Stoichiometric deviations in bond distances in the mixed As2S3-As2Se3 system: Raman spectroscopy and EXAFS studies

Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy have been applied to find evidence for isostructural substitution of chalcogen atoms in pyramidal structural units of network glasses from stoichiometric/pseudo-binary As₂S₃-As₂Se₃ system. The obtained values of the nearest neighbours atomic distance (r_As-Se = 2.37 ± 0.01 Å; r_As-S = 2.33 ± 0.01 Å for As₄₀Se₃₀S₃₀) suggest the existence of mixed AsS₂Se and AsSe₂S pyramids in which each of As atoms is bonded with S and/or Se atoms in relation to S/Se ratio in composition, while Se atoms are bonded only with As ones with the same bond distance in the form of = As-Se-As = bridges that connect the main pyramidal structural units. No evidence for existence of heteroatomic or heteronuclear Sesbnd S bonding is found for the examined glasses.