Reversible devitrification in amorphous $\mathrm{As_2Se_3}$ under pressure

In pressure-induced reversible structural transitions, the term “reversible” refers to the recovery of the virginstructure in a material upon complete decompression. Pressure-induced amorphous-to-crystalline transitionshave been claimed to be reversible, but evidence that amorphous material recovers its virgin amorphous structureupon complete depressurization has been lacking. In amorphous As$_2$Se$_3$ (a-As$_2$Se$_3$) chalcogenide, however, wereport a novel reversible amorphous-to-crystalline transition that provides compelling experimental evidencethat upon complete decompression, the recovered amorphous phase is structurally the same as that of the virgin(as-cast) amorphous phase. Combining the experimental results with ab initio molecular dynamics simulations,we elucidate that the amorphization ismediated by a surplus of total free energy in the high-pressure face-centeredcubic phase as compared to the virgin amorphous phase and that the structural recovery to the virgin amorphousphase is a consequence of an enhancement in covalent bonding character over interlayer forces upon completedecompression. Furthermore, we observed a two-dimensional to three-dimensional network transition undercompression and its reversibility upon decompression.