Atomic-size effects on medium-range order in glasses

Effects of atomic sizes on structural correlations in ${\mathit{AX}}_2$-type disordered systems such as ${\mathrm{SiO}}_2$, ${\mathrm{GeSe}}_2$, and ${\mathrm{Ag}}_2$Se are studied using a charged-hard-sphere model and the hypernetted-chain scheme. Structural change is elucidated as a function of the size ratio through the first sharp diffraction peak in the number-number structure factor (medium-range correlations), the A-X coordination [formation of A(${\mathit{X}}_{1/2}$ ${)}_4$ tetrahedra], and the principal peak in the charge-charge structure factor (charge ordering). We find a gradual change from ${\mathrm{SiO}}_2$-type to ${\mathrm{Ag}}_2$Se-type disordered structure in the range of 0.5<R<1.0, where R (=${\mathrm{\sigma }}_{\mathit{A}}$/${\mathrm{\sigma }}_{\mathit{X}}$) is a ratio of the steric radius of A species to that of X species. As R increases, the medium-range order, which is closely related to the formation of A(${\mathit{X}}_{1/2}$ ${)}_4$ tetrahedra, disappears. In the transition region frustration between the steric and Coulombic interactions depresses the charge ordering.