Modeling the Onset of a Peierls Distortion in Amorphous Ge-Sb Alloys Using an ML Potential

Phase change materials -- particularly alloys containing germanium, antimony, and tellurium -- have received attention as candidates for next-generation memory storage due to the stability of multiple solid phases with different electronic and optical properties that can be realized at ambient conditions. It has recently been shown that the Ge$_{15}$Sb$_{85}$ alloy, the eutectic composition of Ge-Sb, exhibits a Peierls distortion that diminishes under compression or heating, and it has been speculated that these competing structural motifs may give rise to an amorphous-amorphous phase transition. We develop a machine learned interatomic potential using the Atomic Cluster Expansion (ACE) model for Ge-Sb alloys, transferable across densities and composition stoichiometry. Applying this model to the Ge₁₅Sb₈₅ system, we reproduce experimentally measured changes of structure and confirm the presence of a Peierls distortion that is suppressed at high pressures and temperatures. We define a structural parameter to quantify the strength of a Peierls distortion and classify the dependence of this parameter on pressure, temperature, and stoichiometry. We find that the Peierls distortion depends strongly on thermodynamic variables, but that the variation in both density and local order under compression is smooth and continuous, leading us to conclude that this is a gradual crossover as opposed to a discontinuous phase transition.