Color-tunable visible photoluminescence of Eu:CaF single crystals: variations of valence state and local lattice environment of Eu ions.

In this article, Eu-activated CaF single crystals were synthesized by Bridgman-Stockbarge method. The dependence of photoluminescence properties of Eu: CaF crystals in UV-Vis regions on EuF doping concentrations were investigated. While the EuF doping concentration is increased from 0.6% to 6.0%, the CIE (Commission Internationale de L'Eclairage) color coordinates of Eu: CaF crystals can be tuned from (0.28, 0.12) to (0.60, 0.38), corresponding to the luminescence color from blue to orange. XPS (X-ray Photoelectron Spectroscopy) measurements indicated that Eu and Eu ions both existed in the crystals. With EuF doping concentration increasing, the proportion of Eu ions increase from 16.73% to 39.00%, while that of Eu ions decrease from 83.27% to 61.00%. Moreover, the integrated intensity ratio (R) of the 614 nm to 593 nm of Eu ions increase from 0.38 to 0.44, indicating the local lattice environment symmetry of Eu ions become lower with higher EuF doping concentrations. Furthermore, the CIE chromaticity coordinates of Eu: CaF crystals greatly depend on the excitation wavelength. The warm white-light emission has been realized in 0.6%Eu: CaF crystal when the excitation wavelength is around 322 nm.