Pure and Pr³⁺-doped BaYF₅ nanoparticles were synthesized by the microwave hydrothermal method. The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical spectroscopy. The XRD and TEM confirm that the average size of nanoparticles is in the range of 26-37 nm. The optical excitation and luminescence spectra of BaYF₅:Pr³⁺ nanoparticles are presented in the visible and ultraviolet (UV) range. It has been verified that Pr³⁺ ions are capable of emitting UV-C photons when excited by a 444 nm laser. This emission arises from a two-photon energy-transfer upconversion mechanism. The concentration dependence of the upconversion luminescence of BaYF₅:Pr³⁺ was studied.

The recent pandemic has intensified the search for new ultraviolet C (UVC) phosphors excited by low-intensity visible light that can be used for disinfection or therapeutic purposes. Currently, the most promising phosphor with efficient upconversion was thus far Y₂SiO₅ (YSO) doped with Pr³⁺. However, we have studied a new material Sr₃(BO₃)₂:Pr³⁺ (SBO), whose upconversion emission is 10 times stronger than YSO, despite the high phonon energy possessed by SBO and despite the lack of optimization of synthesis and dopant concentration and the absence of co-dopant that should be added to compensate for the charge. Such an efficient upconversion is achieved by engagement of the ¹D₂ level, which is populated by both the multiphonon non-radiative transition and the closed feedback loop. From this level, blue excitation can reach the 4f¹5d¹ electronic configuration. At the same time, due to a small Stokes shift, the 5d levels emit exclusively in the UVC and partially in the ultraviolet B (UVB) region.

Fluorides have a wide bandgap and therefore, when doped with the appropriate ions, exhibit emissions in the ultraviolet C (UVC) region. Some of them can emit two photons in the visible region for one excitation photon, having a quantum efficiency greater than 100%. In a novel exploration, praseodymium (Pr³⁺) ions were introduced into KLaF₄ crystals for the first time. The samples were obtained according to a high-temperature solid-state reaction. They exhibited an orthorhombic crystal structure, which has not been observed for this lattice yet. The optical properties of the material were investigated in the ultraviolet (UV) and visible ranges. The spectroscopic results were used to analyze the Pr³⁺ electronic-level structure, including the 4f5d configuration. It has been found that KLaF₄:Pr³⁺ crystals exhibit intense luminescence in the UVC range, corresponding to multiple 4f → 4f transitions. Additionally, under vacuum ultraviolet (VUV) excitation, distinct transitions, specifically ¹S₀ → ¹I₆ and ³P₀ → ³H₄, were observed, which signifies the occurrence of photon cascade emission (PCE). The thermal behavior of the luminescence and the thermometric performance of the material were also analyzed. This study not only sheds light on the optical behavior of Pr³⁺ ions within a KLaF₄ lattice but also highlights its potential for efficient photon management and quantum-based technologies.