Impact of crystallinity and local disorder on the luminescence properties of solvothermally synthesized LuPO4:Pr3+ nanocrystals†

In this study, we present the solvothermal synthesis of LuPO₄:Pr³⁺ (1%) nanocrystals in a dimethyl sulfoxide–water solvent system. The as-prepared nanocrystals exhibit a relatively high degree of crystallinity, along with significant short-range disorder, particularly in the anionic sublattice, as revealed by XRD and Raman spectroscopy data. Low temperature luminescence spectroscopy under VUV excitation showed that this disorder hinders the energy transfer from the host lattice to the Pr³⁺ ions, resulting in the absence of the 4f¹5d¹→4f² emission under high energy excitation. Thermal annealing of as-prepared nanocrystals significantly reduces the degree of local disorders enhancing UV-C luminescence in the 4.4–5.5 eV range by improving energy transfer processes from the LuPO₄ host to Pr³⁺ ions. Time-integrated luminescence spectra reveal up to a five-fold increase in UV-C emission intensity after annealing. UV-C emissions due to Pr³⁺ ions in LuPO₄ are efficiently excited in the deep intrinsic absorption region at 45 eV, which simulates the energy conversion process for radiotherapy applications. Time-resolved studies identify two types of Pr³⁺ ions. One group consists of strongly perturbed ions near the nanoparticle surface, influenced by structural imperfections, while the other includes weakly perturbed ions located in the bulk. These results highlight the critical role of local structural imperfections almost undetectable by XRD in energy transfer processes involving the host lattice.