Nonthermal phase transitions in irradiated oxides

It is predicted theoretically that various oxides (Al₂O₃, MgO, SiO₂and TiO₂) under ultrafast excitation of the electronic system exhibit nonthermal phase transitions. In the bulk, Al₂O₃transiently forms a superionic phase via nonthermal phase transition, MgO and SiO₂disorder, TiO₂experiences solid-solid phase transition while thermal effects lead to melting. In the finite-size samples and near-surface regions, MgO undergoes solid-solid phase transition at lower doses than those required for atomic disorder. All studied oxides but TiO₂, if allowed to expand, exhibit a lower damage threshold, whereas in TiO₂expansion releases the stress and prevents solid-solid phase transition thereby increasing the damage threshold up to the melting one. The results suggest that a nonthermal phase transition is a general response of oxides to sufficiently high ultrafast electronic excitation. A comparison with nonadiabatic simulations demonstrates that Born-Oppenheimer approximation systematically overestimates damage thresholds, and in some cases misses a phase transition entirely.