The layered structure of MAX phases is associated with a number of functional properties and is the subject of extensive research. While the unit-cell layers of these structures have been well studied, much less is known about the distribution and manipulation of point defects within them. Here, we selected the prototype Cr2AlC system and, using variable energy positron beams, observed Doppler broadening and positron annihilation lifetimes to track the evolution of defects caused by the penetration of energetic transition metal ions (Co+ and Mn+) and noble gas ions (Ar+ and Ne+). In all cases an overall reduction of the open-volume defect concentration is observed post-irradiation. Atomic displacements induced by the penetrating ions drastically modify the defect distribution: the concentration of agglomerates of 9–15 vacancies (corresponding to positron lifetimes of 335–450 ps) in the precursor [Cr2C/Al]n layers is suppressed, whereas Al mono- and Al-Cr di-vacancy (lifetimes 217–231 ps) concentrations are enhanced. This breakdown of large defects into point defects scales with atomic displacements and is largely independent of the penetrating ion species, providing insights into the manipulation of point defects in nano-layered systems.

中文翻译：

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在 Cr2AlC 纳米层系统无序过程中修饰空位缺陷


MAX 相的层状结构与许多功能特性有关，是广泛研究的主题。虽然这些结构的晶胞层已经得到了很好的研究，但对它们内部点缺陷的分布和操纵知之甚少。在这里，我们选择了原型 Cr2AlC 系统，并使用可变能量的正电子束，观察了多普勒展宽和正电子湮没寿命，以跟踪由高能过渡金属离子（Co+ 和 Mn+）和惰性气体离子（Ar+ 和 Ne+）渗透引起的缺陷的演变。在所有情况下，照射后观察到开放体积缺陷浓度的总体降低。由穿透离子引起的原子位移极大地改变了缺陷分布：前驱体 [Cr2C/Al]n 层中 9-15 个空位（对应于 335-450 ps 的正电子寿命）的团聚体浓度被抑制，而 Al 单空位和 Al-Cr 二位位（寿命 217-231 ps）浓度得到增强。这种将大缺陷分解为点缺陷的过程与原子位移成比例，并且在很大程度上与穿透离子种类无关，从而为纳米层系统中点缺陷的操纵提供了见解。