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 similar to 18 vol.% (Na-0) to similar 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 (λmax = 481 → 499 nm at λex = 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.