In Situ Synchrotron Radiation and Neutron Diffraction Reveal A-Site Substitution Pathways and Structural Reconstruction in Mo<sub>2</sub>Ga<sub>2</sub>C MAX-Like Phase.
basic_science · Level V
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- Also identified by DOI 10.1002/adma.202518712.
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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<sub>2</sub>Ga<sub>2</sub>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<sub>2</sub>SnC and Mo<sub>2</sub>Ga<sub>0.5</sub>Sn<sub>0.5</sub>C (molar ratio 1:2.72). In situ synchrotron radiation XRD uncovers a multi-step A-site substitution pathway involving intermediate Mo<sub>2</sub>(Ga<sub>x</sub>Sn<sub>1-x</sub>)<sub>2</sub>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.