Two-dimensional (2D) abnormal stoichiometric crystals represent an emerging class of atomically ordered materials characterized by their abnormal stoichiometries, which are stabilized within 2D confined spaces or at specific interfaces. Historically, such non-equilibrium phases were only accessible under extreme high-pressure conditions, severely restricting their practical exploration. A breakthrough occurred with the discovery that cation-π interactions, electrostatic attractions between cations and π-electron systems like graphene, can induce the crystallization of unsaturated salt solutions on carbon-based interfaces under ambient conditions. This mechanism facilitates the spontaneous formation of various 2D crystals with unconventional stoichiometries, such as Na2Cl, Na3Cl, and notably CaCl, where calcium exhibits an atypical +1 oxidation state. These materials not only challenge the fundamental principles of traditional crystallography and valence theory but also exhibit a remarkable suite of physical properties absent in their conventional counterparts. Key demonstrations include room-temperature ferromagnetism in CaCl@rGO systems, metallic conductivity in Li2Cl and CaCl crystals, and piezoelectricity coexisting with metallicity. The stabilization and emergence of these exotic properties are attributed to cation enrichment at the carbon interface driven by cation-π interactions, followed by charge transfer and self-assembly. This review systematically summarizes the discovery and research progress of these materials across alkali, alkaline earth, and transition metal halides. It delves into their unique structural features characterized via cryo-electron microscopy, XRD, and XPS, and discusses formation mechanisms rooted in interfacial electrostatics. Furthermore, the article highlights their significant potential for applications in next-generation magnetic materials, ultra-sensitive sensors, energy storage, and neuromorphic computing. The study of 2D abnormal stoichiometric crystals establishes a novel paradigm for designing functional materials under mild conditions, opening vast avenues for fundamental research and technological innovation.
Li et al. (Fri,) studied this question.