Optical properties and electronic structure of ordered and disorderedCu3Au

We have carried out measurements of the reflectance spectrum from 1 to 13 eV on ordered and on disordered ${\mathrm{Cu}}_3$Au. Optical constants were computed via Kramers-Kronig analyses. In addition, we used a sensitive comparison technique to measure directly the reflectance difference between ordered and disordered alloys. This is the first time that such a modulation technique has been applied to the optical study of an order-disorder transformation. As a result, spectral detail is observed which is not apparent in other measurements. Bulk samples were utilized for our studies of ordered and of quench disordered ${\mathrm{Cu}}_3$Au. For the comparison technique, thin-film samples of ordered, disordered, and amorphous ${\mathrm{Cu}}_3$Au of varying stoichiometry were prepared. We have found that the second principal maximum in ${\epsilon }_2$ for ordered ${\mathrm{Cu}}_3$Au is located at 3.28 eV, in contrast to 3.6 eV in the earlier measurements of Nilsson and Norris. They believe that this peak arises from a new gap upon ordering whereas we attribute it to new transitions in the same region of $k$ space. Electron transitions involved with various optical features of the ordered alloys are based principally upon folding considerations applied to the copper and gold band structures.