Evidence and Practical Applications of Site Occupancy Theory (SOT) of Eu³⁺ in Scheelite Compounds

The determination of the site occupancy of activators in phosphors is essential for precise synthesis, understanding the relationship between their luminescence properties and crystal structure, and tailoring their properties by modifying the host composition. Herein, one simple method was proposed to help determine the sites at which the doping of rare earth ions or transition metal ions occupies in the host lattice through site occupancy theory (SOT) for ions doped into the matrix lattice. SOT was established based on the fact that doping ions preferentially occupy the sites with the lowest bonding energy deviations. In order to provide detailed experimental evidence to prove the feasibility of SOT, several scheelite-type compounds were successfully synthesized using a high-temperature solid-phase method. When Eu³⁺ ions occupy a similar surrounding environment site, the photoluminescence spectra of the activators Eu³⁺ are similar. Therefore, by comparing the intensity ratio of photoluminescence spectra and the mechanism of all transitions of KEu(WO₄)₂, KY(WO₄)₂:Eu³⁺, Na₅Eu(WO₄)₄, and Na₅Y(WO₄)₄:Eu³⁺, it was proved that SOT can successfully confirm the site occupation when doped ions enter the matrix lattice. SOT was further applied to the sites occupied by Eu³⁺ ion-doped LiAl(MoO₄)₂ and LiLu(MoO₄)₂.