Host-dependent Eu2+/Eu3+ co-luminescence and excitation-tunable multicolor emission of Eu-doped AeF2 (Ae = Ca, Sr, Ba, Ca0.5Sr0.5, Ca0.33Sr0.33Ba0.33) single crystals

doi:10.1088/1674-1056/ae0562

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.

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

Received: 12 May 2025
Revised: 27 August 2025
Accepted manuscript online: 10 September 2025

PACS:	81.10.-h	(Methods of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation)
	87.15.mq	(Luminescence)
	87.16.dt	(Structure, static correlations, domains, and rafts)

Fund: This work is partially supported by the National Key Research and Development Program of China (Grant Nos. 2022YFB3605701 and 2023YFB3507401) and the National Natural Science Foundation of China (Grant Nos. 62275198, 62075166, and 62450134).

Corresponding Authors: Wudi Wang, Qingguo Wang
E-mail: 25310031@tongji.edu.cn;wqingguo_83@yahoo.com.cn

Cite this article:

Xiaobo Huang(黄孝波), Ying Quan(全颖), Wudi Wang(王无敌), Qingguo Wang(王庆国), Xiaodong Xu(徐晓东), Huili Tang(唐慧丽), and Jun Xu(徐军) Host-dependent Eu²⁺/Eu³⁺ co-luminescence and excitation-tunable multicolor emission of Eu-doped AeF₂ (Ae = Ca, Sr, Ba, Ca_0.5Sr_0.5, Ca_0.33Sr_0.33Ba_0.33) single crystals 2026 Chin. Phys. B 35 048102

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Host-dependent Eu2+/Eu3+ co-luminescence and excitation-tunable multicolor emission of Eu-doped AeF2 (Ae = Ca, Sr, Ba, Ca0.5Sr0.5, Ca0.33Sr0.33Ba0.33) single crystals

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Host-dependent Eu²⁺/Eu³⁺ co-luminescence and excitation-tunable multicolor emission of Eu-doped AeF₂ (Ae = Ca, Sr, Ba, Ca_0.5Sr_0.5, Ca_0.33Sr_0.33Ba_0.33) single crystals