Reversible Crystallization in Amorphous As₂Se₃ under Pressure

In pressure-induced reversible structural transitions, the term ‘reversible’ refers to the recovery of the virgin structure in a material upon complete decompression. Pressure-induced amorphous-to-crystalline transitions have been claimed to be reversible, but evidence of the fact that amorphous material recovers its virgin amorphous structure upon complete depressurization has been lacking. In amorphous As₂Se₃ (a-As₂Se₃) chalcogenide, however, we report a novel reversible amorphous-to-crystalline transition providing compelling experimental evidence that upon complete decompression the recovered amorphous phase is structurally the same to that of the virgin (as-cast) amorphous phase. Combining the experimental results with ab initio molecular dynamics simulations, we elucidate that the amorphization is mediated by a surplus of total free energy in the high pressure face-centered cubic phase as compared to the virgin amorphous phase, and the structural recovery to the virgin amorphous phase is a consequence of an enhancement in covalent bonding character over interlayer forces upon complete decompression. Furthermore, we also observed a 2-dimensional-to-3-dimensional network transition under compression, and its reversibility upon decompression.