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.