Localization of electrons in ionic liquids: Nuclear magnetic resonance in Cs-CsI and CsI-I solutions

We describe a ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ and ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ nuclear-magnetic-resonance investigation of liquid solutions in the system Cs-CsI-I. Measurements of the resonance shifts and nuclear relaxation rates were carried out as functions of composition and, to a limited extent, of temperature. Excess electrons introduced by dilute amounts of Cs in CsI were found to be localized on the time scale of the underlying liquid structure. The spatial distribution of these states is represented well by $F$-center analogs. Excess I in CsI produces paramagnetic centers which are analogs of $V_K$ centers, i.e., ${\mathrm{I}}_2^{-}$ molecular ions. Addition of higher concentrations of Cs to CsI rapidly increases the electronic mobility and the system passes through a continuous metal-nonmetal transition in the range 5-20 at.% excess Cs. On the metallic side of the transition, electronic transport is initially of strong-scattering diffusive character which gives way to nearly-free-electron conditions when the Cs content exceeds about 60 at.%. High concentrations of I in CsI lead to increased concentrations of localized paramagnetic species.