Environmental influence of high Na2O additions on microstructure and ligand field parameters of Cr3+ inside borosilicate glass

Abstract

A glass system based on the Na2O/B2O3-doped CrO3 borosilicate has been prepared by the melt quenching technique. The structure, color, optical absorbance and ligand field parameters were investigated for a wide range of Na2O additives (20–60 mol%). All X-ray photoelectron spectroscopy (XPS) profiles were used to study the chemical shift states of the glass-constituting elements. Fourier transform infrared (FTIR) analyses explored the internal structure and subnetwork units. Furthermore, from the FTIR results, we concluded the transformation of trigonal borate units (BO3) to tetrahedral borate units (BO4) and the possibility of transformation from B3-O-Si linkages to B4-O-Si linkages. Despite the fixed CrO3 content, the doped glasses showed a color transition from green to yellow with additional Na2O content. The increased intensity of the band at 451–427 nm and the decreased intensity of the band at 619–627 nm are the main reasons for this color transformation. The optical absorption spectra confirmed the existence of Cr3+ and Cr6+ states. A decreasing behavior for the crystal field splitting (10Dq) and an increasing behavior for Racah parameter (B) were obtained with further Na2O additives. The decreasing behavior of 10Dq was attributed to reduced oxygen concentrations with more Na2O/B2O3 substitutions. The increasing behavior of B reflects the tendency of the bond between the Cr cations and their oxygen ligands towards an ionic nature. Moreover, the Dq/B values indicated that Cr3+ cations are in high-field positions for the glass sample containing 20 mol% Na2O, and Cr3+ cations are in intermediate field positions for the glass sample containing 30 mol% Na2O. However, for the glass samples doped with 40, 50 and 60 mol% Na2O glass samples, Cr3+-cations are in weak field positions. These results of (Dq/B) recommend the glass sample doped with 20 mol% Na2O for tunable laser applications.