Determination of Opto‐Structural Parameters to Optimize Blue‐to‐UV Upconversion in Pr ³⁺ ‐Activated RE BO ₃

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.