Mechanochemically driven amorphization of nanostructurized arsenicals, the case of β-As₄S₄

The amorphization is studied in mechanically activated β-As₄S₄ using high-energy ball milling in a dry mode with 100–600 min⁻¹ rotational speeds, employing complementary methods of X-ray powder diffraction (XRPD) related to the first sharp diffraction peak, positron annihilation lifetime (PAL) spectroscopy, and ab initio quantum-chemical simulation within cation-interlinking network cluster approach (CINCA). The amorphous substance appeared under milling in addition to nanostructurized β-As₄S₄ shows character XRPD halos parameterized as extrapolation of the FSDPs, proper to near-stoichiometric amorphous As–S alloys. The structural network of amorphized arsenicals is assumed as built of randomly packed multifold cycle-type entities proper to As₄S₄ network. The depressing and time-enhancing tendency in the PAL spectrum peak is direct indicative of milling-driven amorphization, associated with free-volume evolution of interrelated positron- and Ps-trapping sites. At lower speeds (200–500 min⁻¹), these changes include Ps-to-positron trapping conversion, but they attain an opposite direction at higher speed (600 min⁻¹) due to consolidation of β-As₄S₄ crystallites. In respect of CINCA modeling, the effect of high-energy milling is identified as destruction–polymerization action on monomer cage-type As₄S₄ molecules and existing amorphous phase, transforming them to amorphous network of triple-broken As₄S₄ derivatives. These findings testify in a favor of "shell" kinetic model of solid-state amorphization, the amorphous phase continuously generated under speed-increased milling being identified as compositionally authentic to arsenic monosulfide, different in medium range ordering from stoichiometric As₂S₃.