Defining Resonant Bonding: Why Are Phase Change Materials Unique ? (Invited)

Many of the most extraordinary properties of phase change materials (PCMs) can be attributed to the specific type of bonding found in the crystal structure and called ‘resonant bonding’. However, if experimental studies have shown (using FTIR) the unique character of bonding in several PCMs, the ‘resonant’ nature of bonds remains a rather general concept. In this presentation, we use ab initio (DFT) methods to address the bonding properties of various types of crystalline compounds, among which popular phase change materials, and quantify the electronic properties using several indicators. We show the direct correlation between the high polarizability of the bonds and the localization properties of electrons close to the Fermi level which allow to better define resonant bonding. This allows to place resonantly bonded materials, thus PCMs, at a specific location on a scale that extends from ionic compounds to regular semiconductors and finally metals. Our indicator also illustrates clearly the difference between amorphous and crystalline PCMs.