PNNL

Abstract

Two-step crystallization mechanisms based on spinodal decomposition followed by nucle- ation are commonly observed both in the laboratory and in nature.1,2 While this pathway may require chemical reactions, subsequent nucleation and growth are often considered as separate, discrete events from the reaction itself.3 Recent work has also shown a distinct in- termediate step involving the formation of an amorphous aggregate.4 Here, using both exper- imental measurements and atomistic computational modeling, we report another step in the aqueous synthesis of sodium yttrium fluoride of solid state chemical diffusion, thus showing a mechanism of 1) the segregation of aqueous ions into a dense liquid phase, 2) the forma- tion of an amorphous aggregate, 3) solid-state diffusion of sodium and fluoride ions into the amorphous aggregate toward a NaYF4 stoichiometry, and 4) the crystallization of a stable nonstoichiometric cubic NaYF phase. The penultimate step involves a continuous, gradual change of the solid phase’s chemical stoichiometry from YF3 toward NaYF4. Unlike previ- ously studied nucleation and growth mechanisms, the stoichiometry of the final solid phase evolves throughout the crystallization process rather than being determined at the time of the initial separation from solution. This novel four-step mechanism provides a new perspec- tive into the nucleation and growth of many other crystalline materials given the ubiquity of nonstoichiometric compounds in nature.