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 SiO2-type to Ag2Se-type disordered structure in the range of 0.5 < R < 1.0, where R (=sigma(A)/sigma(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(X1/2)4 tetrahedra, disappears. In the transition region frustration between the steric and Coulombic interactions depresses the charge ordering.