Thermodynamics of the Nonmetal‐Metal Transition of Sodium‐Ammonia Solutions

Abstract

Losunyeti 1 M i~~u l l cI Nic.litmetrrll-Metcill-Uheryany 1 Thermodynamik binarer SystemeMetal-ammonia solutions (MAS) pass with increasing metal concentration through a nonmetal-metal transition (NMT). Determination of the ammonia vapour pressure of sodium-ammonia solutions around -40°C and its temperature dependence and application of the Gibbs-Duhem equation lead to the chemical potentials and the partial molar enthalpies and entropies of both components. Whereas the chemical potentials are almost concentration independent in the concentration range of the NMT, the other partial molar quantities display pronounced extrema. The evaluation of the data leads to the conclusions. a) the N M T is accompanied by an increase of enthalpy and entropy of the dissolved metal. due to the transition of the electrons from the solvated to the free state; b) the N M T is favoured by temperature increase; c) the miscibility gap at lower concentrations is a consequence of the NMT. Introduction Metal-ammonia solutions (MAS) which contain dissolved or excess electrons behave at low metal concentrations (<0.5 mol percent metal (MPM)) like an electrolyte solution, whereas the high concentrated solutions ( > 10 MPM) display the characteristics of a liquid metal [l]. Somewhere in the intermediate concentration range the nonmetal-metal transition (NMT) occurs. An NMT has been observed also in other systems, e.g. in supercritical mercury by increasing the mass density [2] or in solid binary mixtures, e. g. tungsten bronzes [3] by changing the composition. Metall-Ammoniaklosungen (MAS) durchlaufen mit steigender Metallkonzentration einen Nichtmetall-Metalliibergang (NMTAlthough the NMT is of increasing interest, e.g. with respect to semiconductors, little is known about its thermodynamics. This should be mainly due to experimental difficulties because the thermodynamic measurements have to be done at high pressures and temperatures, as with supercritical mercury, or with solid multicomponent mixtures, as with tungsten bronzes which mostly are not in an equilibrium state. Only for MAS a few thermodynamic data have been published : precision vapour pressure measurements by Marshall [4] at only one temperature ( -35 "C) from which