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In Situ Synchrotron Radiation and Neutron Diffraction Reveal A-Site Substitution Pathways and Structural
Kefu Zhu1,2, Feiran Shen2, Hongwei Shou1
1National Synchrotron Radiation Laboratory, State Key Laboratory of Precision and Intelligent Chemistry, School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China.
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 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 Mo2Ga0.5Sn0.5C (molar ratio 1:2.72). In situ synchrotron radiation XRD uncovers a multi-step A-site substitution pathway involving intermediate Mo2(GaxSn1-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.
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