Structural model of amorphous arsenic sulfide

Modeling studies of amorphous (a-) semiconductors have recently enjoyed renewed interest because of increasing access to computing power and experimental developments that have revealed more information about local atomic structure. Amorphous arsenic sulfide (a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$) is a paradigm of an amorphous material that exhibits structural bistability, and therefore is a prime candidate for investigations of models. Previous studies of this material have suggested that a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$ consists of either helical or planar structures or bridged parallel chains. Experimental investigations alone have not led to a resolution of which of these proposed structures most accurately represents the actual material. We have built a model of a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$ that consists of 1790 atoms and agrees relatively well with all experimental studies of the material that can be directly compared with a computer model. These include the density, the (infrared) vibrational properties, the radial distribution function, pair distribution functions, neutron (and x-ray) diffraction spectra, and elastic moduli. By studying the dihedral-angle relationships in the model, it can be determined that a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$ consists of randomly oriented segments of helical chains. The model does not contain planar structures or bridged parallel chains.