Luminescences of Bi3+ and Bi2+ ions in Bi-doped CaAl4O7 phosphor powders obtained via modified Pechini citrate process

In the present work, bismuth-doped CaAl4O7 phosphor powders were obtained via modified Pechini citrate process and their photoluminescence, radioluminescence and thermally stimulated luminescence properties were studied down to 20 K. At room temperature excitation in UV led to broad-band P-3(0,1) -> S-1(0) Bi3+ luminescence with three maxima located in blue-green part of spectrum as well as Bi2+ emission around 760 nm assigned to the P-2(3/2) -> P-2(1/2) transition. At 150 K an additional Bi3+ emission band peaking at 370 nm appeared and its intensity increased significantly upon further cooling. The presence of a few luminescent centers of Bi3+ in CaAl4O7 was assigned to the presence of defects disturbing the local symmetry of activator ions. The radioluminescence spectra showed only two bands, an intense one at 760 nm clearly related to Bi2+ and a very weak component at 555 nm associated with one of the Bi3+ sites. These two emissions were also observed in the thermoluminescence of the material following its exposure to X-rays. The Bi3+ luminescence is suitably positioned to contribute the cyan color to white-LEDs. Its quantum yield is satisfactory, close to 40%. The Bi2+ 720-850 nm emission is attractive for bioimaging as it appears in the Ist biological window. (C) 2019 Elsevier B.V. All rights reserved.