Atomic Radial Distribution Functions of As-Se Glasses

X-ray-diffraction data from a series of arsenic-selenium glasses with compositions in the range Se---50-at.% As have been used to obtain atomic radial distribution functions for these materials. The first-neighbor correlation peak shifts linearly from 2.34 \AA{} for Se to 2.41 \AA{} for 36-at.% As and remains constant at higher As concentrations. Peak areas increase linearly with arsenic concentration and the magnitude agrees well with a model which assumes twofold coordination of selenium and threefold coordination of arsenic. At 40-at.% As and at 50-at.% As the first-neighbor peak is very similar to that obtained from the crystals ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ and ${\mathrm{As}}_{4}$${\mathrm{Se}}_{4}$. However, beginning with the second-neighbor peak at about 3.70 \AA{}, there are significant differences between the crystalline and amorphous distributions. There is a smaller second peak in the glasses, the third crystalline peak is completely absent, and there are shifts in the positions of higher-order shells. Consequently, structural models based on microcrystalline regions of ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ for 40-at.% As or ${\mathrm{As}}_{4}$${\mathrm{Se}}_{4}$ for 50-at.% As did not give satisfactory agreement with the experimental distributions. In particular, the results do not support the existence of ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ layers in the 40-at.% As structure.