Mixed alkali effect in the physical and optical properties of xK2O-(25-x)Na2O-12.5MgO-12.5BaO-50B2O3 glasses

Borate glasses with the composition xK20-(25-x)Na2O-12.5MgO-12.5BaO-50B(2)O(3) (x = 0, 5, 10, 15, 20 and 25 mol%) were prepared by the melt-quenching technique and were characterized using X-ray diffraction at room temperature. From the optical absorption spectra, the direct forbidden, direct allowed, indirect allowed, and indirect forbidden energy gaps; Urbach energy; refractive index (n); dielectric constant (epsilon); reflection loss (R); molar refraction (R-m); and molar polarizability (alpha(m)) values of all of the glass samples were evaluated. R-m, alpha(m) and Lambda(th) increase with the increase of the K2O content. The values of R, epsilon and n increase up to x = 10 mol% and then decrease above x =10 mol%, which may be due to the mixed alkali effect. The density and molar volume values show the opposite behaviour and vary nonlinearly with the K2O content, which manifests the mixed alkali effect in the present glass system. The infrared (IR) spectra of the borate glasses reveal the existence of 3- and 4-coordinated boron atoms. The specific vibrations of the Na-O, K-O, Mg-O, and Ba-O bonds were observed in an IR study. (C) 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University.