Chalcogenide materials are investigated for various applications because of their tunable properties. In this work, we compare the Cu-induced microstructural evolution of epitaxial and polycrystalline $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ thin films using advanced transmission electron microscopy. Cu sputtering onto epitaxial (001) $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ at room temperature results in Cu intercalation into the layered structure. This expands the $c$-lattice parameter, particularly the van der Waals-like gaps, while largely preserving the layered framework. Chemical analysis reveals Cu enrichment at grain boundaries, which act as preferential diffusion pathways, followed by anisotropic in-plane Cu redistribution. Local Cu enrichment promotes transformation into layered Cu–Sb–Te phases with Sb/Te antisite defects, structurally related to hexagonal triple-layered $\mathrm{C}{\mathrm{u}}_7{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_4$ and trigonal double-layered $\mathrm{C}{\mathrm{u}}_7\mathrm{T}{\mathrm{e}}_4$ structures. The transformation proceeds through local reconstruction of the Te sublattice, involving changes in the stacking sequence and in-plane shifts of Te planes. Resistive switching of a Cu/(001) $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$/(111) Si memory stack further promotes Cu redistribution and conversion into layered Cu–Sb–Te structures. In contrast, polycrystalline $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ transforms into tetragonal $\mathrm{C}{\mathrm{u}}_{3-x}\mathrm{T}{\mathrm{e}}_2$-related and $\mathrm{C}{\mathrm{u}}_7{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_4$-type structures upon Cu sputtering. Additional Pt deposition onto the Cu layer promotes the growth of large $\mathrm{C}{\mathrm{u}}_3{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_2$-type grains, structurally related to rickardite $\mathrm{C}{\mathrm{u}}_{3-x}\mathrm{T}{\mathrm{e}}_2$ but containing Sb/Te antisite defects. These results show that Cu-induced phase formation in $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ is influenced by Cu diffusion pathways, the initial $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ microstructure, and thermal activation, which together affect the local Cu distribution and resulting phase evolution. The findings provide insight into the structural factors governing Cu–Sb–Te phase formation and are of potential relevance to the design of chalcogenide thin films for thermoelectric and memory applications.

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• Crystal structure
• Intercalation
• Structural phase transition
• Chalcogenides
• Layered semiconductors
• Thin films
• Scanning transmission electron microscopy
• X-ray diffraction