Optical, Electron-Spin-Resonance, and Electrical Studies of Colloidal and F -Aggregate Centers in Highly Pure KCl Crystals. I. Colloidal Centers

In additively colored highly pure H and "pure" B crystals (background impurity concentration {~}1 and {~}10 ppm, respectively) quenched from 300^∘{}C in the dark, a colloidal optical absorption band is formed. The peak position (730 nm) and half-width (0.25 eV) of the band do not change with heat treatment in the dark in the H crystals. In B crystals the colloidal band has a larger half-width (0.30 eV) and moves to longer wavelengths on heating the crystal to higher temperatures. On quenching the crystals from 150 to 250^∘{}C in light, the F band decreases, and a new X band in the H crystals at 700-710 nm and the Scott R^' band in the B crystals at the same wavelengths are formed. Most of the X centers and some of the Scott R^' centers are converted to colloidal centers at 300^∘{}C. The colloidal band in H crystals and the composite colloidal and Scott R^' bands in B crystals move rapidly to longer wavelengths and increase in half-width on heating the crystals to higher temperatures in light. A narrow symmetrical ESR line due to colloidal centers, superimposed on the well-known ESR line due to F centers, is observed in crystals containing the colloids. The line is Lorentzian in shape, has a half-width of 2.5 G at room temperature, and has a g value of 1.9998. This value of g agrees with the value obtained theoretically by Beinenstock and Brooks for potassium-metal conduction electrons neglecting "polarization correction" and including the "Hartree term." A rough upper limit of 700 atoms on the size of the colloidal particles is obtained by the observed symmetrical shape and half-width of the CESR line. In the B crystals, only a small fraction of the absorption under the "colloidal band" is found to be due to colloidal particles. Even in the H crystals, a part of the optical absorption under the "colloidal band" is due to residual background impurity present in the crystals. No colloids are formed in crystals containing large concentrations of Ba or Cd. The X band does not influence the electrical conductivity, the Scott R^' band suppresses the conductivity by a large factor, and colloids enhance the conductivity at higher temperatures. The results show that the colloidal nature of the centers, investigated by Scott, Smith, and Thompson, is essentially correct, and contradictory results were obtained by several subsequent workers because it is not possible to understand the effect of light and background impurity on the optical absorption bands by optical measurements alone.