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

Abstract

The mixed alkali effect (MAE) has been investigated in the glass system (30− x)Li 2O· xK 2O·20Bi 2O 3·50B 2O 3 (0≤ x≤30, mol%) 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 (Δ 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% 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 ( ε) have been evaluated from the reflection spectra. n and ε also show nonlinear variation in their values with maxima at x=15 mol%, which supports the existence of MAE in the optical properties of the present glass system.