Defect center–tailored calcium aluminate glasses for photonic applications: Unveiling structure–diffusivity correlation

Abstract

Calcium aluminate (CA)-based glasses have drawn significant attention owing to their pronounced optical, mechanical, and chemical properties for potential applications in IR photonics. Nonetheless, limitations related to their poor glass-forming ability was addressed by incorporating ZnO that has formed negatively charged [ZnO₄]²⁻ and [AlO₄]⁻ units, thereby generating the oxygen-excess defects. These defects impart a detrimental effect on the optical properties. Thus, the present study explores the impact of Li₂O, Na₂O, K₂O, and GeO₂ inclusion in the barium–zinc–calcium–aluminate (BZCA) glass to eliminate the oxygen-excess defects. The molecular structure of the glass matrix was examined by Raman, NMR, and XPS along with the bulk diffusivity of the glasses through MD simulation and are correlated to elucidate the origin of the defect centers. Due to higher diffusivity, Li⁺ and Na⁺ cations decrease Al–O hole centers (Al–OHC) and $O_3^{-}$ defects but promote F⁺ (Farbe) centers while K⁺ ions having lower diffusivity and enhance the $O_3^{-}$ defects. Interestingly, all the defect centers are substantially reduced in GeO₂-containing glasses. Further, multivalent Cr ions are doped to optically probe the defect centers through UV–Vis–NIR absorption and photoluminescence spectra. Cr³⁺ and Cr⁴⁺ are found dominant in Li⁺-and Na⁺-containing glasses due to the presence of less $O_3^{-}$ defect, whereas excess $O_3^{-}$ converts all Cr³⁺ into Cr⁶⁺ for K⁺-containing glasses. Additionally, broadband NIR emission from Cr⁴⁺ can functionalize selective glasses for laser and telecommunication applications.