Crystals characterized by "metavalent" bonds have specific properties
Jean-Yves Raty, a FNRS Senior Research Associate at the Solid State Physics Interfaces and Nanostructures Laboratory (UR CESAM, SPIN Lab of ULiège), has just published two outstanding scientific articles in the international journal Advanced Materials that were selected as an editor’s choice in Science Magazine . These studies were carried out with the team of Professor Wuttig from the University of Aachen and collaborators in Belgium, France and Italy. The researchers present an original theoretical study (ab initio) of chemical binding in solids, which allows all types of crystals to be located on a two-dimensional "map".
« The particularity of this map is that, while it very clearly separates semiconductor, metallic, ionic and resonant crystals (such as graphite), a significant part of the map is occupied by crystals in which the bonds are "incomplete", explains Jean-Yves Raty. These bonds, which however ensure the cohesion of the crystal, are indeed too numerous for the number of electrons available, which is why we have called them "meta-valent", or MVB for Metavalent Bonding. »
This two-dimensional map is even more interesting when considering certain electronic, optical or vibrational properties. Indeed, it appears that metavalent bonds are responsible for quite unique behaviours and that the compounds presenting these bonds are most often good materials for heat recovery and transformation into electricity (thermoelectricity), topological insulators, superconductors, or materials that can be used for phase change data recording in new types of fast and non-volatile memories.
« Our calculations have already been partially confirmed by experiments conducted by Professor Wuttig; he will present our results at ULiège on Thursday 7 February » (link to agenda)
« Obviously, our work needs to be continued in order to better explain these meta-valent bonds and to know to what extent they can be manipulated to create innovative materials with enhanced functionality. The question has just been asked by a leading editorialist in Chemistry World magazine », concludes Jean-Yves Raty.
On a two-dimensional map where the axes represent the quantity of electrons transferred between atoms and those actually shared between neighbors, the different types of crystals occupy distinct regions (semiconductors, metals, ionic). An area shown in green contains crystals with incomplete bonds, referred to as 'meta-valent'. When we look at the atoms, we see that these bonds are formed by the delocalization (in green) of initially covalent bonds (red), to form softer bonds (the "hardness" of the bonds is symbolically represented here by springs), but also much more polarizable.
Scientific reference
A Quantum-Mechanical Map for Bonding and Properties in Solids, J.Y. Raty, M. Schumacher, P. Golub, V. Deringer, C. Gatti and M. Wuttig, Advanced Mater. (2018) 1806280
Incipient Metals: Functional Materials with a Unique Bonding Mechanism, M. Wuttig, V. Deringer, X. Gonze, C. Bichara and J.Y. Raty, Advanced Mater. (2018) 1803777
