Evidence for Quasi Tetrahedral S=As(S$_{1/2}$)$_{3}$ local structures in As$_{x}$S$_{1-x}$ glasses

Raman scattering and modulated DSC on bulk As$_{x}$S$_{1-x}$ glasses in the 0.08 $<$ x $<$ 0.41 range is studied$^{1}$. Vibrational spectroscopy results supported by first principles cluster calculations reveal that modes near 335, 380 and 490 cm$^{-1}$ represent respectively symmetric stretch, asymmetric stretch and As=S stretch of Quasi- Tetrahedral (QT) S=As(S$_{1/2})_{3}$ local structures. Mode at 365 cm$^{-1}$ is attributed to both symmetric and asymmetric stretch of pyramidal (PYR) As(S$_{1/2})_{3}$ units. In addition, modes of S$_{n}$ chains near 460 cm$^{-1}$ and those of S$_{8 }$ rings near 475, 430, 217,and 150 cm$^{-1}$ are observed at x $<$ 0.25. Scattering strengths of modes of QT units are found to display a broad global maximum in the 0.20 $<$ x $<$ 0.30 composition range, while modes of PYR units increase monotonically with x, and modes of S$_{n}$ and S$_{8}$ rings monotonically decrease with x in the range examined. These findings are in harmony with the existence of a reversibility window (RW) in the 0.20 $<$ x $<$ 0.30 range. $^{1}$ D.G. Georgiev Ph.D. Thesis, Univ. of Cincinnati (unpublished) 2003. $^{2}$ D.G.Georgiev and P.Boolchand Phil. Mag.\underline {83}, 2941(2003). * Supported by NSF grant DMR 04-56472