Exploring the plasmonic and optical response of gold-silver alloys through evolutionary structure prediction

This study aims to delve into the exploration of stable bulk phases of Ag--Au by employing a synergistic combination of evolutionary algorithms and density functional theory. Through our comprehensive investigations, we uncover the existence of stable phases, specifically the cubic Pm3m and trigonal R3m crystals, which exhibit remarkable mechanical and dynamic stabilities. Our study reveals the presence of metastable hexagonal (P63/ mmc, P 6 m2) and tetragonal (P42/mmm) structures within the Ag--Au system, expanding our understanding of its phase behavior. The calculations demonstrate that the dominant negative contribution on the overall formation energy arises from the chemical energy arising through Ag--Au charge exchange. As a consequence, this chemical effect plays a pivotal role in promoting strong bonding interactions and significantly enhances the thermodynamic stability of AgAu alloys. Furthermore, we conduct an extensive analysis of the plasmonic and optical properties manifested by these stable phases. Our findings unequivocally demonstrate the manifestation of plasmon resonances in the visibleviolet and visible-blue regions for cubic Ag3Au and AgAu3 crystals, respectively. Notably, the trigonal-AgAu phase exhibits a robust excitation of plasmons exclusively within the visible-blue spectral domain. Moreover, Ag3Au, AgAu, and AgAu3 show strong absorption behavior in the near-infrared and ultraviolet -C spectral regions. Additionally, Ag3Au exhibits a pronounced reflectivity behavior within the near-infrared (IR) and ultraviolet -B spectral regions. AgAu, on the other hand, displays strong reflectance within the visible -violet spectral region. Finally, AgAu3 manifests a notable reflection within the visible-blue electromagnetic spectral region.