Optical Properties of the Alpha-Phase Alloys Ag-Zn and Ag-Cd

Abstract

Reflectance measurements at near-normal incidence have been performed on chemically polished, annealed, polycrystalline samples of Ag-Zn and Ag-Cd alloys. Data were obtained for Ag and Ag-Zn and Ag-Cd alloy samples with 2.5, 5.0, 10.0, 20.0, and 30.0% $\mathrm{II}B$ atom concentrations. The measurements were made with a precision of 2% of measured reflectance over a spectrum 1.2-13.5 eV at temperatures of 110, 295, and 400 °K. Dielectric constants were computed by the Kramers-Kronig method. Trends of the dielectric constants with either a Zn or Cd concentration are basically similar. The principal ${\epsilon }_2$ peak of Ag at 4.0 eV moves to higher energies with an increase of $\mathrm{II}B$ atom concentration; however, a secondary peak, observed only in the alloys, moves to lower energies. Transitions probably occur in the alloys from the $d$ band to the Fermi surface and also from the Fermi surface to the $s$ band. The transition to the Fermi surface is dominant in Ag. Trends of the ${\epsilon }_2$ peaks for alloys with a concentration of Zn or Cd follow an approximate free-electron model. Energy gaps in Ag deduced from the present data are in approximate agreement with the deductions of Berglund and Spicer from photoemission experiments. Marked differences exist between the Ag-Zn and the Ag-Cd alloys. The dilute (2.5%) Ag-Zn alloy has a distinctly sharper ${\epsilon }_2$ edge than the corresponding Ag-Cd alloy. On the other hand, the low-energy absorption peak is seen first in the Ag-Cd alloy. These observations suggest that in the Ag-Zn alloy the additional electron contributed by the Zn tends to remain localized, whereas in the Ag-Cd alloy, it tends to contribute to the total number of free electrons and thereby increases the probability of transition to the $s$ band. At 10% or higher concentrations, the optical characteristics are less sharply defined and the temperature effects are weaker for the Ag-Zn alloy than for the Ag-Cd alloy.