Phase dependent luminescence properties of Pr3+ doped BaLu2F8–BaLuF5 materials

This work presents a comparative luminescence study of Pr$³⁺$ doped phase-pure BaLuF$_5$ and mixed-phase BaLu$_2$F$_8$ – BaLuF$_5$ materials synthesized via a hydrothermal route. The spectroscopic studies were mainly performedfor phase-pure BaLuF$_5$ and two mixed-phase materials, which, according to structural and phasecompositional analysis, contained 90 or 70 wt% of orthorhombic BaLu$_2$F$_8$ and 10 or 30 wt% of cubic BaLuF$_5$,respectively. Low-temperature time-integrated photoluminescence measurements under both host excitation at 45 eV and intracenter excitation of Pr$³⁺$ ions at 6.75 eV revealed pronounced differences in luminescence betweenthe two systems. The phase-pure BaLuF$_5$ material exhibited only narrow 4f2 → 4f$^2$ transitions of Pr$³⁺$,because the 4f15d1states are energetically situated above the highest lying 4f$^2$ $^1$S$_0$ level. In contrast, the BaLu$_2$F$_8$rich mixed-phase material showed multiple intense broad bands in the UV-C spectral region due to the 4f$^1$5d$^1$ → 4f$^2$ transitions, in addition to intense 4f$^2$ → 4f$^2$ emissions in the visible spectral range, indicatingenhanced energy transfer processes influenced by the orthorhombic host matrix. A cascade emission mechanismin Pr$³⁺$ starting from the 1S0 level was revealed in both materials by the presence of characteristic emissions at 3.04 eV ($^1$S$_0$ → $^1$I$_6$) and 2.57 eV ($^3$P0 → $^3$H4) appearing in two sequent radiative relaxation stages. Time-integratedexcitation spectra highlighted distinct excitation bands associated with different Pr$³⁺$ energy levels populatedbelow the fundamental absorption edge. Time-resolved spectroscopy of the 4f$^1$5d$^1$ → 4f$^2$ emissions revealed biexponentialdecay kinetics, distinguishing surface-perturbed and bulk Pr$³⁺$ centers, with significantly differentlifetimes in each phase. The varying intensity ratio of the 4f$^1$5d$^1$ → 4f$^2$ and 4f$^2$ → 4f$^2$ emissions demonstrates that,depending on the phase composition, it is possible to control the nature of Pr$³⁺$ emissions, enabling the design ofmultifunctional luminescent nanomaterials for diverse applications of deep UV emissions in phototherapy andoptoelectronics.