Intrinsic nanoscale phase separation of bulk As2S3 glass

Raman scattering on bulk AsxS1-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.