molecular snapshots (bottom inset). At saturation (about 20MPM) the solutions are better conductors of electricity than liquid mercury, and have a lower density than any liquids except the cryogens. Potential industrial applications include battery electrolytes, waste disposal, low temperature heat exchangers, and energy storage. The nonmetal-metal transition is shown in the montage of lithium-ammonia solutions below.

2MPM

3MPM

4MPM

6MPM

8MPM

10MPM

15MPM

20MPM

Our aim is to understand the properties of metal-amine solutions in terms of their microscopic structure. To this end, we are conducting the first high resolution diffraction studies of these systems.

Recent Publications

• The structure of lithium-ammonia and sodium-ammonia solutions by neutron diffraction.
  J. Chem. Phys. 118, 7486-7494 (2003). PDF

• Structural studies of ammonia and metallic lithium-ammonia solutions.
  J. Am. Chem. Soc. 125, 2572-2581 (2003). PDF

• Neutron diffraction study of the structure of saturated sodium-ammonia solutions.
  J. Mol. Liquids. 96-97, 341-352 (2002). PDF

• X-ray diffraction studies of solutions of lithium in ammonia: The structure of the metal-nonmetal transition.
  J. Chem. Phys. 116, 2991-2996 (2002). PDF

• Structure of Solutions of lithium in methylamine across the metal-nonmetal transition.
  J. Phys. Chem. B 106, 11-14 (2002). PDF

• The solvation structure of lithium in ammonia.
  ILL Annual Report 2001: Scientific Highlights. HTML

• The structure of liquid methylamine and solutions of lithium in methylamine.
  Mol. Phys. 99, 779-786 (2001). PDF

• X-ray absorption spectroscopy investigations of metal-ammonia solutions.
  ESRF Annual Report 1999: Scientific Highlights. HTML

• Structure of a metallic solution of lithium in ammonia.
  Phys. Rev. B 61, 11993-11997 (2000). PDF

• The structure of saturated lithium- and potassium-ammonia solutions as studied by using neutron diffraction.
  J. Chem. Phys. 112, 7147-7151 (2000). PDF