An emerging class of 2D materials: AMX2 compounds

Two-dimensional (2D) AMX 2 compounds, in which A is a monovalent metal ion, M is a trivalent metal, and X is an oxygen-group element, have recently emerged as a distinctive class of 2D materials beyond conventional van der Waals semiconductors. Owing to the coexistence of mobile monovalent cations, variable coordination environments, and diverse symmetry and structural dimensionality, these compounds constitute an intrinsically ionic-electronic coupled materials platform with rich structure-property correlations. Recent studies have shown that ultrathin AMX 2 compounds can exhibit a broad spectrum of functionalities, including superionic transport, ferroic order, magnetism, optoelectronic response, and electrically reconfigurable device behavior. In this Review, we summarize the current understanding of AMX 2 compounds from their structural and compositional landscape to their theoretical mechanisms, synthesis strategies, representative physical properties, and device opportunities. Particular emphasis is placed on how structural features, ion dynamics, and reduced-dimensional effects collectively govern emergent functionalities in this family. We further discuss the major challenges that remain in controllable synthesis, mechanism clarification, thickness-dependent regulation, and device integration, and outline future directions for advancing AMX 2 compounds as a multifunctional platform for next-generation electronics, optoelectronics, and neuromorphic systems.