Composites based on Eu²⁺-activated LiSr₄(BO₃)₃ as broadband red components for LEDs

White light-emitting diodes (WLEDs) require thermally stable broadband red components to achieve high color quality. In this work, Eu²⁺-activated LiSr₄(BO₃)₃ (LSBO) phosphor was synthesized via a solid-state reaction and investigated not only as a powder but, importantly, as polymer-phosphor composites designed for practical LED encapsulation. In this paper, we report the synthesis, structural characterization, and spectroscopic properties of Eu²⁺-doped LiSr₄(BO₃)₃ (LSBO) phosphors prepared via a conventional solid-state reaction. LSBO:Eu²⁺ exhibits strong blue-light absorption and a broad red emission band centered at ∼640 nm, arising from Eu²⁺ ions occupying two Sr²⁺ sites. To evaluate application-oriented stability, the phosphor was incorporated into three matrices: silicone (Sylgard), polyurethane (Kisling ST 44), and epoxy resin (Ciech Epidian). Long-term thermal aging tests (10,000 h at 100 °C) were performed, with particular attention to potential spectral shifts caused by matrix-phosphor interactions. Silicone-based composites demonstrated superior optical durability, maintaining both emission intensity and spectral shape, whereas polyurethane and epoxy systems showed reduced stability. For the optimized Sylgard composite, temperature-dependent photoluminescence measurements (-40 °C to 150 °C) confirmed high thermal resilience without significant spectral modification. The results highlight the successful transition from a known phosphor powder to a spectrally stable and thermally robust composite system suitable for red compensation in next-generation WLEDs.