Microvoids in chalcogenide glasses studied by positron annihilation

Microvoids have been detected in glassy As₂Se₃ by positron lifetime spectroscopy. The positron lifetime in perfect and defected crystalline As₂Se₃ was calculated and the results correlated with the size of the (open volume) defects. This correlation suggests that the observed mean lifetime τ = 345(2) ps for the glassy material corresponds to microvoids of on average about three missing atoms (i.e., 2.5-3 Å radius). The lower limit of the microvoid concentration per atom is estimated to lie between 0.1 and 100 ppm (corresponding to a volume fraction 10^-5-10^-2%), well below the detection threshold of small-angle X-ray scattering. The microvoids are stable on heating to 260°C, i.e., after annealing at temperatures 80°C above the glass transition temperature. The present results are in conflict with earlier suggestions that positrons are trapped only in the vicinity of point-like negatively charged defects in amorphous As₂Se₃. Frequent observation of long positron lifetimes leads to the conclusion that microvoids are a feature common to a wide range of chalcogenide glasses. The consequence of these results for the recent model of Elliott, in which the first sharp diffraction peak in the total structure factor of covalently bonded glasses is attributed to the chemical ordering of microscopic voids around cation-centred units, is discussed.