Molecular sp ³ -like Reactivity of Metastable Au ₄ Si near Its Deep Eutectic Point Enables Low-Temperature SiC Formation

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 Au₄Si near its deep eutectic temperature exhibit molecule-like reactivity associated with an sp³-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.