Characterizing the Peierls-Like Distortion and Phase Behavior in Amorphous Ge-Sb Mixtures Using a Machine-Learned...

Phase-change materials have received attention as candidates for next-generation memory storage due to the robustness of multiple solid phases with different electronic and optical properties under ambient conditions. The phase change material Ge₁₅Sb₈₅, the eutectic composition of Ge-Sb, also exhibits a Peierls-like distortion which opens a pseudogap in the electronic density of states that can be exploited for materials design. It is known that this distortion diminishes on heating, and it has been speculated that the competition between distorted and nondistorted states may give rise to an amorphous-amorphous phase transition. To explore these possibilities, we developed a machine-learned interatomic potential for Ge-Sb mixtures that is transferable across densities and compositions using the atomic cluster expansion (ACE) model. Applying this potential to the Ge₁₅Sb₈₅ system, we reproduce experimentally measured structural changes and confirm the presence of a Peierls-like distortion that is suppressed at high temperatures and high pressures. We define a scalar structural order parameter to quantify the strength of the distortion and classify the dependence of this parameter on the pressure, temperature, and stoichiometry. We find that the Peierls-like distortion depends on thermodynamic conditions and is most prominent at low temperature and low density. However, energy minimization of equilibrium configurations reveals that density is the predominant driver of the underlying motif governing the local structure, with temperature affecting solely the sharpness of structural features. We demonstrate that the variation in both density and distortion strength under compression is smooth and continuous, leading us to conclude that structural changes occur via a gradual crossover as opposed to a discrete phase transition.