Mixed alkali effect in optical properties of lithium-potassium bismuth borate glass system

摘要

The mixed alkali effect (MAE) has been investigated in the glass system (30-x)Li_2O centre dot xK_2O centre dot 20Bi_2O_3 centre dot 50B_2O_3(0 <= x <= 30, mol percent) through optical properties. The optical absorption and reflection spectra were recorded at room temperature in the wavelength range 400-800 and 450-800 nm, respectively. From the absorption edge studies, the values of optical band gap (E_(opt)) and Urbach energy (DELTA E) have been evaluated. The values of E_(opt) lie between 2.21 and 2.55 eV for indirect allowed transitions, and for indirect forbidden transitions, the values vary from 1.86 to 2.31 eV. The values of E_(opt) show nonlinear behavior upon substitution of one alkali ion by another with a minima at x= 15 mol percent because the glass structure in the mixed alkali region may be having more nonbridging oxygens than in the dilute foreign alkali regions. The theoretical analysis of optical absorption indicates that the present glass system behaves as an indirect gap semiconductor. The refractive index (n) and optical dielectric constant (epsilon) have been evaluated from the reflection spectra; n and epsilon also show nonlinear variation in their values with maxima at x= 15 mol percent, which supports the existence of MAE in the optical properties of the present glass system.