Temporary Cohabitation: The Metastable Phase Au(4)Si

[Image: see text] The prediction, identification, and characterization of phases away from equilibrium conditions remain difficult challenges for material science. Herein, we demonstrate how systems whose phase diagrams contain deeply incising eutectics can offer opportunities to address these challenges. We report the synthesis of a new compound in the Au–Si system, a textbook example of a system with a deep eutectic. Au(4)Si crystallizes in a complex √18×√2×1 superstructure of the PtHg(4) type, based on the distortion of vertex-sharing Si@Au(8) cubes into bisdisphenoids. Au(4)Si decomposes upon heating and at room temperature even in high vacuum, highlighting its metastability. Electronic structure analysis reveals a pseudogap at the Fermi energy, which is enhanced by the superstructure through the relief of Au–Au antibonding interactions. The pseudogap is associated with a Zintl-type bonding scheme, which can be extended to the locally ordered liquid. These results highlight the potential for metastable phases to form in deep eutectics that preserve the local structures of the liquid.