Charge transfer states in alkali metals

Optical and transport properties of high valence impurities in alkali metals are described. The principal phenomena revealed by both types of measurement is charge transfer from the alkali conduction band to localized orbitals bound on the impurities. The excitation spectra of halogen impurities in alkali metals have been investigated throughout the photon energy range from 5 eV to 12 eV. Threshold energies for charge transfer excitations on the impurity centers were found to agree quantitatively with earlier predictions by Flynn. The deduced Stokes shifts are almost independent of impurity species and are compatible in magnitude with the observed threshold broadening. Fano-like interference with continuum absorption below threshold is also observed. In a number of cases the absorption profiles above threshold bear a close resemblance to those of the analogous salts. Most features remain poorly understood, however, and in particular, the form.of the profile near threshold cannot be predicted reliably using current theories of the electron gas response. The charge transfer configurations adopted by a variety of impurities in alkali metals have been further investigated by measurements of the composition and temperature dependence of the electrical resistivity. Charge transfer effects are observed to cause metal-to-insulator transitions at specific critical concentrations of Xe, I, Te, Sb, and Sn impurities in the Cs host lattice. These transitions are identified as being of the percolative type rather than the Mott type. The temperature dependence of conductivity near these transitions indicates that both hopping and metallic components are present. Enhanced superconducting transition temperatures were observed at low temperatures in certain Sn and In based alloys. The trend of residual resistivity with impurity valence, for elements of the silver row of the Periodic Table, exhibits a sharp discontinuity between Sn and Sb. This discontinuity is interpreted as due to the formation of the charge transfer configuration for Sb, Te, and I impurities.U of I OnlyThesi