Structural Transformation Behind the Deep Eutectic Phenomenon in the Au–Si System

The unusual deep eutectic phenomenon in the Au–Si system has been studied for decades due to its fundamental and practical importance; however, its structural origin remains unresolved. In this work, we employed density functional theory (DFT) to investigate the formation of two metastable Au4Si crystals with compositions in the eutectic range (18–21 atom % Si). Phonon density of states (PDOS) and energy analyses confirm that Si diffusion into the Au lattice is energetically favorable. The diffusion results in the formation of Au4Si clusters, which reorient and transform into a body-centered cubic (bcc) structure and further evolve into an orthorhombic structure. The bcc-structured Au4Si is key to understanding the eutectic behavior. Crystal orbital Hamilton population (COHP) and Hirshfeld population analyses reveal that both Au–Au and Au–Si bonds in the bcc-structured Au4Si are significantly weaker than Au–Au bonds in crystalline Au. This bond weakening is attributed to valence electron transfer from Au to Si. At elevated temperatures, intensified atomic vibrations further weaken the elongated Au–Si bonds and promote the melting of Au4Si. This study provides a structural and bonding-based explanation for the deep eutectic behavior in the Au–Si system.