Structural and optical properties of CuO-doped calcium borate glasses

CuO-modified calcium borate glasses with the composition (70–x)B₂O₃–xCuO–30CaO (x = 0–6 mol%) were prepared via the melt–quenching technique to examine the structural and optical effects of CuO incorporation. X-ray diffraction (XRD) confirmed the fully amorphous nature of all samples, while fourier transform infrared spectroscopy (FTIR) analysis revealed pronounced network depolymerization through BO₃→BO₄ conversion and the formation of non-bridging oxygens. Density decreased slightly with composition, whereas molar volume and free volume increased, indicating the development of a more open and flexible borate network. UV–V is spectra showed a characteristic Cu
2
⁺ d–d absorption band (700–900 nm) whose intensity increased and peak position shifted from 807 to 782 nm, reflecting enhanced ligand-field strength and more distorted Cu-O coordination environments.


The refractive index calculated using the Lorentz–Lorenz relation, increased from 1.700 to 1.740 despite decreasing density, highlighting the dominant influence of Cu
2
⁺ electronic polarizability and NBO-rich local environments. The strong agreement among structural and optical measurements demonstrates that CuO acts as an efficient network modifier, enabling tunable optical density and stability. These findings establish a clear structure–property framework for designing CuO-enriched borate glasses for advanced photonic and high-index optical applications.