Diffusion-driven and size-dependent phase changes of gallium oxide nanocrystals in a glassy host

Abstract

Phase transformations at the nanoscale represent a challenging field of research, mainly in the case of nanocrystals (NCs) in a solid host, with size-effects and interactions with the matrix. Here we report the study of the structural evolution of γ-Ga₂O₃ NCs in alkali-germanosilicate glass – a technologically relevant system for its light emission and UV-to-visible conversion – showing an evolution drastically different from the expected transformation of γ-Ga₂O₃ into β-Ga₂O₃. Differential scanning calorimetry registers an irreversible endothermic process at ∼1300 K, well above the exothermic peak of γ-Ga₂O₃ nano-crystallization (∼960 K) and below the melting temperature (∼1620 K). Transmission electron microscopy and X-ray diffraction data clarify that glass-embedded γ-Ga₂O₃ NCs transform into LiGa₅O₈via diffusion-driven kinetics of Li incorporation into NCs. At the endothermic peak, β-Ga₂O₃ forms from LiGa₅O₈ dissociation, following a nucleation-limited kinetics promoted by size-dependent order–disorder change between LiGa₅O₈ polymorphs. As a result of the changes, modifications of UV-excited NC light emission are registered, with potential interest for applications.