Optical properties of lithium barium haloborate glasses

Lithium haloborate glasses have been prepared according to the formula (70-y)B(2)O(3)-30BaF2-yLiX where y = 0, 5,10,15 and 20 mol% and X = F, Cl and Br. The UV absorption spectra have been recorded at room temperature. The optical energy band gap E(opt) and Urbach energy E(tail) was determined. Increasing concentration of LiX in the glass shifts the UV cutoff to lower energy and decreases both E(opt) and E(tail). The shift in UV cutoff and the decrease of E(opt) and E(tail) are maximum in the range 0-5 mol% LiX. The decrease of E(opt) increases going from F, Cl to Br, whereas the decrease of E(tail) increases going from Br, Cl to F. At higher LiX concentrations Urbach energy E(tail) increases again. The behavior of E,p, has been correlated with the B-O bond length of the studied glasses. The decrease of E(opt) and E(tail) is due to the increase of nonbridging oxygen (NBO) concentration with increase of LiX content. The increase of E(tail) at higher concentrations is attributed to the halide ion accumulation in the interstitial positions and to the formation of orthoborate groups. Refractive index, which is measured and calculated, molar refraction and polarizability are all correlated with the composition of the glasses. (c) 2008 Elsevier Ltd. All rights reserved.