Determination of Opto‐Structural Parameters to Optimize Blue‐to‐UV Upconversion in Pr 3+ ‐Activated RE BO 3 ( RE = Y, La, Lu) for Phosphor‐Converted UV LEDs

ABSTRACT Although blue light‐emitting diodes (LEDs) are efficient, long‐lived, and technologically mature, extending their performance into the UVC region (<280 nm) remains challenging. To address this limitation we investigate Pr 3+ ‐doped rare‐earth orthoborate ( RE BO 3 , RE = Y, La, Lu) compounds as blue‐to‐UV upconversion phosphors, exploiting the structural diversity of this family, which contains aragonite‐type (LaBO 3 ), pseudo‐wollastonite‐type ( π ‐LuBO 3 and YBO 3 ), and calcite‐type ( β ‐LuBO 3 ) crystal structures, to tune the upconversion performance. Comprehensive UV photoluminescence spectroscopy confirmed that the energy of the lowest 4f 1 5d 1 level, the Stokes shift (∆ S ), and the thermal quenching temperature ( T 50 ) of the 4f 1 5d 1 emission are all strongly dependent on the crystallographic environment of the Pr 3+ ion. Among the studied compounds, YBO 3 :Pr 3 + and π‐LuBO 3 :Pr 3 + exhibit the most favorable opto‐structural parameters: small ∆ S , high T 50 values (652 and 806 K, respectively), and appropriately positioned 4f 1 5d 1 levels, yielding upconversion intensities 4 and 7 times higher than the Y 2 SiO 5 :Pr 3+ reference. In contrast, β ‐LuBO 3 :Pr 3+ and LaBO 3 :Pr 3+ show significantly weaker performance, attributed to unfavorable 4f 1 5d 1 energy and, for β ‐LuBO 3 , suppression of 4f 2 → 4f 2 electric dipole transitions due to the inversion centre at the Lu 3+ site. Excitation with a commercial 450 nm LED confirms the practical potential of YBO 3 :Pr 3+ and π ‐LuBO 3 :Pr 3+ for phosphor‐converted UV‐LED applications.