The role of plasmons and interband transitions in the color of AuAl₂, AuIn₂, and AuGa₂

First principles calculations of the optical properties of the intermetallic compounds AuAl₂, AuIn₂, and AuGa₂ have been performed. Analysis of the dielectric functions showed that AuAl₂ is unique because a bulk plasmon is seen in the optical region and contributes to the purple color of this material. An experimental electron energy-loss spectrum showed excellent agreement with the theoretical prediction and confirmed the presence of the bulk plasmon.