Alkali metals dissolve freely in liquid ammonia, to yield conducting solutions - intensely blue and salt-like when dilute; bronze-gold and truly metallic when concentrated. In the transitional range between non-metallic and metallic, cooling the sample gives rise to a striking liquid-liquid separation of the two phases. Because of these novel electronic and thermodynamic properties, the solutions have been the subject of intense research effort, ever since their discovery by Sir Humphry Davy, in 1808 [1].

Recent research into metal-ammonia solutions has focussed on the transition from localised to itinerant electronic states, and the concomitant liquid-liquid phase separation. In fact, a number of electronic states have been proposed; isolated polarons, spin-paired bipolarons, excitonic atoms, metal anions, and delocalised electrons [1]. To determine which, if any, of these species are present at the various levels of dilution requires high-resolution structural studies of the solutions. In this context, X-ray absorption spectroscopy is an extremely powerful probe, since it can provide ion-centred local information, over the full range of concentrations. Rb-NH3 solutions have been studied on BM29 using the EXAFS technique to measure the fine structure above the Rb K-absorption edge, at photon energies of around 15.2 keV.

The Rb- NH3 solutions were made in situ by condensing a known volume of anhydrous ammonia directly onto pre-weighed rubidium metal. The sample temperature was maintained close to the point at which phase separation was proposed, namely 206K at a concentration of 4 mole per cent metal [2]. By using an X-ray camera, we are able to observe directly the dissolution of the Rb into NH3. The "live" X-ray images of our samples also provided the first direct evidence for liquid-liquid phase separation in Rb- NH3, and enabled a view of the liquid-liquid interface itself.

A typical photograph of the equilibrated liquid-liquid interface is shown in Figure 30. Note that the concentrated bronze-gold phase is less dense than the dilute phase due to the volume occupied by the delocalised electrons. For this reason, bubbles of ammonia vapour form at the liquid-liquid interface, and are seen escaping from the dilute blue region. However, these bubbles do not break the meniscus of the gold phase.

In addition to providing the first image of this very unusual liquid-liquid interface, the data also show directly that phase separation does indeed occur in the Rb- NH3 system [2,3]. This is confirmed by the data presented in Figure 31, in which the ratio of the transmitted to incident beam intensity, I(1)/I(0), is plotted as a function of sample cell height. By moving the sample cell ~ 2 mm (from 17.15 to 18.95 mm) through the fixed energy X-ray beam, the change from a metallic to a non-metallic liquid is identified by the change in the sample absorption on crossing the interface.

Within homogenous solutions, the EXAFS signal provides details of the Rb-centred structure. A well-defined solvation shell around Rb+ is observed, but with evidence of an increased cation-ammonia radius in the dilute phase. This is consistent with a weak tendency for solvated (localised) electrons to associate with a cation.

The aim of future work will be to obtain a better understanding of the microscopic structure and electronic species of the metallic and nonmetallic solutions, thereby allowing the mechanism(s) for the metal, non-metal transition to be elucidated.

[1] P.P. Edwards, Adv. Inorg. Chem., 25, 135 (1982).
[2] G. Lepoutre, J-P. Lelieur, "Properties of concentrated metal-ammonia solutions" in Metal Ammonia Solutions, J.J. Lagowski and M.J. Sienko eds., Butterworths, London, 272 (1970).
[3] A.C. Sharp, R.L. Davies, J.A. Vanderhoff, E.W. LeMaster and J.C. Thompson, Phys. Rev., A 4, 414 (1971). J.V. Acrivos, K. Hathaway, A. Robertson, A. Thompson and M.P. Klein, J. Phys. Chem., 84, 1206 (1980).

J.C. Wasse (a), S. Hayama (a), N.T. Skipper (a), D. Morrison (b), D.T. Bowron (c).

(a) Dept. of Physics, University College London (UK)
(b) Dept. of Chemistry, University College London (UK)
(c) ESRF