Origin of long afterglow in strontium aluminate phosphors: Atomic scale imaging of rare earth dopant clustering

Ceramic pigments emitting long afterglow have enormous potential for emerging lighting applications that consume zero energy from the power grid. One of the most efficient compounds is strontium aluminate, when co-doped with 2 rare-earth elements - an optically active emitter, such as Eu2+, and an auxiliary ion, such as Dy3+ - and B. To date, spectrophotometric methods are commonly used to determine the material structure supporting long afterglow, yielding indirect evidence of energy transfer between the rare-earth co-dopants. Here, atomic resolution HAADF-STEM imaging is used to resolve columns of Sr sub-lattice sites in the (012)-projection of a Sr4Al14O25:Eu,Dy single crystal. Through quantitative STEM image simulations, heavy rare earth dopants are shown to incorporate substitutionally into Sr sites, causing an enhancement in image contrast over that of neighboring atomic columns by 125%. DFT structural simulations demonstrate that Eu2+ and Dy3+ would incorporate into adjacent Sr lattice sites along the [012], enabling energy transfer between them that we see as afterglow. With the help of atomic resolution HAADF-STEM imaging, we also provide direct experimental evidence of clustering of ionic point defects induced by B doping, leading to extremely long (> 14 h) afterglow in strontium aluminate phosphors.