Materials Science

A Matter of Coexistence

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Science  16 Nov 2007:
Vol. 318, Issue 5853, pp. 1039
DOI: 10.1126/science.318.5853.1039c

Alloys such as Ge2Sb2Te5 (GST) find use in nonvolatile electronic memory and as recording media in DVDs because they undergo a fast and reversible transition between amorphous and crystalline phases with distinct optical and electronic properties. It was initially assumed that the amorphous form was simply a disordered version of the metastable cubic (rock salt) form. However, experiments on quenched thin films and simulations have given a different and often conflicting picture. Caravati et al. used ab initio molecular dynamics simulations to probe the amorphous structure. They started with metastable cubic GST with Te occupying one sublattice and Ge, Sb, or vacancies randomly occupying the other. For a quenched and annealed sample at 300 K, the calculated x-ray scattering factor was in good agreement with experimental results from the literature. The Ge and Sb atoms were mostly four-coordinate and the Te atoms mostly three-coordinate in defective octahedral-like sites, resembling cubic crystalline GST. However, about one-third of the Ge atoms occupied a tetrahedral environment, absent in the crystalline phase but supportive of the large number of homopolar Ge-Ge, Ge-Sb and Sb-Sb bonds that formed. The authors believe the coexistence of these two arrangements accounts for the rapid phase changes and strong optical contrast between the phases. — MSL

Appl. Phys. Lett. 91, 171906 (2007).

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