"Purple glory": The optical properties and technology of AuAl2 coatings

Abstract

The application of thin coatings of the purple gold intermetallic compound AuAl2 provides a possible alternative to the use of this material in the bulk form. Here we explore the properties of AuAl2 coatings prepared by vacuum deposition onto heated substrates. Control of the thickness and microstructure of the deposit allowed the optical properties of the film to be tuned over a wide range. The properties were characterised using CIE LAB colour coordinates, and by measurement and calculation of reflection and transmission spectra. Reflection colours could be varied at will between dark-silver through to light purple, and were largely a function of the degree of crystallization of the deposit. Transmission colours could be varied from light to dark greenish-brown, and were a function of both microstructure and film thickness. The complex refractive indices, n(λ) +i.k(λ), and ε 1(λ)+ε2(λ), of AuAl2 were determined from the experimental measurements and are provided in tabular form. Proof-of-concept jewellery items were successfully produced to showcase the decorative aspects of this technology. Finally the possibility of using AuAl2 as a spectrally selective coating on architectural glass was considered. Use of it would certainly provide a degree of solar screening with a somewhat more neutral colour than elemental Au coatings, but the figure-of-merit, Tvis/Tsol, for the pure Au film is always better when compared at the same luminance.