Photochromism and near-infrared persistent luminescence in Eu^2+-Nd^3+-co-doped CaAl_2O_4 ceramics

Abstract

We report near-infrared persistent luminescence and photochromism in Eu2+-Nd3+-codoped CaAl2O4 ceramics, a well-known blue persistent phosphor. After irradiation with UV light, the color of the sample body changed from white to purple. From the reflectance spectrum, the color center created by UV irradiation has a broad absorption band at 500 nm, which results in pink coloration. The purple color of the sample just after stopping UV irradiation is caused by mixing the blue persistent luminescence, due to the Eu2+:4f65d1→4f7 transition and the pink photochromic coloration. The sample also shows near-infrared persistent luminescence originating from the Nd3+:4F3/2→4IJ/2 transitions in addition to the blue persistent luminescence at 440 nm. Because of the similarity between Eu2+ and Nd3+ afterglow decay profiles, the electron traps contributing to both persistent luminescences are regarded as identical. The role of photo-oxidation, electron trapping and de-trapping are also discussed with regard to the persistent luminescence and photochromism.