Synchrotron infrared spectroscopy of the pressure-induced insulator-metal transitions in glassy As2S3 and As2Se3

Optical reflectivity and absorption measurements of glassy As2S3 and As2Se3 from 0.18 to 1.4 eV to 57 GPa reveal closure of the optical gap in both materials. The transition to the metalliclike response is continuous with pressure. The insulator-metal transition can be understood in terms of the delocalization of nonhybridized S and Se p states (lone pairs), as supported by the optical reflectivity data and theoretical calculations that show delocalization and evolution of three-dimensional electronic states in crystalline As2Se3.