Progress and prospects in MAB phases: A comprehensive review of structure, synthesis, and multifunctional properties

MAB phases comprise a diverse family of layered ternary and quaternary transition metal borides and related 211-type MAX borides. Conventional MAB phases contain transition-metal–boron structural units separated by A-containing layers; in many Al-containing members, strong M–B and, where present, B–B interactions coexist with relatively weak A-related interactions, producing anisotropic mechanical and transport behavior. Selected MAB phases combine high hardness, damage tolerance, electrical and thermal conductivity, oxidation resistance, crack-healing capability, or radiation tolerance, although these properties depend on composition, structure, phase purity, and processing history. Some of these phases can also serve as precursors for topochemical synthesis of two-dimensional MBenes, which have been investigated for energy storage, electrocatalysis, sensing, and optoelectronic applications. This review traces the historical development of the MAB phase field, establishes an operational scope and classification framework for conventional MAB phases and related subfamilies, and describes the structural diversity arising from stoichiometry, crystal symmetry, boron-network topology, and chemical ordering. It surveys computational approaches for evaluating thermodynamic, dynamical, and mechanical stability and for predicting intrinsic properties, and critically examines experimental synthesis routes for powders, dense bulks, single crystals, thin films, coatings, and selected MBene derivatives. The review further evaluates mechanical, thermophysical, high-temperature, tribological, radiation-related, and functional properties with attention to sample state and measurement conditions. The review relates composition, structure, bonding, processing, and performance to the opportunities and limitations for practical applications of MAB phases and MBenes.