The increasing electromagnetic pollution necessitates the development of advanced microwave absorbers. Although MAX phases exhibit chemical stability and electrical conductivity, their absorption performance is limited by a singular loss mechanism. Here, we propose a “pre-placed vacancy and isomorphous occupancy” strategy to engineer A-site high-entropy (HE) MAX phases, achieving unprecedented incorporation of large-radius elements (Ag and Bi). The optimized absorber delivers exceptional microwave absorption performance, with a minimum reflection loss of −71.6 dB (at 3.05 mm) and a broad effective absorption bandwidth of 4.1 GHz (at just 1.25 mm), outperforming both reported MAX phase variants and commercial absorbers. These remarkable properties stem from three synergistic mechanisms: A-site composition tailoring optimized impedance matching, HE-induced lattice distortion enhanced dipolar polarization, and A-site entropy engineering increased conduction loss. Our work pioneers a novel method for manipulating electromagnetic response in MAX phases through atomic-scale entropy engineering, paving the way for next-generation electromagnetic protection materials.

High-entropy (HE) MAX phases represent an emerging family of multi-constituent solid solutions that provide large compositional variations and, therefore, a wide variety of properties. Here, we report the experimental realization of M2AlC (M = Ti/Ta/V/Nb/Cr) MAX phase by an energy-efficient self-propagating high-temperature synthesis, which enables facile scalability to an environmentally friendly industrial production. The HE-MAX phase was developed according to crystal size, electronegativity, and valence electron concentration of corresponding metals required to form a substitutional single-phase material. Variations in initial mixture composition, inert gas pressure, additive amount and sample diameter played a decisive role in HE-MAX formation. The combustion of the stoichiometric mixture favors the formation of the HE-carbide. Deviation from the stoichiometry has resulted in the formation of 211 and/or 413 type HE-MAX phases. Fine-tuning the aluminum and carbon content in the initial mixture, facilitated the formation of a layered structure, characteristic of MAX phases. DSC/TG analysis proved an enhanced oxidation resistance of HE-MAX phases, which outperforms conventional MAX phases and several previously studied HE-MAX phases.