The structure of molten and glassy 2:1 binary systems: An approach using the Bhatia-Thornton formalism

A systematic analysis of those liquid binary 2:1 systems (denoted MX ₂), for which experimental partial structure factors are available from the isotopic substitution method in neutron diffraction, is made using the Bhatia-Thornton (BT) formalism. Particular attention is paid to the origin of the first sharp diffraction peak (fsdp), which occurs in the measured diffraction patterns for some of the MX₂ systems, since it appears, from recent studies, that this feature is a signature of directional bonding. It is found that fsdps can occur in all three BT partial structure factors Sαβ(k). A fsdp feature in the concentration-concentration partial structure factor Scc(k) is not, however, pronounced except in the case of MgCl₂ and the glass forming network melts ZnCl₂ and GeSe₂. To the extent that these systems can be regarded as ionic melts a fsdp in Scc(k) implies a non-uniformity in the charge distribution on the scale of the intermediate-range order (iro). The structure of molten GeSe₂ is compared with the structures of molten ZnCl₂, glassy GeS₂ and glassy SiO₂. Although the GeSe₂ and ZnCl₂ melts have different short-range order, there are similarities in the observed iro which can be attributed to the arrangement of the electropositive species M. The essential features of the measured total structure factor for glassy GeS₂ can be reproduced by using the molten GeSe₂ Sαβ(k). This result lends support to the notion that the Sαβ(k) for liquid GeSe₂ (and ZnCl ₂) are characteristic of both the liquid and glassy states of other network glass forming systems. The structures of molten GeSe₂ (or ZnCl₂) and glassy SiO₂ are, however, found to be different. The observed discrepancies are largest in the region of the fsdp which signifies pronounced differences in the nature of the iro for these systems.