Abstract
The neutron diffraction (ND) measurements and the detailed structural modeling by reverse Monte Calro (RMC) simulation have been performed for l-BiBr3 and l-45 and 55 mol % Bi mixtures. The partial structure factor Sij(Q), partial radial distribution function gij(r), and snapshot produced from RMC configurations have been examined. The structure of l-BiBr3 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-BiBr3 and by chains in l-mixture. The packing density is 0.59 for l-BiBr3 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 ∼1.2 Å−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.
