Multiple optical features in binary-transition-metal borate glasses

Moukhtar A Hassan,F M Ebrahim,M M El-Hady,Zakaria M Abd El-Fattah,F Ahmad,M A Ashoush

doi:10.1007/s11082-021-03095-4

Abstract

Glasses doped with transition metal (TM) ions exhibit rich optical transitions spanning the entire ultraviolet, visible, and infrared spectral regimes. Here we utilize the melt-quenching technique to synthesis binary-TM-doped alkali-borate glasses of composition xCuO-(75-x)B2O3-24.7Li2O-0.3Cr2O3, with variable copper ion concentrations (x = 0, 0.2, 1.0 and 2.0 mol %) and a fixed amount of chromium ions (0.3 mol %). We identify several optical transitions from Cr3+, Cr6+, and Cu2+ ions, the latter manifests exclusively at longer wavelengths and gains noticeably higher intensity with Cu additives. Despite the Cr concentration being fixed, the Cr6+ peaks are quenched and Cr3+ signals are barely attenuated. This behavior rules out possible phase separation and suggests a non-trivial interplay between the two types of TM ions and their ligand, as supported by probing their oxidation states from electron spin resonance. The crystal field and Racah parameters followed an opposite behavior, while the optical band gap was reduced upon doping. These changes are correlated with the structural modifications introduced by Cu additives, where we anticipate homogenous and preferential proximity of Cu-Cr ions within the network.

Highlights

Transition metal (TM) ions offer unique structural probes for the local environments within amorphous glasses, due to their distinguished wide-radial-distribution of the outer d-shell electronic configurations, which renders transition metal (TM) ions highly sensitive to minimal changes in compositions and/or structures of the surrounding ligands [1,2,3,4,5]
The mathematical summation of the two spectra in (a) yields the yellow spectrum in Fig. 1(b), which obviously contains contributions from Cr6+, Cr3+, and Cu2+ ions. This is compared to a prepared sample (green spectrum in (b)) consisting of the respective Cu and Cr amounts, i.e., 1.0 mol % Cu and 0.3 mol% Cr. For such hybrid TM sample, Cr3+ transitions are only slightly attenuated while Cr6+ are entirely suppressed, despite the Cr amount is fixed for both samples in (a)
In order to follow these spectral changes in such homogenous and binary TM-doped systems, we present in Fig. 2 the optical absorption spectra for a series of Cu-doped samples (x = 0, 0.2, 1.0, 2.0 mol %) with fixed Cr concentration (0.3 mol %)