X-site element dependence of radiation resistance in Zr2SeC and Zr2SeP covalent MAX phases

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.