Growth and luminescence properties of Eu-doped (Na 0.425− x Lu 0.575+ x )F 2.15+2 x single crystals
(Na0.425−xLu0.575+x−yEuy)F2.15+2x (x=0, ±0.05, y=0.001, 0.01, 0.05) single crystals were grown from the melt by the precise atmosphere control type micro-pulling-down (μ-PD) method. The grown crystals were single-phase materials that were confirmed by X-ray diffraction, and their lattice constants increased when substitution of Lu with Eu increased. The crystals demonstrated about 90% transmittance in the 400–900nm region that decreased with greater doping with Eu or Na. In all the crystals, strong absorption resulted from Eu2+ 5d–4f transition was observed around 200–350nm. Eu2+ 5d–4f photoluminescence and radioluminescence under α-ray excitation were also detected. The crystals with low content of Eu or Na had high photoluminescence intensity; however, the radioluminescence intensity was extremely low. Intensity of the Eu3+ 4f–4f emission increased with increase in Eu concentration. Formation of Eu2+ in the grown crystals was also detected especially in those that had low content of Eu and Na.