Mixed alkali effect in borate glasses-optical absorption studies in ${Ho}^{3+}$ doped $x(Na_2O) . (30 - x )(K_2O) . 70(B_2O_3)$ glasses

Abstract

Mixed alkali effect (MAE) in $x(Na_2O) . (30 - x )K_2O . 70B_2O_3$ (x =5, 10, 15, 20 and 25) glasses doped with $0.5{Ho}_2O_3$ has been investigated by measuring the optical properties of ${Ho}^{3+}$. From the optical absorption spectra, optical band gaps $(E_{opt})$ for both direct and indirect transitions have been calculated using Davis and Mott theory and are found to exhibit a minimum when the two alkalies are in equal concentration (due to mixed alkali effect). Spectroscopic parameters like Racah $(E^1, E^2, E^3)$, spin-orbit $({\xi}_{4f})$, configuration interaction (\alpha,\beta) and Judd-Ofelt intensity parameters $({\Omega}_2, {\Omega}_4 and {\Omega}_6)$ have been calculated as a function of x. Also radiative and non-radiative transition probabilities $(A_T and W_{MPR})$, radiative lifetimes $({\tau}_R)$, branching ratios (\beta) and integrated absorption cross sections ( \Sigma ) have been obtained. The spectral profile of the hypersensitive transition has been correlated to the site symmetry of the rare earth ion. The trends observed in the intensity parameters, radiative lifetimes and stimulated emission cross sections as a function of x in these borate glasses have been discussed, keeping in view the mixed alkali effect.