In Situ Synchrotron Radiation and Neutron Diffraction Reveal A‐Site Substitution Pathways and Structural Reconstruction in Mo 2 Ga 2 C MAX‐Like Phase

ABSTRACT Layered MAX‐like carbides, which integrate metallic and ceramic characteristics, hold great promise for applications in harsh environments, yet the atomic‐scale mechanisms of A‐site substitution remain insufficiently understood. Here, we synthesize a heterostructured MAX phase through Lewis molten‐salt treatment of Mo 2 Ga 2 C, a 221‐type layered precursor. Rietveld refinements of X‐ray diffraction (XRD) and neutron diffraction data, along with high‐resolution scanning transmission electron microscopy and energy‐dispersive X‐ray mapping, reveal that the hetero‐MAX phase consists of Mo 2 SnC and Mo 2 Ga 0.5 Sn 0.5 C (molar ratio 1:2.72). In situ synchrotron radiation XRD uncovers a multi‐step A‐site substitution pathway involving intermediate Mo 2 (Ga x Sn 1−x ) 2 C phases and a transformation from double to single A‐layers. Density functional theory calculations confirm the thermodynamic stability and formation mechanism of the final structure. The Ga‐to‐Sn substitution drives A‐site reconstruction and local chemical optimization, resulting in significantly improved corrosion resistance in acidic, alkaline, and saline solutions. This work reveals previously unrecognized A‐site dynamics and offers a viable design strategy for chemically robust MAX phases under harsh conditions.