Compositional dependences of average positron lifetime in binary As–S/Se glasses

Free-volume correlations in positron-sensitive annihilation modes in chalcogenide vitreous semiconductors: on the path from illusions towards realistic physical description

“…(2) was not the best to fit the known experimental data in the probed polymers for small positron lifetimes  2 and void radii R (where Ps does not appear because of geometrical constraints [1,8,9]), it was proposed to replace it by the simplified linear equation:…”

“…So, positrons become indeed trapped similarly to Ps atoms (which is a necessary condition to validate the sequent  3 - 2 -R estimation procedure [16]) only in essentially narrow range of ChVS compositions of the same structural bonding. In case of As-S(Se) glasses built of AsS/Se 3 pyramids interlinked with Se-chains of different lengths [3,11], this is valid near stoichiometric g-As 2 S(Se) 3 stretched mainly in S/Se-rich side [7,8]. Fortunately, to parameterize Eq.…”

“…This C parameter (included also in Table 1) occurs to be 5.15 and 5.73 Å for g-As 2 Se 3 and g-As 2 S 3 , respectively. These estimations seem to be quite realistic, if we accept that in case of ChG the most effective positron traps represent themselves as geometrically arrested free volumes surrounded by BFSA emerging from neighboring chalcogen atoms at the bottom of AsS/Se 3 pyramids [3,4,8]. Being involved into heteropolar covalent bonding between chalcogen (S, Se) and pnictogen (As) atoms possessing different electronegativities [3,28], these BFSA carry an effective negative charge forming relatively thick walls of free-volume voids.…”

“…Recently, this method has been employed even for such complicated disordered solids as chalcogenide vitreous semiconductors (ChVS) [3][4][5][6][7][8][9][10]. These glassy-like alloys of chemical elements from IV-V groups of the Periodic table (typically As, Ge, Sb, Bi, etc.)…”

“…Thus, in [23], the modified sequent  3 - 2 -R correlation was utilized as a basis of the generalized void-size-determination procedure for a wide range of materials, involving molecular substrates, such as polymers, as well as inorganic disordered solids like ChVS. Ignoring the principal difference in the chemical nature of these materials led to evidently misleading conclusion on expected positron trapping void volumes, which occur to be in apparent contradiction with proved experimental data for a great variety of ChVS [3][4][5][6][7][8][9][10]. The time is come to analyze critically these discussible diversities, asking a simple question: "How far can we really go with free-volume parameterization in amorphous semiconductors such as ChVS aided by PAL technique?".…”