Univerzita PardubiceFakulta chemicko- technologická

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Local vs. global order in GeTe-based phase-change alloys

Autoři: Krbal Miloš

Rok: 2012

Druh publikace: ostatní do riv

Strana od-do: nestránkováno

Tituly:

Jazyk	Název	Abstrakt	Klíčová slova
cze	Local vs. global order in GeTe-based phase-change alloys	In the low temperature GeTe crystalline phase, Ge and Te atoms are sixfold coordinated by each other with subsets of three shorter 2.83 Å and three longer 3.15 Å bonds often described as a Peierl distortion. Based on Bragg-diffraction studies, it was concluded that rhombohedral GeTe undergoes a displacive ferroelectric-paralectric transition with the Curie temperature,Tc, around 705 K transforming to the rocksalt structure (Fm3m). Alloying of GeTe with Sb2Te3 generates vacancies which, together with new Sb-Te bonds, gradually decreases the value of the Curie temperature. When Sb2Te3 exceeds 0.137 in (GeTe)x(Sb2Te3)1−x, only the cubic phase can be observed using the Bragg diffraction2. Here, we demonstrate through the combined use of x-ray Bragg diffraction and EXAFS that although the global structure of Ge-Sb-Te undergoes a rhombohedral to rock-salt-like structural transition, locally the material maintains a 3 + 3 bond length splitting configuration at temperatures above Tc, that is, that an order-disorder rather than a displacive phase transition3,4 takes place at Tc.	EXAFS; slitta
eng	Local vs. global order in GeTe-based phase-change alloys	In the low temperature GeTe crystalline phase, Ge and Te atoms are sixfold coordinated by each other with subsets of three shorter 2.83 Å and three longer 3.15 Å bonds often described as a Peierl distortion. Based on Bragg-diffraction studies, it was concluded that rhombohedral GeTe undergoes a displacive ferroelectric-paralectric transition with the Curie temperature,Tc, around 705 K transforming to the rocksalt structure (Fm3m). Alloying of GeTe with Sb2Te3 generates vacancies which, together with new Sb-Te bonds, gradually decreases the value of the Curie temperature. When Sb2Te3 exceeds 0.137 in (GeTe)x(Sb2Te3)1−x, only the cubic phase can be observed using the Bragg diffraction2. Here, we demonstrate through the combined use of x-ray Bragg diffraction and EXAFS that although the global structure of Ge-Sb-Te undergoes a rhombohedral to rock-salt-like structural transition, locally the material maintains a 3 + 3 bond length splitting configuration at temperatures above Tc, that is, that an order-disorder rather than a displacive phase transition3,4 takes place at Tc.	phase-change alloys; EXAFS; local order; global order