Bonding in Chalcogenides: Characterization and Implications for Applications in Phase Change Materials, Thermoelectric and Photovoltaic Compounds

In the last two decades, Phase Change Materials have emerged as active components of non-volatile
memories thanks to their ability to switch extremely rapidly from a conducting crystal to a semiconducting
glass. Ab Initio simulations helped understanding the structure and some properties of the glassy phase,
like aging, but also led us to reinvestigate the nature of chemical bonding in the crystalline phase.
Using a two electron (pair density) formalism, we develop a two-dimensional map based on a quantum
topological description of electron sharing and electron transfer in binary solids. This map intuitively
identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and
binary materials. More interestingly, it highlights a distinct region where phase change materials are
found and for which bonding has been qualified as ‘metavalent’. Extending this map into the third
dimension by including physical properties interesting for applications, we show that bonding in
metavalent compounds differs from the usual views of bonding. This map could then be used to help
designing new materials: by searching for desired properties in a 3D space and then mapping this back
onto the 2D plane of bonding. Indeed, the metavalent region of the map encompasses compounds with
other enhanced properties, such as high thermoelectric performance or photovoltaic efficiency. We
illustrate metavalent bonding for lead chalcogenides and V-VI compounds, and the transition between
covalent and metavalent regions of the map is described.
Remarkable, metavalent bonding in crystal does not survive upon disordering, which makes their glassy
phase exceptions to the Zachariasen’s rule.