Electronic states and the metal–insulator transition in caesium–ammonia solutions

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Abstract
The nature of the electronic states in caesium–ammonia solutions is examined from the insulating to the metallic regime using two different microscopic models (Z. Deng, G. J. Martyna and M. L. Klein, J. Chem. Phys., 1994, 100, 7590.). In the first model, the ammonia molecules are treated via a classical point-charge model and the cations as a positive neutralizing background. In the second model, the ammonia solvent is made fully polarizable and the cations, here caesium, are explicitly included. The solvent and ions are treated classically and the electronic degrees of freedom are handled using the Car–Parrinello method and density functional theory. At 260 K, the models give the following picture of the electronic states as a function of caesium/electron concentration: The dilute solution behaves like an electrolyte in which the electrons exist as polarons, on average spherical states localized in solvent cavities, far from the counter ions. At ca. 0.5 mol % metal (MPM), the solvated electrons are spin-paired and form large peanut-shaped species called bipolarons. The electrons still exist as bipolarons at ca. 1 MPM but well separated from each other. The bipolarons exhibit no strong tendency to be oriented either parallel or perpendicular to each other. We have investigated the effect of system size on the structure and dynamics of the bipolarons at ca. 1 MPM and found it to be small. At higher concentration, ca. 2 MPM, the electrons exhibit a tendency to cluster and the electron density oscillates between localized and delocalized states. At much higher concentration, ca. 9 MPM, the solution behaves as a good liquid metal in which the electron density forms multi-tunnel-like extended states. The caesium cations are always solvated by the ammonia and are thereby isolated from close contact with the electron density. The role of the cations is assessed through a comparison of the results of the two models. At low metal concentration, the effect of the cations turns out to be rather small. However, the explicit inclusion of the ions is found to increase the metallic character of the solution at ca. 9 MPM. Our findings rationalize a large body of experimental data on this system.