The elemental diversity is crucial to screen out ternary MAX phases with outstanding properties via tuning of bonding types and strength between constitutive atoms. As a matter of fact, the interactions between M and A atoms largely determine the physical and chemical properties of MAX phases. Herein, Se element was experimentally realized to occupy the A site of a MAX phase, Zr 2 SeC, becoming a new member within this nanolaminated ternary carbide family. Comprehensive characterizations including Rietveld refinement of X-ray Diffraction and atom-resolved transmission electron microscopy techniques were employed to validate this novel MAX phase. The distinct thermal conduction behaviors emerged are attributed to the characteristic interactions between Zr and Se atoms.
Radiation tolerance is a crucial parameter for MAX phases in nuclear structural applications. While metallic A-site nanolayers are traditionally regarded as the primary contributors due to their low M A -A M antisite defect formation energies, which facilitate defect accommodation and energy dissipation, recent advances in covalent MAX phases have opened new avenues for enhancing radiation damage mechanisms through X-site elements. In this work, we demonstrate that nonmetallic A-site and X-site elements play a decisive role in governing radiation response behavior. Unlike conventional metallic A-site MAX phases, which predominantly exhibit c -axis swelling under irradiation, the covalent variants Zr 2 SeC and Zr 2 SeP show anisotropic lattice expansion, characterized by a -axis-dominated distortion. This distinct behavior arises from strain redistribution mediated by variations in M-A-M bond angles, offering a mechanistic pathway to mitigate radiation-induced dimensional changes. Notably, these covalent MAX phases exhibit divergent structural evolution: Zr 2 SeC undergoes significant lattice swelling and partial amorphization, whereas Zr 2 SeP retains its hexagonal crystalline structure under identical irradiation conditions. We attribute this difference to the unique bonding configuration of Zr 6 X octahedra, where X’s electronegativity modulates Zr-Se bond cohesion and the energetics of Zr Se -Se Zr antisite defects. The combined effects of reduced antisite defect formation energy and lowered vacancy migration barriers collectively confer exceptional intrinsic radiation tolerance to Zr 2 SeP grains. These findings provide a design framework for identifying radiation-resistant MAX phases with optimized structural stability for advanced nuclear applications.
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