Phase-Transition-Driven Regional Distribution of Rare-Earth Ions for Multiplexed Upconversion Emissions

Phase transition of the polymorphs is critical for controlled synthesis and property modulation of functional materials. Upconversion emissions from an efficient hexagonal sodium rare-earth (RE) fluoride compound, β-NaREF₄, which is generally obtained from the phase transition of the cubic (α-) phase counterpart, are attractive for photonic applications. However, the investigation of the α → β phase transition of NaREF₄ and its effect on the composition and architecture is still preliminary. Herein, we investigated the phase transition with two kinds of α-NaREF₄ particles. Instead of a uniform composition, the β-NaREF₄ microcrystals exhibited regionally distributed RE³⁺ ions, in which the RE³⁺ with a smaller ionic radius (smaller RE³⁺) sandwiched the RE³⁺ with a larger ionic radius (larger RE³⁺). We unravel that the α-NaREF₄ particles transformed to β-NaREF₄ nuclei with no controversial dissolution, and the α → β phase transition toward NaREF₄ microcrystals included nucleation and growth steps. The component-dependent phase transition is corroborated with RE³⁺ ions from Ho³⁺ to Lu³⁺ and multiple sandwiched microcrystals were obtained, in which up to five kinds of RE components were distributed regionally. Moreover, with rational integration of luminescent RE³⁺ ions, a single particle with multiplexed upconversion emissions in wavelength and lifetime domains is demonstrated, which provides a unique platform for optical multiplexing applications.