Free volume studies on mechanochemically milled β-As4S4 arsenical employing positron annihilation lifetime spectroscopy

Atomic-deficient free-volume transformations in directly synthesized β-As4S4 arsenical subjected to nanostructurization due to high-energy speed-increased (0–200–500–600 rpm) milling are comprehensively studied employing lifetime spectroscopy of annihilating positrons. These data testify that nanocrystalline and amorphous forms coexist among milling products, testifying in favor of milling-driven solid-state amorphization due to “shell” kinetic model. The depressing and time-enhancing trend in the detected positron lifetime spectra serves as a direct indication for continuous generation of amorphous phase, accompanied by free-volume evolution of coupled positron- and positronium (Ps) traps obeying x3-x2-CDA (coupling decomposition algorithm). At the stage of quick accumulation of size-reduced non-interacting nanoparticles characteristic of low speed (200 rpm), triple junctions equivalent to bi-/tri-atomic vacancies appear instead of Ps-traps. At higher rotational speed (500 rpm) exemplified by pelletized arsenicals composed of aggregated nanoparticles due to reversibly enhanced adhesion, these traps grow to characteristic sizes of quadruple vacancies and reduced Ps-to-positron trapping conversion occurs in more amorphous environment. In β-As4S4 subjected to milling at the highest speed (600 rpm), where irreversibly agglomerated nanograins dominate, the positron-to-Ps trapping conversion occurs due to elimination of triple junctions between crystallites and appearance of Ps-traps in more crystalline environment.