Direct observation of partial disorder and zipperlike transition in crystalline phase change materials

Chalcogenide phase change materials, such as Ge₁Sb₂Te₄ (GST), are of tremendous importance in emerging data storage technology, which takes advantage of rapid and reversible switching of GST between the amorphous and crystalline phases. To date, however, the atomic arrangement of the crystalline GST structure has not been fully resolved, resulting in a controversial understanding of the polymorphic transition mechanism. Here, the atomic and chemical arrangements of stable hexagonal structures of GST are determined by state-of-the-art aberration-corrected scanning transmission electron microscopy. A partially ordered Ge/Sb atomic stacking is resolved in the hexagonal structure to balance the enthalpy and entropy, differing from completely disordered Ge/Sb intermixing arrangement in a metastable rock-salt structure. The transition mechanism between these two phases is proposed, achieved by opening the van de Waals gap like a zipper triggered by the Ge/Sb hopping near vacancy grooves rather than the interplanar random atomic migration. The present results shed light on the understanding of the atomic arrangement and polymorphic phase transitions as well as the control of disorder in GST phase change memory.