Growth and characteristics of long persistent SrAl2O4- and CaAl2O4-based phosphor crystals by a floating zone technique

Long persistent phosphor crystals, SrAl2O4 doped with Eu and Dy, and CaAl2O4 doped with Eu and Nd have been grown by a floating zone technique. The intensities and the persistent times of the phosphorescences are found to depend on the growth atmosphere. SrAl2O4 crystals grown under H-2-Ar atmosphere and CaAl2O4 crystals grown under Ar atmosphere exhibit strong and long persistent green and blue phosphorescences peaking at lambda = 520 and 450 nm, respectively. The spectra of the phosphorescence from the crystals are quite similar to those obtained with sintered powders used for luminous pigments. The persistent time of the phosphorescence from CaAl2O4 crystals is similar to that obtained with sintered powders, while in SrAl2O4 crystals, it is shorter than in sintered powders. The difference in the persistent time is found to be due to Dy and/or Nd dopant concentration added as auxiliary activators. In SrAl2O4, the Dy concentration in the crystal is about 1/100 of the sintered powders. The Dy3+ ion is responsible to the persistent time of the phosphorescence. (C) 1998 Elsevier Science B.V. All rights reserved.