Effect of halides addition on the ligand field of chromium in alkali borate glasses

• The 10 KM–64.7 B2O3–25 Na2O–0.3 Cr2O3 (M = Cl, Br and I) glassy system was prepared by a quenching method.
• Optical basicity, ligand field theory optical band gap energy, refractive index, ESR and IR were studied.
• The electronegativity plays an important role in deforming the crystal field around the transition metal ions.
• The crystal-field sites of KCl or KBr sample are very strong compared to the very weak one in the KI sample.

Borate oxide glass system of composition 10 KM–64.7 B2O3–25 Na2O–0.3 Cr2O3 (M = Cl, Br and I) was prepared by conventional melt quenching technique. The amorphous nature of the investigated glasses was checked by the X-ray diffraction (XRD) technique. The optical basicity of the system has been calculated, and was found to increase by going from KCl to KBr and to KI. Optical absorption spectra were recorded in the UV–visible range. Through a careful analysis of the data, the ligand field parameters (crystal field strength Dq, Racah parameters B and nephelauxetic functions h) and the optical parameters (optical band gap, Urbach tail band width, and refractive index) have been estimated. The obtained results reveal a strong correlation between that ligand field parameters and the type of halogen atom; the crystal-field strength of KCl or KBr samples are very pronounced but it is rather weak in the KI sample. Electron spin resonance (ESR) has been used to probe the valency of the Chromium ions. The resulting ESR parameters revealed that chromium ions are predominantly in the trivalent state with traces of hexavalent state. Using Infrared spectroscopy (IR) information on the boron structural units has been obtained. The N4 ratio increases by replacing the KCl by KBr or KI, and it was found that the tetrahedral coordination of Cr+ ions becomes preferential in the host glasses with increasing the optical basicity.