Disorder in M _n+1 AX _n phases at the atomic scale

Atomic disordering in materials alters their physical and chemical properties and can subsequently affect their performance. In complex ceramic materials, it is a challenge to understand the nature of structural disordering, due to the difficulty of direct, atomic-scale experimental observations. Here we report the direct imaging of ion irradiation-induced antisite defects in M _n+1 AX _n phases using double C _S -corrected scanning transmission electron microscopy and provide compelling evidence of order-to-disorder phase transformations, overturning the conventional view that irradiation causes phase decomposition to binary fcc-structured M _n+1 X _n . With the formation of uniformly distributed cation antisite defects and the rearrangement of X anions, disordered solid solution γ-(M _n+1 A)X _n phases are formed at low ion fluences, followed by gradual transitions to solid solution fcc-structured (M _n+1 A)X _n phases. This study provides a comprehensive understanding of the order-to-disorder transformations in M _n+1 AX _n phases and proposes a method for the synthesis of new solid solution (M _n+1 A)X _n phases by tailoring the disorder.