Charge transfer and percolation in alkali-metal alloys

We report a comprehensive investigation of charge transfer from host to solute atoms in alloys based on alkali metals. The residual resistances of strongly electronegative impurities in quench-condensed dilute alloys clearly indicate the formation of the charge-transfer state. The same effects cause percolative transitions to random insulating phases in more concentrated alloys. In these materials, named charge-transfer insulators, charge transfer depletes the conduction-electron density below the level needed for charge percolation. Bounded domains of the insulating phase are observed in CsSn, NaSn, and CsAu disordered solids. Values of the minimum metallic conductivity close to the theoretical prediction (3×10⁴ Ω)^-1 are identified by the temperature dependence of conductivity close to the transitions.