Abstract
Two alkaline-borate glass systems of composition x Li2O · 0.2 Cr2O3 · (99.8–x) B2O3 (x = 20, 30 and 40 mol%) and y K2O · (20-y) Li2O · 0.2 Cr2O3 · 79.8 B2O3 (y = 5, 10, 15 and 20 mol%) were prepared by traditional melt-quenching technique. The tunability of their optical spectra, offered by different alkaline contents, allowed disentangling the absorption edge from the characteristic Cr6+ optical transitions. Specifically, the optical band gap is barely affected (3.55 ± 0.15 eV), whilst Cr6+/Cr3+ optical transitions undergo subtle intensity variation with the alkaline content. With increasing the alkaline content, or type, the crystal field strength (10 Dq) is increased, Racah parameters are decreased, and the ligand environment facilitates the crossover from weak into strong ligand field regimes. Electron spin resonance spectroscopy confirmed the existence and quantified the percentage of both Cr6+ (through charge transfer into Cr5+ O−) and Cr3+ oxidation states. Finally, the number of non-bridging oxygen, obtained from Infrared spectroscopy, is increased noticeably for both type and percentage of alkaline, while BO4 structural units increase with Li content and are barely affected for K-doped system.
•Successful fabrication of optically-active borate glasses containing chromium ions.•Simultaneous detections of ultraviolet/visible Cr6+/Cr3+ optical transitions•Multifunctional glass host with potential applications in ultraviolet and visible spectral regimes.•Linking ESR signals to specific Cr6+ and Cr3+ absorption bands