  This is a critical review of MAX-phase carbides and nitrides from an
electronic-structure and chemical bonding perspective. This large group of
nanolaminated materials is of great scientific and technological interest and
exhibit a combination of metallic and ceramic features. These properties are
related to the special crystal structure and bonding characteristics with
alternating strong M-C bonds in high-density MC slabs, and relatively weak M-A
bonds between the slabs. Here, we review the trend and relationship between the
chemical bonding, conductivity, elastic and magnetic properties of the MAX
phases in comparison to the parent binary MX compounds with the underlying
electronic structure probed by polarized X-ray spectroscopy. Spectroscopic
studies constitute important tests of the results of state-of-the-art
electronic structure density functional theory that is extensively discussed
and are generally consistent. By replacing the elements on the M, A, or X-sites
in the crystal structure, the corresponding changes in the conductivity,
elasticity, magnetism and other materials properties makes it possible to
tailor the characteristics of this class of materials by controlling the
strengths of their chemical bonds.
