Local atomic order of crystalline Ge₈Sb₂Te ₁₁ across the ferroelectric to paraelectric transition: The role of vacancies and static disorder

The local structure of crystalline Ge₈Sb₂Te ₁₁ over a temperature range from 10 K to 480 K was studied. While a rhombohedral to rock-salt-like structural transition is clearly observed at 385 K via x ray (Bragg) diffraction, an x-ray absorption study shows that the local structure associated with the rhombohedral configuration at 10 K, where six Ge(Sb)-Te bonds are split into three shorter and three longer bonds, remains unchanged across the transition temperature, T_c. Based upon a fit of the experimental data to a correlated Einstein model, the shorter Sb-Te bond was found to be stiffer than its Ge-Te counterpart. Using DFT calculations, a tendency for the shorter bonds to localize around vacancies was noted, clearly displaying an increase in static disorder in Ge-Sb-Te alloys with respect to pure GeTe. This increase in static disorder may be directly related to the decrease in T_c observed when GeTe is alloyed with Sb ₂Te₃. Total energy calculations found the coupling of Sb-Te and a vacancy to be energetically the most favorable in Ge-Sb-Te compounds, where vacancies serve to buffer internal stresses caused by the Sb-Te bonds.