Structural, optical and luminescence properties of Fe³⁺-doped mixed alkali zirconia-borate glasses for warm orange-red photonic applications

A novel series of Fe³⁺ doped mixed alkali zirconia-borate glasses with the composition 60B₂O₃–25Na₂O–10Li₂O–(5 − x)ZrO₂–xFe₂O₃ (x = 0–1 mol%) were synthesized using the melt-quenching technique. The novelty of this work lies in combining the structural modifications with enhanced optical properties and orange-red luminescence in Fe³⁺ doped mixed alkali zirconia-borate glasses for warm photonic applications. X-ray diffraction confirmed the non-crystalline nature of glass, while SEM images and EDS spectra were utilised for morphological and elemental analysis. The density of the glasses decreased from 2.4197 to 2.4168 g cm⁻³ before gradually increasing to 2.4324 g cm⁻³. FTIR and Raman spectral studies showed the formation of non-bridging orthoborate units from pentaborate and di-pentaborate units. The optical absorption band at 450 nm is associated with the Fe³⁺ transition ⁶A_1g (⁶S) → ⁴A_1g (⁴G); ⁴E_g (⁴G). The decrease in direct bandgap from 3.90 to 3.10 eV and indirect bandgap from 3.44 to 2.77 eV, along with an increase in Urbach energy from 0.245 to 0.273 eV, indicates the disorder in structure. An increase in refractive index leads to an increase in third-order susceptibility and nonlinear refractive index. The photoluminescence spectra exhibited orange-red emission at (⁴T_2g(⁴G) → ⁶A_1g(⁶S)) with 550 nm, 560 nm and 570 nm excitation wavelengths. The CIE chromaticity and CCT values show that Fe³⁺ doped glasses are suitable for warm-emitting orange-red photonic applications.