Luminescent Afterglow Behavior in the M₂Si₅N₈: Eu Family (M = Ca, Sr, Ba)

Persistent luminescent materials are able to emit light for hours after being excited. The majority of persistent phosphors emit in the blue or green region of the visible spectrum. Orange- or red-emitting phosphors, strongly desired for emergency signage and medical imaging, are scarce. We prepared the nitrido-silicates Ca₂Si₅N₈:Eu (orange), Sr₂Si₅N₈:Eu (reddish), Ba₂Si₅N₈:Eu (yellowish orange), and their rare-earth codoped variants (R = Nd, Dy, Sm, Tm) through a solid state reaction, and investigated their luminescence and afterglow properties. In this paper, we describe how the persistent luminescence is affected by the type of codopant and the choice and ratio of the starting products. All the materials exhibit some form of persistent luminescence, but for Sr₂Si₅N₈:Eu,R this is very weak. In Ba₂Si₅N₈:Eu the afterglow remains visible for about 400 s, and Ca₂Si₅N₈:Eu,Tm shows the brightest and longest afterglow, lasting about 2,500 s. For optimal persistent luminescence, the dopant and codopant should be added in their fluoride form, in concentrations below 1 mol%. A Ca₃N₂ deficiency of about 5% triples the afterglow intensity. Our results show that Ba₂Si₅N₈:Eu(,R) and Ca₂Si₅N₈:Eu(,R) are promising persistent phosphors for applications requiring orange or red light.