  While solid-state materials are commonly classified as covalent, ionic, or
metallic, there are cases that defy these iconic bonding mechanisms. A
prominent example is given by phase-change materials (PCMs) for data storage or
photonics, which have recently been argued to show 'resonant' bonding; a clear
definition of this mechanism, however, has been lacking until the present day.
Here we show that these solids are clearly different from resonant bonding in
the pi-orbital systems of benzene and graphene. Instead, they exhibit a unique
mechanism between covalent and metallic bonding, which we call 'metavalent'
bonding. The materials are on the verge of electron delocalization, which
explains their exceptional property portfolio, and we therefore argue that they
represent 'incipient metals'. This yields deeper, fundamental insight into the
bonding nature of solid-state materials, and is expected to accelerate the
discovery and design of new functional materials including PCMs and
thermoelectrics.
