Pressure-Induced Phase Transitions in GeTe-Rich Ge-Sb-Te Alloys across the Rhombohedral-to-Cubic Transitions

We demonstrate that pressure-induced amorphization in Ge-Sb-Te alloys across the ferroelectric-paraelectric transition can be represented as a mixture of coherently distorted rhombohedral Ge₈Sb₂Te₁₁ and randomly distorted cubic Ge₄Sb₂Te₇ and high-temperature Ge₈Sb₂Te₁₁ phases. While coherent distortion in Ge₈Sb₂Te₁₁ does not prevent the crystalline state from collapsing into its amorphous counterpart in a similar manner to pure GeTe, the pressure-amorphized Ge₈Sb₂Te₁₁ phase begins to revert to the crystalline cubic phase at ∼9 GPa in contrast to Ge₄Sb₂Te₇, which remains amorphous under ambient conditions when gradually decompressed from 40 GPa. Moreover, experimentally, it was observed that pressure-induced amorphization in Ge₈Sb₂Te₁₁ is a temperature-dependent process. Ge₈Sb₂Te₁₁ transforms into the amorphous phase at ∼27.5 and 25.2 GPa at room temperature and 408 K, respectively, and completely amorphizes at 32 GPa at 408 K, while some crystalline texture could be seen until 38 GPa (the last measurement point) at room temperature. To understand the origins of the temperature dependence of the pressure-induced amorphization process, density functional theory calculations were performed for compositions along the (GeTe)_x - (Sb₂Te₃)_1-x tie line under large hydrostatic pressures. The calculated results agreed well with the experimental data.