Local structure and vibrational spectra of vAs2O3

We discuss the atomic displacements of the optically active vibrational modes of vitreous (v-)As₂O₃ through comparisons of the infrared (ir) and Raman response with the corresponding spectra of the two crystalline polymorphs, claudetite, a layer crystal similar to orpiment, and arsenolite, a molecular crystal based on the As₄O₆ molecule. We conclude from these comparisons that the structure of the glass is composed of $\text{AsO}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ pyramidal units that are corner connected to form a continuous random network. The character of the strong ir and polarized Raman modes in the vitreous form, suggests that the interconnection of these pyramidal units cannot be described by a random distribution of dihedral angles, but rather has peaks at angles characteristic of the different ordering in two-dimensional macromolecular layer basis of claudetite, and the As₄O₆ molecule. The comparisons are extended to v-As₂S₃ and vAs₂Se₃ where we conclude the dihedral angle distributions characteristic of an As₄O₆-like local geometry are less prevalent.