Séminaire

Vendredi 6 Mai 2022 à 11h00.

Bonding and properties in phase change materials through ab initio simulations

''Phase Change Materials are emerging as active components of non-volatile memories thanks to their ability to switch extremely rapidly from a conducting crystal to a more semiconducting glassy state. Using Ab Initio molecular dynamics simulations, we address the structure and some properties of the glassy phase but also go back to the description of bonding in the crystalline phase. Simulations are used to explain the aging process in those glasses as well as the metal-semiconductor transition observed in the metastable cubic phase of Ge-Sb-Te PCMs. Using a quantum-topological description of electron sharing and electron transfer in binary solids, we can construct a 2D map and highlight a distinct domain 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 of application interest, we show that bonding in metavalent compounds differs from the our usual views of bonding. Intrinsically, PCMs can be characterized as being covalent in the amorphous but metavalent in the crystal. Finally, we illustrate this concept by considering excitations in ‘regular’ chalcogenide glasses that are used as ovonic threshold switching materials. We show that electronic excitation drives the appearance of metavalent bonds.''