Mechanism of Persistent Luminescence in Eu[sup 2+] and Dy[sup 3+] Codoped Aluminate and Silicate Compounds

A mechanism of persistent luminescence that was proposed in 1996 for SrAl 2 O 4 :Eu 2+ ;Dy 3+ has been widely adopted to explain afterglow in many Eu 2+ and Dy 3+ codoped aluminates and silicates. The mechanism involves the thermally activated release of a hole from Eu 2+ in its excited 5d state to the valence band which is subsequently trapped by Dy 3+ . In this work the location of the lanthanide energy levels relative to the valence and conduction band of various compounds is presented. It is shown that the mechanism of persistent luminescence cannot be correct. An alternative model that involves the ionization of the 5d electron to conduction band states and subsequent trapping by Dy 3+ is proposed. The level schemes are consistent, both qualitatively and quantitatively, with many observations regarding persistent luminescence. They also provide insight into the mechanism of thermal quenching of Eu 2+ 5d-4f emission.