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Séminaire CETHIL (présentiel - La Doua) : Metavalent Bonding : Characterisation and Implications for Applications in Phase Change Materials, Thermoelectric and Photovoltaic compounds

Le 29 mars 2022

Amphi Chappe, bât. Claude Chappe, INSA

Langue / language:
the presentation will be in English

Présenté par : Dr. Jean-Yves RATY Maître de recherches FNRS & Fonds assoc., Faculté des Sciences, Département de physique, University of Liège

Dr. Jean-Yves RATY
Maître de recherches FNRS & Fonds assoc., Faculté des Sciences, Département de physique, University of Liège

Résumé au format pdf

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 [1] 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 [2]. 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 [3].
Interestingly, we show that it is possible to transform regular covalent bonds metavalent through excitation, which can explain the ovonic threshold switching behavior [4] of some chalcogenide glasses, or sub-picosecond laser induced phase transitions [5].
[1] J.Y. Raty et al. Advanced Materials, (2018) 1806280
[2] M. Wuttig et al. Advanced Mater. (2018) 1803777
[3] L. Guarneri et al. Advanced Materials (2021) 2102356
[4] P. Noé et al. Science Advances (2020) 9eaay2830
[5] P. Martinez et al. Advanced Materials (2021) 2102721