Multi-Step Crystallization and Chemical Evolution of Sodium Yttrium Fluoride

Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are commonly observed both in the laboratory and in nature. While this pathway may require chemical reactions, subsequent nucleation and growth are often considered as separate, discrete events from the reaction itself. Recent work has also shown a distinct intermediate step involving the formation of an amorphous aggregate. Here we report a novel four-step mechanism in the aqueous synthesis of sodium yttrium fluoride involving 1) the segregation of aqueous ions into a dense liquid phase, 2) the formation of an amorphous aggregate, 3) nucleation of a cubic YF3 phase, and 4) subsequent solid-state diffusion of sodium and fluoride ions to form a final NaYF4 phase. The final step involves a continuous, gradual change of the solid phase’s chemical stoichiometry from YF3 toward NaYF4. Unlike previously studied nucleation and growth mechanisms, the stoichiometry of the final solid phase evolves throughout the crystallization process rather than being determined at nucleation. This novel four-step mechanism provides a new perspective into the nucleation and growth of many other crystalline materials given the ubiquity of nonstoichiometric compounds in nature.

Enhanced up-conversion emission in Er3+ doped barium–natrium–yttrium–fluoride phosphor by alkali ions introduction under 1550 nm excitation

Multi-component fluoride phosphors BaxNay-nMnYzF2x+y+3z+3m: Er3+m (M=Li, K) sensitive to 1550 nm were synthesized by the low temperature combustion synthesis (LCS) method. The optimum batch formula was determined by the orthogonal...

Yttrium speciation in subduction-zone fluids from ab initio molecular dynamics simulations

The rare Earth elements (REEs) are important geochemical tracers for geological processes such as high-grade metamorphism. Aqueous fluids are considered important carriers for the REEs in a variety of geological environments including settings associated with subduction zones. The capacity of a fluid to mobilize REEs strongly depends on its chemical composition and on the presence of suitable ligands such as fluoride and chloride. In this study, we present structural and thermodynamic properties of aqueous yttrium–chloride and yttrium–fluoride species at a temperature of 800 ∘C in a pressure range between 1.3 and 4.5 GPa derived from ab initio molecular dynamics simulations. The total yttrium coordination by H2O and halide ions changes from seven to eight within the pressure range. For the yttrium–chloride species, a maximum number of three chloride ligands was observed. The derived thermodynamic data show that aqueous yttrium–fluoride complexes are more stable than their yttrium–chloride counterparts in chloride- and fluoride-rich environments at conditions relevant to slab dehydration. Mixed Y(Cl,F) complexes are found to be unstable even on the molecular dynamics timescale. Furthermore, in contrast to field observations, thermodynamic modeling indicates that yttrium should be mobilized at rather low fluoride concentrations in high-grade metasomatic systems. These results suggest a rather low fluoride activity in the majority of subduction-zone fluids because yttrium is one of the least-mobile REEs. Additionally, the simulations indicate that yttrium drives the self-ionization of hydration water molecules as it was observed for other high-field-strength elements. This might be a general property for highly charged cations in aqueous solutions under high-temperature and high-pressure conditions.