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

Abstract

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.