Unusually Effective Blue-to-UVC Upconversion of Pr³⁺-Doped Sr₃Lu(PO₄)₃ and Ba₃Lu(PO₄)₃ Phosphors: A Comparative Study

Many pathogens, including bacteria and viruses, are increasingly developing resistance to conventional disinfectants. As a result, new approaches to disinfection are being explored, such as the use luminophores that convert visible light into ultraviolet C radiation (UVC). In this work, we present novel UVC phosphors, A₃Lu(PO₄)₃ (A = Sr, Ba), activated by Pr³⁺. These phosphates were synthesized by the Pechini method with different activator concentrations and crystallized in a cubic structure with the space group I4̅3d. The emission and excitation spectra, as well as decay times under synchrotron and visible-light excitation, were measured. Both phosphates exhibited efficient and fast 5d-4f emission from 240 to 340 nm, along with a very weak f-f emission line around 600 nm upon ultraviolet excitation. Multiphonon relaxation from the ³P₀ to ¹D₂ state causes the phosphors to emit weak blue (³P₁ → ³H₄) and intense orange (¹D₂ → ³H₄) light upon direct excitation of the ³P₂ level. The upconversion properties of A₃Lu(PO₄)₃:Pr³⁺ crystallites were investigated under 444 nm laser excitation, and the effect of Pr³⁺ concentration on these properties was evaluated. Compared to the YPO₄:Pr³⁺ reference material studied earlier, the Ba₃Lu(PO₄)₃:Pr³⁺ host showed a 20-fold enhancement in UVC upconversion emission, making this phosphate one of the most efficient visible-to-ultraviolet upconversion matrices to date. Thus, the studied phosphates have potential use in sterilization, disinfection, photocatalysis, and phototherapy.