The roles of the Ge-Te core network and the Sb-Te pseudo network during rapid nucleation-dominated crystallization of amorphous Ge ₂Sb ₂Te ₅

Ge ₂Sb ₂Te ₅ (GST) has demonstrated its outstanding importance among rapid phase-change (PC) materials, being applied for optical and electrical data storage for over three decades. The mechanism of nanosecond phase change in GST, which is vital for its application, has long been disputed: various, quite diverse scenarios have been proposed on the basis of various experimental and theoretical approaches. Nevertheless, one central question still remains unanswered: why is amorphous GST stable at room temperature for long time while it can rapidly transform to the crystalline phase at high temperature? Here it is revealed for the first time, by modelling the amorphous structure based on synchrotron radiation anomalous X-ray scattering data, that germanium and tellurium atoms form a "core" Ge-Te network with ring formation. It is also suggested that the Ge-Te network can stabilize the amorphous phase at room temperature and can persist in the crystalline phase. On the other hand, antimony does not contribute to ring formation but constitutes a "pseudo" network with tellurium, in which the characteristic Sb-Te distance is somewhat longer than the covalent Sb-Te bond distance. This suggests that the Sb-Te pseudo network may act as a precursor to forming critical nuclei during the crystallization process. The findings conclude that the Ge-Te core network is responsible for the outstanding stability and rapid phase change of the amorphous phase while the Sb-Te pseudo network is responsible for triggering critical nucleation. In the rapid phase change material Ge ₂Sb ₂Te ₅, the Ge-Te core network stabilizes the amorphous phase at room temperature. This network is also present in the crystalline phase, thus enabling rapid phase change. The Sb-Te pseudo network, defined by atoms that are somewhat further apart than the covalent Sb-Te distance, is responsible for triggering critical nucleation.