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

Binary As(x)Se(100-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 As(4)Se(2) stoichiometry, realized in glassy g-As(65)Se(35), glassy-crystalline g/c-As(67)Se(33) and glassy-crystalline g/c-As(70)Se(30). From the XRPD results, the number of rhombohedral As and cubic arsenolite As(2)O(3) 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 As(4)Se(n) (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 As(4)Se(2)-bearing arsenoselenides are recognized. Over-constrained As(6·(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 As(4)Se(2) composition, while an activated decomposition process for creating layered As(2)Se(3) structures is responsible for the nanomilling-driven molecular-to-network transition.