2. The M_n+1AX_n Phases: The Precursors for MXenes

This chapter gives an overview of the class of layered, machinable, M_n+1AX_n or MAX, phases (where M = early transition metal; A = A-group element, e.g., Al or Si; and X = C or N) precursor to MXene. These materials exhibit an unusual combination of metallic and ceramic properties: good electrical and thermal conductivity, machinability, and resistance to thermal shock. Some are also quite oxidation and creep resistant and plastic at high temperatures. These remarkable properties originate from their layered structure and the mixed metallic-covalent nature of the strong M-X bonds together with M-A bonds that are relatively weak. They are also the common precursors for the class of 2D materials known as MXenes, which are typically formed by selectively etching, mostly Al, from the MAX phases. Within a couple of years of the first report, MXenes have established themselves as a fascinating class of 2D materials with remarkable possibilities for composition variations and property tuning.