This title appears in the Scientific Report : 2011 

Polymorphism in phase-change materials: melt-quenched and as-deposited amorphous structures in Ge_2 Sb_2 Te_5 from density functional calculations
Akola, J.
Larrucea, J. / Jones, R. O.
Quanten-Theorie der Materialien; PGI-1
Physical review / B, 83 (2011) S. 094113
College Park, Md. APS 2011
Journal Article
Grundlagen für zukünftige Informationstechnologien
Physical Review B 83
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The as-deposited (AD) amorphous structure of the prototype phase change material Ge2Sb2Te5 (GST-225) has been studied by density functional calculations for a 648-atom sample generated by computer-aided deposition at 300 K. The AD sample differs from a melt-quenched (MQ) sample in essential ways: (1) Ge atoms are predominantly tetrahedrally coordinated, and (2) homopolar and Ge-Sb bonds are more common and reduce the number of ABAB squares (A = Ge, Sb; B = Te), the characteristic building blocks of the material. The first observation resolves the contradiction between measured (EXAFS) and calculated Ge-Te bond lengths, and the latter explains the very different crystallization speeds. Sb and Te have higher chemical coordination than suggested by the "8-N rule" of covalent networks (N is the number of valence electrons). The EXAFS signal calculated for AD agrees much better with experiment than that calculated for MQ.