Influence of nickel ion concentration on the free volume defects entrenched in an alkali sulphophosphate glass system by means of positron annihilation characterization technique

In this study we have evaluated the concentration of defect centers like voids, their size (up to nanoscale) and the fraction of free volume space integrated in Li2SO4-MgO - P2O5:NiO glass system as a function of NiO content. Positron annihilation lifetime (PAL) technique with Na-22 positron source of radio activity 100 kBq is used for this study. The size, shape and the concentration of entrenched defects in the glass matrix are predicted to influence profoundly the physical properties of the glass. Cavity radius (R) and the entrenched free volume fraction (f(v)) are found to be the maximal in the sample doped with 0.8 mol% of NiO. This observation suggested that Ni2+ ions predominantly occupied octahedral positions (acted as modifiers along with SO42- ions) and induced large concentration of structural defects. The obtained information is found to be in concurrent with inferences drawn from dielectrics and spectroscopic studies of these glasses reported earlier.