Structural investigation of ${\mathrm{GeSb}}_2{\mathrm{Te}}_4:$ A high-speed phase-change material

Pseudobinary ${\mathrm{G}\mathrm{e}\mathrm{T}\mathrm{e}-\mathrm{S}\mathrm{b}}_2{\mathrm{Te}}_3$ is in wide use today as a memory material in phase-change optical disks such as DVD-RAMs. ${\mathrm{GeSb}}_2{\mathrm{Te}}_4,$ one of the intermetallic compounds used in this binary system, when in thermal equilibrium, shows a complex cubic close-packed structure with a 21-layer period. However, when an amorphous thin film of this compound is heated by laser irradiation and then suddenly cooled, it crystallizes into a simple NaCl-type structure as the metastable phase. These two structures are so different from each other that it is difficult to imagine that they have the same chemical composition. To reveal the relationship between the two, in this paper we examine the structures of both phases of ${\mathrm{GeSb}}_2{\mathrm{Te}}_4$ at various temperatures by x-ray powder diffraction using synchrotron radiation facilities. The results of our investigation are as follows: the metastable phase has a very open structure, which contains one vacancy per eight atoms. The structure transition from the metastable to the stable phase is due to the vacancy diffusion. Contrary to what was previously thought, the crystal structure of the stable phase is similar to that of ${\mathrm{PbBi}}_2{\mathrm{Se}}_4.$