On the Chemical Diversity of the MAX Phases

The M_{n +1}AX_n, or MAX, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, MAX phases known to date. This review covers ≈155 MAX compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in- and out-of-plane ordered MAX phases opens the door to the discovery of many more. The chemical diversity of the MAX phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer quaternary (and higher)phases have yet to be characterized, much work remains to be done.