Structural origins of the unusual thermal stability of amorphous Cu _x Ge_50-x Te₅₀ (0=x=33.3)

We have investigated the local atomic structures of several compositions of the amorphous phase of the system Cu x Ge50-x Te50 (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.