关键词: matrix effect, Eu$^{2+}$/Eu$^{3+}$, energy transfer, multicolor emission, Eu:$Ae$F$_{2}$ crystals

Abstract: This study successfully grew a series of mixed-valence Eu$^{2+}$/Eu$^{3+}$ co-activated alkaline-earth fluoride single crystals (CaF$_{2}$, SrF$_{2}$, BaF$_{2}$, Ca$_{0.5}$Sr$_{0.5}$F$_{2}$, and the medium-entropy Ca$_{0.33}$Sr$_{0.33}$Ba$_{0.33}$F$_{2}$) using the vertical Bridgman method. During the high-temperature growth process, Eu$^{3+}$ undergoes spontaneous reduction to form Eu$^{2+}$. X-ray photoelectron spectroscopy (XPS) confirmed the coexistence of both valence states, and their relative ratio exhibits a strong host-dependence; for instance, Eu$^{2+}$ is predominant in SrF$_{2}$, while Eu$^{3+}$ is dominant in BaF$_{2}$. Under ultraviolet excitation, all crystals simultaneously exhibit the broadband emission of Eu$^{2+}$ (400—550 nm) and the narrow-line emission of Eu$^{3+}$ (585—710 nm). The Eu$^{2+}$ emission peak systematically red-shifts from 440 nm in CaF$_{2}$ to 485 nm in BaF$_{2}$ as the host cation radius increases. Efficient resonant energy transfer from Eu$^{2+}$ to Eu$^{3+}$ is evidenced by spectral overlap and opposing fluorescence lifetime trends. By controlling the host's composition, the emission chromaticity of the crystals can be continuously tuned from the deep-blue of CaF$_{2}$ (0.1720, 0.0382) to the warm-white of BaF$_{2}$ (0.4135, 0.3516) under 299 nm excitation. Notably, the medium-entropy Eu: Ca$_{0.33}$Sr$_{0.33}$Ba$_{0.33}$F$_{2}$ crystal can achieve a shift in emission tone from neutral white to warm white simply by changing the excitation wavelength. These findings provide a new design strategy for developing single-activator, wide-gamut, tunable luminescent materials for next-generation solid-state lighting and display technologies.

Key words: matrix effect, Eu$^{2+}$/Eu$^{3+}$, energy transfer, multicolor emission, Eu:$Ae$F$_{2}$ crystals