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

Abstract

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.