Enhanced Vis-to-UVC Upconversion in Pr ³⁺ -Doped A ₃ (BO ₃ ) ₂ (A = Sr, Ca) under 444 nm Excitation: Suppression of Cluster Formation via Alkali Ion Co-doping

Upconverting materials represent a diverse and significant class of luminophores, with those capable of converting visible (vis) light into ultraviolet C (UVC) playing a particularly important role. This is due to the germicidal properties of UVC radiation, which is widely used for disinfection, sterilization, and purification applications. In this study, we investigate the vis-to-UVC upconversion process in A₃(BO₃)₂:Pr³⁺ (A = Sr, Ca) compounds, with a particular focus on the effects of Li⁺ and Na⁺ co-doping as charge compensation. Our findings reveal that Na⁺ co-doping significantly enhances upconversion luminescence in both Sr₃(BO₃)₂ (SBO) and Ca₃(BO₃)₂ (CBO). Moreover, the co-doped samples exhibited improved crystallinity as a result of the flux effect. Interestingly, we observed no concentration quenching of the ¹D₂ level in singly doped samples, while co-doping with Na⁺ led to a reduction in ¹D₂ decay time with increasing Pr³⁺ content. Numerical simulations based on a multilevel model revealed that when cross-relaxation (CR) dominates over radiative decay, as in clustered Pr³⁺ ions, concentration quenching effects on decay time are minimal. Co-doping with Na⁺ or Li⁺ suppresses cluster formation, reducing nonradiative losses and improving upconversion efficiency. A more pronounced effect was observed for Na⁺ ions, attributed to their closer ionic radius match with the substituted Sr²⁺ and Ca²⁺ sites in the host lattice. Overall, this study provides a detailed analysis on the influence of CR processes on the decay kinetics of the ¹D₂ level in compounds with high energy of phonons. Furthermore, the obtained results bring valuable insight into the roles of charge compensation and cluster formation in Vis-to-UVC upconversion materials, an area that remains largely underexplored.