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, beta-NaREF4, which is generally obtained from the phase transition of the cubic (alpha-) phase counterpart, are attractive for photonic applications. However, the investigation of the alpha -> beta 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 alpha-NaREF4 particles. Instead of a uniform composition, the beta-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 alpha-NaREF4 particles transformed to beta-NaREF4 nuclei with no controversial dissolution, and the alpha -> beta 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.