Comparative study of free-volume nanostructurization in glassy and crystalline As2S3 re-examined with annihilating positrons

Evolution of free-volume atomic scale structure under crystal-to-glass transition was re-examined in arsenic trisulfide As₂S₃ in glassy and crystalline state, employing mutually correlated positron annihilation lifetime studies treated in a framework of two-state simple positron trapping model. In crystalline state represented by mineral orpiment As₂S₃ supplied from Wanshan mine (China), the trapping rate is restricted to ~1.14 ns^-1, associated with 0.240 ns defect-free bulk and 0.402 ns defect-specific lifetimes due to free volumes stabilized preferentially within inter-layer spaces. The positron trapping rate is notably enhanced to ~1.51 ns^-1 in glassy state presented by melt-quenched As₂S₃, the positron traps being smaller with decreased bulk (0.278 ns) and defect (0.380 ns) lifetimes due to nanostructurized free volumes stabilized within corrugated quasi-layers. In general, vitrification in As₂S₃ produces depressing and time-enhancing trend in the recorded PAL spectra peaks. The ratio of bulk positron lifetimes for glassy and crystalline states serving as criterion of vitrification completeness reaches 1.16, meaning that some additional volume is available for annihilation in a glass. The ratio of mean positron lifetimes approaching 1.12 reflects a degree of realized crystal-to-glass transformation. Vitrification-related changes in As₂S₃ can be imagined as fragmentation of existing free-volume voids due to crumpling of intrinsic inter-layer spaces.