Tuning Eu²⁺ emission in oxynitride glass-ceramics via Na₂O modification

Eu-doped oxynitride glass-ceramics with varying Na₂O content (0–12 wt.%) were synthesized by spark plasma sintering (SPS) to investigate the influence of sodium-induced structural evolution on Eu²⁺ photoluminescence. Although europium was introduced as Eu₂O₃, photoluminescence measurements confirmed Eu²⁺ as the dominant emitting species, stabilized by the intrinsically reducing conditions of SPS. X-ray diffraction revealed a systematic Na₂O-induced amorphous-to-crystalline transition, with crystalline phase fraction increasing from ∼18 vol.% (Na-0) to ∼60 vol.% (Na-12). Rietveld refinement of the most crystalline composition confirms the formation of multiple nanocrystalline silicate and aluminosilicate phases embedded within a residual amorphous matrix. All samples exhibit broadband Eu²⁺ emission (4f⁶5d¹→4f⁷), and increasing Na₂O causes a systematic red shift of the emission maximum (λ_max = 481 → 499 nm at λ_ex = 320 nm) and spectral broadening (FWHM = 113 → 138 nm), which correlate directly with increasing crystallinity and the coexistence of multiple Eu²⁺ coordination environments. Complementary FTIR reveals Na₂O-driven silicate network depolymerization and structural heterogeneity, supporting the luminescence trends. Emission intensity shows a non-monotonic dependence on Na₂O, with a maximum for the intermediate composition (Na-3, 31 vol.% crystalline). Corresponding CIE 1931 chromaticity coordinates shift from blue toward blue-green emission, confirming effective compositional tunability. Overall, this study establishes a clear quantitative structure–property relationship linking Na₂O-induced crystallization, local Eu²⁺coordination environments, and emission characteristics, demonstrating that controlled compositional tuning provides an effective strategy for adjusting emission bandwidth and color in broadband UV-excited glass-ceramic phosphor materials.