These colourful ‘metal solutions’ are in a class of their own because they contain solvated electrons, and therefore offer us the opportunity to study fundamental physical phenomena.
Key to their utility is their ‘tunability’; through varying the electron density the solution can be continuously changed from an electrolyte to a liquid demonstrating genuine metallic behaviour. Chemical tunability is also available by varying the amine and metal used. In experiments now published in Angewandte Chemie, neutron diffraction on the SANDALS instrument was used to explore the microscopic structure of a new class of concentrated metal-amine liquid:, Li-NH3-MeNH2. In this new system, the amine solvent is a 50:50 mixture of ammonia and methylamine. Remarkably, the resulting solution is truly homogenous and metallic and appears unique amongst current expanded metal-amine solutions.
In 1808, whilst attempting to demonstrate that potassium is an element, Sir Humphry Davy discovered that a concentrated potassium-ammonia solution has a striking bronze/gold appearance, whilst a more dilute solution has an intense blue colour. There’s also a large increase in volume with metal concentration, meaning that, extraordinarily, a more concentrated solution will float above a more dilute one. Even now, these ‘metal solutions’ offer us the romance of alchemy, although we are now equipped to understand their unique structure and properties.
In the 1940s, R. A. Ogg claimed to have discovered evidence of high-temperature superconductivity in glassy metal-amine solutions, although his results have never been consistently replicated. The phase diagram for these solutions is similar to that for metals that have superconductor phases, and their tunability suggests that we may, one day, unlock their secret and potential.
In the meantime, their unique properties offer us unrivalled opportunities to study fundamental chemical and physical phenomena. Neutron diffraction at ISIS has been used to study metal-amine solutions since the late 1990s, and research now published in Angewandte Chemie has explored the structure of a solution formed with a mixed amine solvent, which adds another fascinating mechanism for tunability, and another way to control the phase diagram for fundamental investigations.
Neutrons and Metal-Amine Solutions
Ammonia and amines are hydrogen-based solvents, with a unique ability to accommodate high concentrations of metal (and electron) solutes. Neutron diffraction is a uniquely powerful method for determining the liquid structure of metal-amines, allowing us to probe free electrons in disorder, and SANDALS - the Small Angle Neutron Diffractometer for Liquids and Amorphous Samples – is the perfect instrument for this type of investigation. In these experiments, NMR (Nuclear Magnetic Resonance) and ESR (Electron Spin Resonance) were used alongside neutron diffraction to investigate the structure of the new solvent. Using a mixed solvent gives a novel type of electron solvation and delocalization, and the results show that this solution is truly homogenous, with stronger longer-range order in which the solvated electron acts as a structural template. It therefore appears unique amongst current expanded metal-amine solutions.
These discoveries open up new avenues for fundamental research. There are also applications in nanomaterials processing, as metal-ammonia solutions have been shown to enable the dissolution of carbon nanotubes. The resulting solutions or ‘inks’ can then be used to scalably print the nanotubes into transparent conductive films for flexible displays.
Could these results lead to a metal-amine high–temperature superconductor down the line? With these fascinating and quixotic metal solutions, only time - and study - will tell.