Phase transformation pathways of ultrafast-laser-irradiated Ln2O3(Ln=Er–Lu)

Abstract

Ultrafast laser irradiation causes intense electronic excitations in materials, leading to transient high temperatures and pressures. Here, we show that ultrafast laser irradiation drives an irreversible cubic-to-monoclinic phase transformation in ${\mathrm{Ln}}_{2}{\mathrm{O}}_{3}\phantom{\rule{4pt}{0ex}}(\mathrm{Ln}=\mathrm{Er}--\mathrm{Lu})$, and explore the mechanism by which the phase transformation occurs. A combination of grazing incidence x-ray diffraction and transmission electron microscopy are used to determine the magnitude and depth-dependence of the phase transformation, respectively. Although all compositions undergo the same transformation, their transformation mechanisms differ. The transformation is pressure-driven for $\mathrm{Ln}=\mathrm{Tm}--\mathrm{Lu}$, consistent with the material's phase behavior under equilibrium conditions. However, the transformation is thermally driven for $\mathrm{Ln}=\mathrm{Er}$, revealing that the nonequilibrium conditions of ultrafast laser irradiation can lead to novel transformation pathways. Ab initio molecular-dynamics simulations are used to examine the atomic-scale effects of electronic excitation, showing the production of oxygen Frenkel pairs and the migration of interstitial oxygen to tetrahedrally coordinated constitutional vacancy sites, the first step in a defect-driven phase transformation.