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

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 Mo₂Ga₂C, 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 Mo₂SnC and Mo₂Ga_0.5Sn_0.5C (molar ratio 1:2.72). In situ synchrotron radiation XRD uncovers a multi-step A-site substitution pathway involving intermediate Mo₂(Ga_xSn_1-x)₂C 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.