Stability and formation pathways of compositionally complex M-layer MAX phases

MAX phases exhibit unusual combinations of metallic and ceramic properties and offer broad opportunities to tailor those properties through incorporation of multiple elements on the M, A, and X sites. However, conventional synthesis from elemental or binary powders often involves complex reaction pathways, intermediate phase formation, competing reactions, and deviations from targeted stoichiometry. Here, we synthesize 26 targeted M 2 AlC compositions containing two to five equiatomic M-site elements using high-purity single-element MAX phase precursors (T 2 AlC, Ta 2 AlC, V 2 AlC, Nb 2 AlC, and Cr 2 AlC). This approach minimizes intermediate-phase formation and promotes solid-solution formation. Coupling experimental results with composition-based machine learning analysis reveals that only nine compositions form phase-pure M 2 AlC phases, while the remaining compositions produce non-targeted chemistries, competing phases, or undergo decomposition upon cooling. The observed phase-formation behavior correlates strongly with a coupled geometric–electronic balance, providing a data-driven framework for assessing phase-stability trends and supporting future predictive screening of multicomponent MAX-phase chemistries.