Atomic-size effects on medium-range order in glasses

Effects of atomic sizes on structural correlations in AX₂-type disordered systems such as SiO₂, GeSe₂, and Ag₂Se 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(X1/2{)}₄$ tetrahedra], and the principal peak in the charge-charge structure factor (charge ordering). We find a gradual change from ${SiO}₂$-type to ${Ag}₂$Se-type disordered structure in the range of 0.5R1.0, where R (=${{{σ}}}A$/${{{σ}}}X$) 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(${X}1/2)₄ tetrahedra, disappears. In the transition region frustration between the steric and Coulombic interactions depresses the charge ordering.