Equilibrium model for the interpretation of the conduction properties of metal–ammonia solutions

Abstract

Can. J. Chem. 55.221 l(1977). Recently we (1) presented experimental data about the electrical conductivity of metalammonia solutions (MAS) in the temperature and pressure range from 240 to 420 K and 200 to 1000 bar, respectively. As shown in Fig. 1, in which the equivalent conductance A at collstallt pressure (A = o / c (R-I cm2 mol-I); o specific conductivity (R-I c n~-l ) , c molar concentration (mol/cm3)) is plotted rs. molar concentration with temperature as parameter, the steep increase of the conductivity is shifted to smaller concentrations with increasing temperature; pressure increase on the other hand leads to a shift to higher concentrations. The electrical conductivity o f metal-ammonia solutions can be described by an equilibrium o f solvated electrons o f low mobility and o f free electrons o f high mobility. With proper choice o f the equilibrium constant and itsNo matter what the conduction mecha~lism in these solutions is and how the nonn~etal-metal transition (NMT) is defined the data suggest that this transition is favoured by temperature increase and opposed by pressure increase.For the interpretation of the NMT in MAS two contrasting theories were presented by Cohen and Jortner (2) and Mott (3), respectively, based on the percolation model and on the concept of the Anderson transition. Although the authors gave a series of arguments to support their concepts it seemed that no general agreement is obtained on the nature of the NMT and the conduction mechanism in moderate to high coilcentrated solutions. Only for the dilute range (c < 0.05 mol/f or 0.1 mol"7, metal (mpiu)) in wh~ch the equivalent conductance varies only slightly with concentration (A = 500-1000 !X1 cn12 mol-' at 240 K) is it not doubted that the electrical current is carried by individual moving solvated electrons and metal cations (4).In the intermediate range (0.5-2 mol/C or 1-4 mpm. depending on temperature) with the beginning steep conduction increase and the still unexplained maxima of the posit~ve temperature coefficient (5) and the negat~ve pressure coefficient (6) of the conductiv~ty (see also Fig. 3), seiuiconduction is assumed. This is supported by the peculiar temperature dependence of the thermopower (7, 8) (see also Fig. 4) which in the intermediate range decreases with temperature as is to be expected for activated transport (3), whereas in lower and higher concentrated MAS it increases with temperature.Besides a more or less quantitative description of the conduction behaviour of the MAS at normal temperature and pressure, no explanation was given or could be given on the basis of the percolation model or the Anderson concept for the observed shifts of the N M T with temperature and pressure.Two experimental observations prompted us Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.245.55.244 on 05/10/18For personal use only.