Crystal Orientation Memory during Ag2S Phase Transformation

Phase transformations govern material properties, enabling applications in medical devices, data storage, aerospace components, and more. However, the understanding and controlled manipulation of the phase microstructures during the reversible transformation remains incomplete due to the formation of diverse variants, particularly for nonmetallic materials. Here, using in situ transmission electron microscopy (TEM), we revealed that single-crystalline Ag₂S could be controlled to expand and contract along the same direction during its phase transition cycles. This allows its orientation to be memorized, ascribed to the nonuniform distribution of Ag⁺ in a high-temperature body-centered cubic (BCC) phase of β-Ag₂S, which is a fast-ion-conductor. Intriguingly, by controlling the Ag⁺ ion distribution in β-Ag₂S, we are able to manipulate the orientation memory effect and accordingly to control the orientation and shape during phase transformation back to α-Ag₂S, a typical semiconductor. These findings open the possibility for the development of unconventional shape memory material for multifunctional electronic devices.