Short- and Intermediate-Range Order in Liquid Bi-BiBr_3 Mixtures : Importance of Voids(Condensed matter: structure and mechanical and thermal properties)

The neutron diffraction (ND) measurements and the detailed structural modeling by reverse Monte Cairo (RMC) simulation have been performed for l-BiBr_3 and l-45 and 55 mol% Bi mixtures. The partial structure factor S_<ij>(Q), partial radial distribution function g_<ij>(r), and snapshot produced from RMC configurations have been examined. The structure of l-BiBr_3 is dominated by the network of trigonal pyramidal units. The l-mixtures consist of the -Br-Bi-Br- chain fragments terminated by Br. A Voronoi-Delaunay analysis has been applied to characterize the voids within network or chain. We find that the voids are surrounded by six pyramids in l-BiBr_3 and by chains in 1-mixture. The packing density is 0.59 for l-BiBr_3 and 0.45 for l-45 mol% Bi mixture which is substantially low compared with that of dense random packing. The void-void structure factor S'_<V-V>(Q) involving void centers have the pre-peak at 〜1.2A^<-1> which is in common with the pre-peak in experimentally observed S(Q). It is concluded that the spatial distribution of voids couples to the appearance of a pre-peak in S(Q) indicative of the intermediate range order. The disappearance of the miscibility gap in Bi-rich concentration region under pressure is briefly discussed in connection with void concentration.