Electronic Processes in Solutions of Alkali Metals in Liquid Ammonia. I. A Model Including Two Diamagnetic Species

The various models which have been proposed for metal—ammonia solutions are reviewed in the light of the available experimental data. It is shown that, in order to account quantitatively for the electrical and magnetic properties of these solutions, it is necessary to assume two different species containing paired electrons. A new model postulated for metal—ammonia solutions consists of the following five species in equilibrium: metal ions, polarons or e centers, M centers, M2 centers, and M′ centers. An M′ center is assumed to consist of an electron trapped in the field of an M center. The equilibrium concentrations of the species which are assumed to be present in metal—ammonia solutions are calculated for different metal concentrations. The calculated values agree well with experimental transference numbers and magnetic susceptibilities. The expressions for the equilibrium constants are related to the binding energies of electrons trapped by the various centers, and the relative values of the trapping energies are estimated from the magnetic and transference data. It is concluded that the main contribution to the binding in all centers arises from the polarization of the dielectric. The estimate of the concentration at which a narrow conduction band may begin to form agrees with the experimental temperature coefficient of conductivity which has a sharp maximum at 0.8M.