Atomistic origins of the phase transition mechanism in $Ge2Sb2Te5$

The fast and reversible phase transition mechanism between crystalline and amorphous phases of $Ge2Sb2Te5$ has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from $Sb2Te3$⁠, lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate configuration can be obtained experimentally by applying an intense laser beam, which overcomes the thermodynamic barrier from the octahedron to tetrahedron sites. The high figure of merit of $Ge2Sb2Te5$ is achieved from the optimal combination of intrinsic vacancies provided by $Sb2Te3$ and the instability of the tetrahedron sites provided by GeTe.