Origin of short- and medium-range order in supercooled liquid Ge3Sb2Te6 from ab initio molecular dynamics simulations

Phase-change materials such as Ge-Sb-Te compounds have attracted much attention due to the potential value for electrical data storage. In contrast to the amorphous and crystalline phases, supercooled liquid is far from to be deeply understood despite of the inevitable role in both amorphization and crystallization process. To this end, we have studied the dynamics property and structural characteristic of liquid and supercooled liquid Ge3Sb2Te6 during the fast cooling process. The results revealed that Ge3Sb2Te6 is still a strong liquid as the temperature is above 723 K. As the temperature decreases, chemical bonds become more homogeneous, but coordination numbers of Ge, Sb and Te atoms change very little. Meanwhile, the order of short-range configurations strengthens notably. Further studies suggested that Ge-centered, Sb-centered and Te-centered configurations change to the more ordered defective octahedrons mainly by adjusting the bond-angle and bond length, rather than just change the coordinated environment. It is the more ordered octahedrons that promote the formation of medium-range order. Our finding provides a deep insight on the origin of local structural order in supercooled liquid Ge3Sb2Te6, which is of great importance for the comprehensive understanding of amorphization and crystallization process.