Structural studies of glassy CuAs${\mathrm{Se}}_2$ and Cu-${\mathrm{As}}_2$${\mathrm{Se}}_3$ alloys

X-ray-diffraction radial-distribution, electron-spectroscopy-for-chemical-analysis (ESCA), crystallization, and differential-thermal-analysis (DTA) studies of glassy ${\mathrm{Cu}}_x{\left({\mathrm{As}}_{0.4}{\mathrm{Se}}_{0.6}\right)}_{100-x}$ alloys, with 0 ≤ × ≤ 30, as well as CuAs${\mathrm{Se}}_2$ have been performed. The alloy radial-distribution-function (rdf) results indicate that the average nearest-neighbor coordination number increases from 2.4 for $x=0\mathrm{} \mathrm{to} 3.83$ for $x=30\mathrm{}$. The CuAs${\mathrm{Se}}_2$ rdf is similar to those of amorphous Ge and Si and indicates an average nearest-neighbor coordination number of a little less than 4, indicating that the atomic arrangement in the compound is similar to that in the amorphous elements. The rdf results, when coupled with the ESCA, DTA, and crystallization studies, imply that large regions of the Cu-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ alloys have an atomic arrangement and composition close to that of glassy CuAs${\mathrm{Se}}_2$. These results are used to rationalize the marked decrease of the band gap, increase of the conductivity, and much of the observed $T_g$ variation with increasing Cu concentration in the alloys. Also included in this paper is a derivation of a new random covalent model which corrects inconsistencies in that model which has been used frequently to analyze the structures of amorphous germanium-chalcogen alloys.