Direct atomic identification of cation migration induced gradual cubic-to-hexagonal phase transition in Ge₂Sb₂Te₅

GeTe-Sb₂Te₃ pseudobinary system, especially Ge₂Sb₂Te₅ alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge₂Sb₂Te₅ during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge₂Sb₂Te₅ alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below: -Te-Sb_x/Ge_y-Te-Ge_x/Sb_y-Te-Ge_x/Sb_y-Te-Sb_x/Ge_y-Te- (x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge₂Sb₂Te₅ alloy.