Modeling and first-principles calculation of low-frequency quasi-localized vibrations of soft and rigid As–S nanoclusters

Experimental and theoretical studies were performed on Boson peak of binary AsxS100−x glasses and As–S polycrystalline composites (mixtures of glass with polycrystallites) of different compositions. Low-frequency Raman spectra of six different compositions ranging from As6S94 to As60S40, including the stoichiometric As40S60 composition, were measured in the region of 5–100 cm−1. The Fourier-transform Raman spectra of the As–S samples in the range of 50–600 cm−1 were also measured to reveal the structure of the materials at nanoscale. In addition, density functional theory calculations were performed on different As–S nanoclusters for determination of their low-frequency vibrational modes. The effect of the structural interconnection of the clusters on their vibrational mode frequencies was modeled by attaching different numbers of heavy dummy hydrogen atoms to the dangling bonds of branchy—As2+4/3S5 and 12-membered ring-like As6S6+6/2 nanoclusters. It was found that the vibrational mode frequencies have a U-shaped dependence on the level of interconnection, which correlates with experimental findings on compositional dependence of the Boson peak position in AsxS100−x glasses. The composition dependence of spectral behavior and very low-frequency features detected at low-energy side of the Boson peak in the Raman spectra of As–S samples were also analyzed and their structural origin is discussed.