Atomistic origins of the phase transition mechanism in Ge$_2$Sb$_2$Te$_5$

The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge$_2$Sb$_2$Te$_5$ has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the meta-stable 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. Due to the instability of the tetrahedra, the Ge atoms shift away from those sites, giving rise to the formation of local-ordered 4-fold motifs coupled with long-range structural disorder. The high figures of merit of Ge$_2$Sb$_2$Te$_5$ are achieved from the optimal combination of intrinsic vacancies provided by Sb2Te3 and the instability of the tetrahedron sites provided by GeTe.