Melting-induced electron localization:Cs133NMR study of solid and liquid CsAu

We describe a ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ NMR study of the ionic intermetallic semiconductor CsAu and the transition to its saltlike liquid state. Measurements of the resonance shift and nuclear relaxation rates extend from room temperature through the melting point to 650 and 700 °C, respectively. A strongly-temperature-dependent spin-lattice relaxation rate and accompanying shift in the solid are attributed to extrinsic, nondegenerate, delocalized electrons which are excited from a donor band about 0.25 eV below the conduction band. A sharp 25-fold increase in the relaxation rate at the melting point is not accompanied by a similar jump in the shift, showing that the conduction electrons become localized immediately upon passing to the liquid state.