Band gap analysis and correlation with glass structure in phosphate glasses melted with various allotropes of carbon

Highlights: • Phosphate glasses melted with graphite, MWCNTs, and nano-diamond. • Transmission, photoluminescence, and ³¹P NMR data analyzed and compared. • Optical property-structure relationship supported. A rigorous numerical optical band gap analysis is presented regarding the enhanced ultraviolet light transmission realized for barium-phosphate glasses melted with graphite, multi-wall carbon nanotubes, and nano-diamond powder. The study entails a comparative assessment wherein optimal ultraviolet transparencies were achieved. It is proposed that based on the principle of superposition, the optical absorption spectra of carbon-doped glasses may be considered as solid solutions of the host matrix and an optimal carbon-doped matrix with characteristic POC bonds. ³¹P nuclear magnetic resonance (NMR) was employed for structural analysis to evaluate the relative content of the PO₄ tetrahedra with two (Q²) and one (Q¹) bridging oxygens. Further, the Q²/Q¹ ratio from ³¹P NMR data was correlated with the Urbach energy. In the carbon-doped metaphosphate system, the widening of the band gap can be attributed to a decreased matrix polarizability, structurally promoted by decreased concentration of the non-bridging oxygens in agreement with the effects associated with decreased optical basicity of the matrix.