Unravelling the crystallization mechanism and structural evolution of Yb/Er-doped SiO2-GdF3 nano-glass ceramics

Abstract

The crystallization mechanism of Yb/Er-doped GdF₃ nanocrystals in silica nano-glass ceramics was analyzed using model-free and model-fitting methods and thermal analysis data in correlation with structural data. The formation of GdF₃ nanocrystals occurs at around 300 °C, and their size is temperature dependent, ranging from 14 to 40 nm, depending on the processing temperature. A similar trend is observed for cell volume, where a contraction of up to ≈2.3% (at 600 °C) was assigned to the gradual incorporation of Li and Yb,Er dopants. Model-free analysis showed an increase in activation energy (E_a) and the preexponential factor (log A) up to 175 kJ mol⁻¹ and 14.8 s⁻¹, respectively, until the completion of crystallization. Model-fitting analysis indicated a crystallization process controlled by an autocatalytic-type reaction where a second metastable phase (LiF) acts as a catalyst and facilitates a rapid and self-accelerated crystallization of the main GdF₃ nanocrystalline phase. The ceramization process boosted UC luminescence up to values comparable to those of NaYF₄:18Yb/2Er.