Intrinsic nanoscale phase separation of bulk As₂S₃ glass

Raman scattering on bulk As _x S_{1− x} glasses shows that vibrational modes of As₄S₄ monomer first appear near x = 0.38, and their concentration increases precipitously with increasing x, suggesting that the stoichiometric glass (x = 0.40) is intrinsically phase separated into small As-rich (As₄S₄) and large S-rich clusters. Support for the Raman-active vibrational modes of the orpiment-like and realgar-like nanophases is provided by ab-initio density functional theory calculations on appropriate clusters. Nanoscale phase separation provides a basis for understanding the global maximum in the glass transition temperature T _g near x = 0.40, and the departure from Arrhenius temperature activation of As₂S₃ melt viscosities.