Voids and intermediate-range order in network-forming liquids:: Rb20Se80 and BiBr3

The RMC and Voronoi-Delaunay analyses were applied to study the void structure in the network-forming liquids: Rb20Se80 and BiBr3. The distribution function of the relative positions of atoms and void centers in space, the coordination number of atoms and voids around a void and the size distribution of voids were examined using the previous experimental data for S(Q) to characterize the connectivity of the bonding network supporting voids and the intermediate-range correlation as manifested in the pre-peak of S(Q). The voids in 1-Rb20Se80 are formed by the 8-membered ring-like Se chains. A Rb cluster composed of 3-4 Rb atoms replaces one of five void sites and binds similar to 4 Se rings involving voids around it through the interactions between Rb and Se and between Rb and Rb. In 1-BiBr3 the hexagonal cages involving voids are formed by the corner-sharing of the trigonal pyramidal BiBr3 blocks. The strong preference of Bi ion for three-fold coordination helps to bind between the neighboring cages and to construct a honeycomb network in highly correlated fashion. The pre-peak of the observed S(Q) indicative of the intermediate-range order arises from the correlation between the neighboring Se rings in 1-Rb20Se80 and between the neighboring hexagonal cages formed by BiBr3 blocks in 1-BiBr3. In conclusion the pre-peak of S(Q) in 1-Rb20Se80 and 1-BiBr3 is related to the pre-peak of the void-based concentration-concentration structure factor S'(CC)(Q) due to an intermediate chemical order in the structure.