Structural origins of the unusual thermal stability of amorphous Cu x Ge 50- x Te 50 (0⩽ x ⩽33.3).

We have investigated the local atomic structures of several compositions of the amorphous phase of the system Cu _x Ge50-xTe ₅₀ (0⩽x⩽33.3), based on extended x-ray absorption fine-structure as well as anomalous x-ray scattering experiments, and discuss the unusual trend regarding their thermal stability as a function of the Cu content. At low concentrations (x⩽15), Cu atoms tend to agglomerate in flat nanoclusters reminiscent of the crystalline phase of metallic Cu, leading to a more and more Ge-deficient Ge-Te host network structure with growing Cu content and an increasing thermal stability. At higher Cu concentrations (x⩾25), Cu is incorporated into the network, leading to an overall weaker bonding situation which is associated with a decreasing thermal stability.
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