The Art of Positronics in Contemporary Nanomaterials Science: A Case Study of Sub-Nanometer Scaled Glassy Arsenoselenides

Nanostructured Molecular-Network Arsenoselenides from the Border of a Glass-Forming Region: A Disproportionality Analysis Using Complementary Characterization Probes

Binary AsxSe100-x alloys from the border of a glass-forming region (65 < x < 70) subjected to nanomilling in dry and dry-wet modes are characterized by the XRPD, micro-Raman scattering (micro-RS) and revised positron annihilation lifetime (PAL) methods complemented by a disproportionality analysis using the quantum-chemical cluster modeling approach. These alloys are examined with respect to tetra-arsenic biselenide As4Se2 stoichiometry, realized in glassy g-As65Se35, glassy-crystalline g/c-As67Se33 and glassy-crystalline g/c-As70Se30. From the XRPD results, the number of rhombohedral As and cubic arsenolite As2O3 phases in As-Se alloys increases after nanomilling, especially in the wet mode realized in a PVP water solution. Nanomilling-driven amorphization and reamorphization transformations in these alloys are identified by an analysis of diffuse peak halos in their XRPD patterning, showing the interplay between the levels of a medium-range structure (disruption of the intermediate-range ordering at the cost of an extended-range one). From the micro-RS spectroscopy results, these alloys are stabilized by molecular thioarsenides As4Sen (n = 3, 4), regardless of their phase composition, remnants of thioarsenide molecules destructed under nanomilling being reincorporated into a glass network undergoing a polyamorphic transition. From the PAL spectroscopy results, volumetric changes in the wet-milled alloys with respect to the dry-milled ones are identified as resulting from a direct conversion of the bound positron-electron (Ps, positronium) states in the positron traps. Ps-hosting holes in the PVP medium appear instead of positron traps, with ~0.36-0.38 ns lifetimes ascribed to multivacancies in the As-Se matrix. The superposition of PAL spectrum peaks and tails for pelletized PVP, unmilled, dry-milled, and dry-wet-milled As-Se samples shows a spectacular smoothly decaying trend. The microstructure scenarios of the spontaneous (under quenching) and activated (under nanomilling) decomposition of principal network clusters in As4Se2-bearing arsenoselenides are recognized. Over-constrained As6·(2/3) ring-like network clusters acting as pre-cursors of the rhombohedral As phase are the main products of this decomposition. Two spontaneous processes for creating thioarsenides with crystalline counterparts explain the location of the glass-forming border in an As-Se system near the As4Se2 composition, while an activated decomposition process for creating layered As2Se3 structures is responsible for the nanomilling-driven molecular-to-network transition.

Nanomilling-driven volumetric changes in multiparticulate As4S4-bearing nanocomposites recognized with a help of annihilating positrons

Employing positron annihilation lifetime (PAL) spectroscopy, nanomilling-driven volumetric changes driven are identified in multiparticulate nanocomposites of As4S4–ZnS–Fe3O4 system, considered in transitions between their respective hierarchical derivatives from triparticulate (1⋅As4S4/4⋅ZnS/1⋅Fe3O4) to biparticulate (1⋅As4S4/1⋅Fe3O4, 1⋅As4S4/4⋅ZnS) and monoparticulate (As4S4) ones. Unconstrained three-component PAL spectra of nanocomposites are parameterized in terms of positron-Ps trapping conversion obeying x3-x2-CDA (coupling decomposition algorithm). Coexistence of nanocrystalline nc-β-As4S4 and amorphous a-AsS phase is shown to be crucial feature of these nanocomposites, the latter being generated continuously due to reamorphization of initial disordered phase and/or vitrification of nc-β-As4S4 phase. The inverse positron-to-Ps trapping conversion prevails in transition from biparticulate (1⋅As4S4/1⋅Fe3O4) and monoparticulate (As4S4) nanocomposites (both dominated by trapping in As4S4-bearing sub-system) to triparticulate (1⋅As4S4/4⋅ZnS/1⋅Fe3O4) one, disappeared positron traps being vacancy defects in a-As–S matrix, and Ps-decay sites formed instead being triple junctions between amorphized nc-β-As4S4 grains. The normal Ps-to-positron-trapping conversion prevails in transition from biparticulate (1⋅As4S4/4⋅ZnS) nanocomposite dominated by positron trapping in ZnS sub-system to triparticulate (1⋅As4S4/4⋅ZnS/1⋅Fe3O4) one, disappeared Ps-decay sites being triple junctions between amorphized nc-β-As4S4 grains, and positron traps formed instead being vacancy-type defects in the packing of the finest ZnS crystallites (~ 2–3 nm).