The method of isotopic substitution in neutron diffraction is used to measure the metal-metal partial structure factor $S_{\mathrm{MM}}\mathrm{}\left(k\right)\mathrm{}$ for the glassy fast ion conductor $g-\mathrm{A}{g}_2\mathrm{A}{s}_3\mathrm{S}{e}_4$ and for the glassy semiconductor $g-\mathrm{C}{u}_2\mathrm{A}{s}_3\mathrm{S}{e}_4$. The remaining partial structure factors are hence separated into two functions which comprise either the $M-\mu (\mu =\mathrm{As}\mathrm{or}\mathrm{Se})$ or $\mu -\mu$ species. It is found that the short range order of the network former $g-\mathrm{A}\mathrm{s}\mathrm{S}\mathrm{e}$ is not destroying on alloying with $M_2\mathrm{Se}$ and that the most significant structural differences occur with respect to the $M$ atom ordering.

• Received 11 January 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.264