Genesis and Effects of Swapping Bilayers in Hexagonal GeSb<sub>2</sub>Te<sub>4</sub>

Disorder plays an essential role in shaping the transport properties of GeSbTe phase-change materials (PCMs) to enable nonvolatile memory technology. Recently, increasing efforts have been undertaken to investigate disorder in the stable hexagonal phase of GeSbTe compounds, focusing on a special type of swapping bilayer defects. This configuration has been claimed to be the key element for a new mechanism for phase-change memory. Here, we report a direct atomic-scale chemical identification of these swapping bilayer defects in hexagonal GeSb<sub>2</sub>Te<sub>4</sub> together with nanoscale atomic modeling and simulations. We identify the intermixing of Sb and Te in the bilayer to be the essential ingredient for the stability of the defects, and elucidate their abundance as due to the small energy cost. The bilayer defects are demonstrated to be ineffective in altering the electron localization nature that is relevant to transport properties of hexagonal GeSb<sub>2</sub>Te<sub>4</sub>. Our work paves the way for future studies of layer-switching dynamics in GeSbTe at the atomic and electronic level, which could be important to understand the new switching mechanism relevant to interfacial phase-change memory.