Positron annihilation Lifetime and Fourier transform infrared spectroscopic studies on Bi2O3–B2O3 glasses

Bismuth borate glasses with compositions x (Bi2O3)×(100−x) (B2O3) (where x is in mol. % ranging from 25 to 50 in steps of 5) have been prepared by using melt quenching technique. The glass transition temperature (Tg) of the prepared samples has been measured. It was found that the glass transition temperature (Tg) decreases with increasing Bi2O3 content which indicates that the degree of disorder increases. Furthermore, the density, molar volume and oxygen packing density have been studied. It has been found that the density increases as well as the molar volume. However, the oxygen packing density decreases by increasing the Bi2O3 content which gives an indication that the structure becomes loosely packed. Positron Annihilation Lifetime (PAL), Fourier Transform Infra-Red (FT-IR) and X-ray Diffraction (XRD) measurements have been performed to study the amorphous and defect structure as well as the vibrating band structure. PAL results have revealed weak formation of ortho-positronium (o-Ps) at amorphous sites. The correlation between the optical properties deduced from FT-IR spectra, and the positron annihilation lifetime parameters has been investigated. The decrease of the annihilation lifetime τ2 of trapped positrons at defects with Bi2O3, suggests a free annihilation of positrons at non-bridging oxygen (NBO) bonds, which is also evidenced by the positive correlation between I1 and the number of NBO atoms. FT-IR spectra revealed the presence of some bonds, that are assigned to vibrations of Bi–O bonds from [BiO3] pyramidal and [BiO6] octahedral units and B–O bonds from [BO3] and [BO4] units. On the other hand, increasing Bi2O3 have led to increase in the optical basicity or polarizability up to 35% concentration because the ionic radii of Bi2O3 is larger than those of B2O3. Consequently, a decrease in the optical band gap energy (Eg) with Bi2O3 is observed. •Bismuth borate glasses have been prepared by using melt quenching technique.•The oxygen packing density decreases by increasing the Bi2O3 content.•The optical band gap energy Eg decreases linearly by increasing the Bi2O3 content.•At concentrations of Bi2O3>35%, the optical basicity decreases.•The reduction of τ2 & I2 due to the increase of electron density at dangling bonds.