Inherent Simple Cubic Lattice Being Responsible for Ultrafast Solid-Phase Change of Ge₂Sb₂Te₅

Crystallization of solid is generally slow in kinetics for atoms trapped in solids. Phase-change materials (PCMs) challenge current theory on its ultrafast reversible amorphous-to-crystal transition. Here by using the stochastic surface walking global optimization method, we establish the first global potential energy surface (PES) for Ge2Sb2Te5. By analyzing all structures on the global PES, we show that an inherent structural pattern of simple cubic lattice is present universally in low-energy structures, either globally in a newly found metastable simple cubic crystal phase or locally in the amorphous structures. Our solid-tosolid reaction pathway sampling reveals that this simple cubic lattice plays a critical role in the rapid amorphous-to-crystal transition, which occurs via dynamic vacancy creation/annihilation, Martensitic-type {100} shearing, and diffusionless local relaxation. This knowledge from global PES allows the prediction of PCMs by linking the phase-change kinetics with the geometry of metastable phases. G (GST) phase-change material has great potentials for data recording in electrical devices. It has a fast (∼ns) amorphous-to-crystal solid-phase transition that works at relatively low temperatures, that is, ∼150 °C, and exhibits the sharp switch of electronic/optical signals (e.g., ∼2 orders of magnitude in resistivity change) in the solid transition. While such rapid reversible solid transitions often imply a diffusionless transformation mechanism typical in Martensitic transition between crystal phases with a particular habit plane and a shape-memory effect, the long-range ordering in GST from amorphous to crystals is apparently not the case, where the amorphous phase of GST is known to have a large structural heterogeneity, as evident by the disordered vacancies and the presence of many new types of bonds (e.g., Ge−Ge and Te−Te). This puzzle on the transition kinetics could be attributed to the lack of knowledge on intermediates between amorphous and crystal phases. New theoretical models are urgently called for to bridge the structure gap and resolve the atomic mechanism of amorphous-to-crystal transition. The potential energy surface (PES) of GST is complex due to the presence of three elements. To date, two stable classes of crystalline structures were observed for GST, hexagonal (hex) and rock-salt (rs) crystals, where rs is known as the product in the rapid amorphous-to-crystal transition. While the vacancy sites aggregate preferentially in hex and rs crystals, the amorphous phase tends to have randomly distributed cavities according to molecular dynamics (MD) simulations. In addition, new bond types, for example, homopolar Ge−Ge, Sb−Sb, Te−Te, and Ge−Sb bonds, which are not present in the two stable crystalline phases, were observed in the amorphous phase and also confirmed using reverse Monte Carlo to fit the experimental data. It is thus expected that the amorphous-to-crystal transition involves atom displacement to annihilate the homopolar bonds and gradual vacancy ordering. This complex structural variation is apparently contradictory to the rapid reversible phase transition kinetics. Consequently, how the crystalline nucleus forms and grows in an amorphous matrix is still highly debated in the literature. Here we utilize novel Stochastic Surface Walking (SSW) global optimization to resolve the global PES of GST. The SSW method developed recently is designed to efficiently explore a complex PES via smooth surface walking along softened random directions. This allows determination of the atomic structures for a huge amount of metastable structures on the global PES, including new metastable crystals and an amorphous phase, and statistical characterization of their common structural features. On the basis of the information from SSW trajectories, we further identify low-energy reaction pathways linking amorphous structures to the rs phase, which helps to clarify the physical origin of the rapid amorphous-to-crystal transition. Our investigation starts by exploring the PES of GST with SSW global optimization in the framework of van der Waalscorrected density functional theory (DFT) calculations (see Received: April 14, 2017 Accepted: May 23, 2017 Published: May 23, 2017 Letter