Local- and Intermediate-Range Structures on Ordinary and Exotic Phase-Change Materials by Anomalous X-ray Scattering

Local- and intermediate-range atomic structures were investigated on amorphous phases of an ordinary phase-change material, Ge2Sb2Te5 (GST), and an exotic one, Cu2GeTe3 (CGT), by using anomalous X-ray scattering close to K absorption edges of each element to find a fast amorphous-crystalline phase-change mechanism. The obtained data were analyzed by using reverse Monte Carlo modeling to obtain partial structure factors, partial pair distribution functions, and three-dimensional atomic configurations. Ring statistics were carefully examined to clarify the similarity and difference compared with the corresponding crystal structures, and it was found that amorphous GST has a number of four-membered rings indicating fragments of crystal structure, and amorphous CGT has a remarkable number of three-membered rings showing a collapse of crystal structures composed of purely six-membered rings. A persistent homology analysis was carried out and long-range ring structures of the constituent elements were observed in the amorphous phase, which may originate from fragments of crystal structures with a long-range periodicity.