Abstract
Metastable states near deep eutectic points are typically regarded as transient intermediates preceding phase separation, yet their potential chemical reactivity remains largely unexplored. Here, we demonstrate that metastable Au-Si bonding configurations derived from Au(4)Si near its deep eutectic temperature exhibit molecule-like reactivity associated with an sp(3)-like local bonding environment, enabling direct Si-C bond formation at temperatures as low as 636 K. Using a high-vacuum coevaporation platform, Au-Si species generated during coevaporation react with carbon clusters to produce SiC accompanied by Au segregation, whereas elemental Si under identical conditions remains chemically inert. Raman spectroscopy and X-ray photoelectron spectroscopy reveal that SiC formation occurs only within a narrow temperature window centered at the eutectic point and displays nonmonotonic temperature dependence inconsistent with conventional catalytic or vapor-liquid-solid mechanisms. These results provide experimental evidence that eutectic metastable bonding configurations can transiently adopt molecule-like characteristics, thereby enabling unconventional low-temperature reaction pathways in metal-semiconductor systems.