Mechanistic Study of the Persistent Luminescence of CaAl2O4:Eu,Nd

CaAl2O4:Eu is a persistent luminescence (PL) material in the blue light region with potentially wide commercial applications. With the doping of Nd, the decay time can be elongated to more than 19 h. These excellent properties are believed to be in close relation with the electronic structures of the dopants, the defects, and the host material. In this work, we attempt to achieve a better understanding of the PL mechanism of CaAl2O4:Eu based on first-principles calculations. The electronic structures of the host CaAl2O4, the luminescent center Eu, the O and Ca vacancies, and the dopant Nd are systematically studied. According to the calculations, the 4f and 5d levels of Eu are located within the band gap and slightly above the conduction band minimum (CBM), respectively. The electrons on the 4f levels can be excited into the Sd levels via ultraviolet radiation. The excited electrons on the 5d levels can move to the conduction bands and become free electron carriers. The electron carriers can be trapped for a short period by the empty defect levels below the CBM if they are very close to the defects and then return back into the conduction band. After the trap release process, the electrons may re-enter the 5d levels of Eu and then move back to the 4f levels accompanied by light emission. The +2 charged-state O vacancies can serve as electron traps. The Ca vacancies cannot contribute to the PL property directly but can assist in stabilizing the +2 charged-state O vacancies. Nd dopants can serve as both electron donors and electron traps. These new insights into the electronic structures are useful for determining which materials may possess good PL properties, thereby motivating more experimental efforts in synthesizing improved PL materials.