Laser excitation-activated self-propagating sintering of NaYbF₄:Pr³⁺/Gd³⁺ white light microcrystal phosphors

We demonstrate that self-propagating sintering reaction could be activated and dramatically enhanced by laser excitation of ion dopants in the solid-state reactants. Near-resonant laser absorption and subsequent nonradiative decays make the solid-state reactants be sintered efficiently while ionic excitations catalyze self-propagating solid-state reactions. As a prototype demo, we synthesized white light upconversion phosphors NaYbF₄:Pr³⁺/Gd³⁺. A continuous-wave laser at 980 nm was used to populate Yb³⁺ ions in YbF₃ to excited level, which react with NaF to preform NaYbF₄ nuclei. The preformed nuclei enhanced laser excitation and energy transfer to those ions that could not be directly excited by the pump laser and thus enabled self-propagating solid-state sintering synthesis of NaYbF₄ microcrystals at quite low laser powers. Laser excitation of Yb³⁺ ions could also benefit facile rare-earth ion doping through activated self-propagating reactions. Gd³⁺ and Pr³⁺ ions were doped in NaYbF₄ by simply adding Gd³⁺ and Pr³⁺ ionic oxides or fluorides in the raw materials. In addition, Gd³⁺ ions doping in F⁻ anions ambient could transform the NaYbF₄ microcrystal phase from cubic to hexagonal and tune upconversion photoluminescence. This synthetic method can be widely applied to synthesize many other solid-state compounds, perovskite solar cells, photocatalysts, solid oxide fuel cells, and so forth.