Defects and persistent luminescence in Eu-doped SrAl${}_2$O${}_4$

Abstract

We investigate native point defects and rare-earth (co)dopants in SrAl${}_2$O${}_4$ using hybrid density-functional defect calculations. Europium (Eu) and dysprosium (Dy) are found to be mixed valence and energetically most favorable at the Sr lattice sites. However, unlike Eu where both Eu${}^{2+}$ and Eu${}^{3+}$ can be realized in synthesis, Dy is stable predominantly as Dy${}^{3+}$, and the divalent Dy${}^{2+}$ may only be photogenerated under irradiation. On the basis of an analysis of Eu-related band-defect (including charge-transfer) and interconfigurational $5d$-$4f$ optical transitions, we assign the characteristic broad blue (445 nm) and green (520 nm) emission bands in Eu${}^{2+}$-doped SrAl${}_2$O${}_4$ to the $4f^{6}5d^1$ $\to$ $4f^7$ transition in Eu${}^{2+}$ incorporated at the Sr1 and Sr2 sites, respectively. Strontium interstitials (not oxygen vacancies, in contrast to what is commonly believed) and Dy${}_{\mathrm{Sr}}$ can act as efficient electron traps for room-temperature persistent luminescence. This work calls for a re-assessment of certain assumptions regarding specific carrier trapping centers made in all mechanisms previously proposed for the persistent luminescence in Eu- and (Eu,Dy)-doped SrAl${}_2$O${}_4$. It also serves as a methodological template for the understanding and design of rare-earth doped phosphors.

15 pages, 14 figures, 4 tables

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