Photoinduced Structural Instability Toward the Superionic Phase Transition in Cu ₂ S

Copper(I) sulfide (Cu 2 S) undergoes a semiconductor-to-superionic phase transition associated with the destabilization of the ordered Cu sublattice. Herein, we investigated the structural dynamics of photoexcited Cu 2 S using time-resolved electron diffraction from picoseconds to milliseconds. The measurements reveal a rapid suppression (80%–90%) of semiconducting phase superlattice reflections in ∼10 ps, indicating prompt destabilization of Cu ordering. This process is accompanied by a slower lattice expansion of approximately 100 ps, observed as a shift of Bragg diffraction peaks corresponding to a lattice strain of ∼0.7%. The suppression of the superlattice reflections exhibits a fluence threshold, indicating a critical excitation density for destabilizing the ordered Cu sublattice. Density functional theory calculations show that photoexcitation populates antibonding states, weakening Cu–S bonds, and driving the observed structural destabilization. These findings reveal the ultrafast microscopic pathway toward the superionic phase in Cu 2 S and provide insights into photoinduced structural dynamics in superionic materials.