New insights into bonding in phase-change materials from ion-driven synthesis

Abstract

Optimizing the storage performance of phase-change memory (PCM) devices critically depends on understanding the chemical-bonding nature of phase-change materials (PCMs). However, significant controversy persists regarding the fundamental bonding characteristics, representing a key bottleneck for the field's advancement. In this study, using a unique ionic-reaction-based approach, we successfully synthesized the PCM materials, crystalline Sb₂Te₃, Bi₂Te₃, and SnTe. Crucially, this pathway is fundamentally distinct from conventional atomic-based routes, yet it yields final reaction products with identical structural characteristics. This observation prompted a thorough investigation into the bonding nature of the products. A comprehensive theoretical analysis, using methods such as the electron localization function (ELF), revealed that the chemical bonds in these materials exhibit distinct hybrid characteristics: clear covalent orbital interactions are accompanied by an ionic contribution and features of multicenter bonding. Our findings indicate that the unique bonding behavior of PCMs—compatible with disparate synthesis pathways—does not necessitate classification as a fundamentally new bond type. Instead, it can be coherently and uniformly described within the existing framework of multicenter (hypervalent) bonding theory. This work provides novel experimental evidence, and a theoretical perspective, for understanding the properties of such important functional materials.