On the Chemical Diversity of the MAX Phases

The layered machinable Mn+1AXn, or MAX, phases (where M is an early transition metal, A is an A-group element, and X is C and/or N and n = 1−3) have attracted considerable attention because they possess an unusual combination of both metallic and ceramic properties. The ease by which one can tune their chemistry while keeping their structures the same is another advantage. This review covers ≈155 MAX compositions synthesized to date. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both in- and out-of-plane ordered MAX phases opens the door to the discovery of many more. A recent factor fueling interest in the MAX phases is the ease by which they can be converted to MXenes, their 2D counterparts. The latter exhibit extraordinary properties and are considered for a host of potential and different applications. The Mn+1AXn, 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. The Mn+1AXn, 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. a term used to describe the top of the collection of electron energy levels at absolute zero temperature. in-of-plane ordered quaternary MAX phases with a unique ordering of the M layers, such that the M′ atoms are arranged in a hexagonal arrangement, with the M′′ atoms positioned at the centers of hexagons, where M′ and M′′ denote an early transition metal. also called lattice constants, these are the physical dimension of the unit cell, and is the smallest repeating unit having the full symmetry of the crystal structure. one of the seven crystal systems in crystallography. Crystals in this system are described by three vectors of unequal lengths that form a rectangular prism with a parallelogram base. Hence, two vectors are perpendicular, while the third vector meets the other two at any given angle. out-of-plane ordered quaternary MAX phases with a unique ordering of the M layers, such that 2 M′ layers ‘sandwich’ either one or two layers of M′′ within each M–X block, where M′ and M′′ denote an early transition metal. in crystallography, crystal structure can be represented and described by symmetry known as space groups. The P63/mmc symmetry is one of the 26 space groups that represent a hexagonal crystal structure. ripplocation is a new strain accommodating defect in layered materials where one or more layers buckle orthogonally to the layers. A ripplocation nucleation is the process associated with the formation and establishment of the defect itself. an infinite number of atom positions for a given crystallographic structure (space group) can be subdivided into sets of so-called Wyckoff sites or Wyckoff positions.