Reevaluation of Cr6+ optical transitions through Gd2O3 doping of chromium-borate glasses

Abstract

Abstract Chromium-doped borate glasses are appealing for potential optical applications as they feature distinct optical transitions in the visible and ultraviolet spectral regimes. However, such intense UV transitions are often screened in borate glasses due to their proximity to the bulk absorption edge. Here, we utilize variable concentrations of the wide band gap Gd2O3 dopant to detach these near-edge optical transitions. The amorphous nature of all Gd2O3-doped borate glass samples, containing a fixed Cr amount, was confirmed by X-ray diffraction. Gd2O3 additives were found to enforce the compactness of the glassy network as inferred from the increased experimental density and calculated average boron-boron separation. Analysis of the infrared spectra revealed the essential vibration modes of structural units, the non-bridge oxygen, and the fraction of tetrahedrally-coordinated boron atoms ratios. These ratios together with a noticeable BO3 to BO4 conversion suggest an open glass structure with enlarged band gap energy. The latter is confirmed by recording the optical absorption spectra for all samples, which exhibited clear blue shifts of the absorption edge and an estimated wide band gap of size ~3.38 eV. In addition to Cr3+ transitions manifesting in the visible region, two well-resolved ultra-violet transitions characteristic for Cr6+ were unambiguously identified down to ~339 nm. These transitions and other relevant quantities, such as crystal field strength and Racah parameters, were accurately determined from a deconvolution process applied to all optical spectra. The present Gd2O3-doped glasses offer wide band gap materials with high energy optical transitions suitable for UV optics, such as ultraviolet digital photography and insect light traps.