Materials Science

Time for Change

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Science  14 Oct 2011:
Vol. 334, Issue 6053, pp. 158
DOI: 10.1126/science.334.6053.158-b

Phase-change materials, such as Ge2Sb2Te5 (GST), show exceptionally fast phase transitions, which renders them useful for optical data storage and nonvolatile memory applications. Ab initio molecular dynamics simulations can explore the early stages of nucleation and crystallization in such materials. Lee and Elliott used models with 180 atoms that were held at constant volume to simulate capped films and annealed from a quenched glassy state. They defined the smallest structural element in the metastable rocksalt phase of GST as a fourfold ring (or square), six of which are required to make a cube. Both these elements are observed in the amorphous phases as transient structures. By tracking the number of rings and cubes, they identified four stages in the crystallization process, with a critical nucleus size of 5 to 10 GST cubes. At the interface between the nucleus and the amorphous phase, they observed a series of quasi-ordered rings, an arrangement facilitated by the near 90° bond angles found in GST. They propose that it is the similarity between the bonds found in the amorphous state and the distorted rocksalt structure, common to most phase-change materials, that allows such rapid crystallization.

Phys. Rev. Lett. 107, 145702 (2011).

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