High‐Entropy Engineering Enables Highly Enhanced Upconversion Luminescence in β‐NaGdF ₄ ‐Structured Fluorides

ABSTRACT

Lanthanide (Ln ³⁺ )‐doped upconversion luminescent materials (UCLMs) have tremendous potential for applications in solar cells, optical sensors, and various other fields. However, their low luminescent efficiency and thermal stability remain key challenges that hinder their practical applications. Here, we introduce an innovative method for enhancing the upconversion luminescent efficiency of Er ³⁺ ‐activated UCLMs by constructing high‐entropy fluoride hosts based on the β ‐NaGdF ₄ framework—NaMF ₄ (M = Na, Yb, Gd, Ca, La, Lu, or Y). The β ‐NaGdF ₄ structure is preserved despite the decrease in the Gd ³⁺ concentration with an increase in the number of principal elements. High‐entropy engineering introduces severe lattice distortion, which enhances the doping tolerance of the host for Er ³⁺ ions and significantly modifies the local crystal field around them. These changes result in enhanced emission intensity and a higher quantum yield. Optimum upconversion luminescence is observed for the high‐entropy six‐element composition at room temperature; the emission intensity and quantum yield are 2.5 times those of NaGdF ₄ :20%Yb/2%Er. In addition, Er ³⁺ ‐activated high‐entropy fluorides exhibit better thermal stability than NaGdF ₄ :20%Yb/2%Er. These results demonstrate that high‐entropy engineering is a promising approach for improving the luminescent performance of Ln ³⁺ ‐doped UCLMs.