Nature of Atomic Bonding and Atomic Structure in the Phase-Change Ge₂Sb₂Te₅ Glass

Using electronic structure calculations, we demonstrate a global valence alternation in the amorphous Ge₂Sb₂Te₅, a prototype phase-change alloy for data storage. The resulting p bonding profoundly influences the local atomic structure, leading to right-angle components similar to those in the crystalline counterpart of this chalcogenide glass. The dominance of p bonding is revealed by (i) distributions of the coordination number (CN) and the bond angle, for truly bonded atoms determined based on the electron localization function, and (ii) a direct evaluation of the p (and s) orbital occupation probability for the CN=3 Ge atoms that form 90° bonds with neighbors.