Structural characteristics of GeTe-rich $GeTe–Sb2Te3$ pseudobinary metastable crystals

The $(GeTe)1−γ–(Sb2Te3)γ$ pseudobinary system has, over almost its entire composition range, two kinds of crystalline phase: one is a metastable phase with a NaCl-type structure and the other is a spectrum of stable phases with homologous structures. In the metastable phase, Ge/Sb atoms and intrinsic vacancies occupy the Na sites; on the other hand, Te atoms are located at the Cl sites. These vacancies are produced by following $γ/1+2γ$ to ensure the stoichiometry of the metastable pseudobinary compound. This metastable phase obstinately holds its NaCl-type structure and resists transformation to stable homologous structures, even at high temperatures on the GeTe-rich side of the system. In GeTe $(γ=0)$⁠, the NaCl-type atomic configuration itself is the stable structure. GeTe has, as is well known, a high-temperature cubic phase and a low-temperature rhombohedral phase. This GeTe and the pseudobinary compounds containing a small quantity of $Sb2Te3$ have their single-phase regions not on the $GeTe–Sb2Te3$ tie line but at Ge-poor sides off the line: in other words, the Na sites of these off-stoichiometric compounds have some excess vacancies besides the intrinsic vacancies. As $Sb2Te3$ is further added to GeTe, however, the structural transformation temperature continuously falls and the single-phase region converges on the tie line as the excess vacancies at the Na site disappear, which change its electrical property from metallic to semiconducting conductivity. The low-temperature rhombohedral phase is present up to near $γ=0.14$⁠. The NaCl-type metastable phase becomes unstable with increased $Sb2Te3$⁠; after subjecting the compound $Ge8Sb2Te11$ $(γ=0.11)$ to heat treatment for 15 days at 773 K, a stable homologous structure appeared.