Modelling the intermediate range order in liquid trivalent-metal halides and oxides

Intermediate range order may be present in the liquid structure of a polyva lent metal salt. It is signalled by a sharp peak in the diffraction pattern of the melt at low wave number. This first sharp diffraction peak (FSDP) h as been observed in both tetrahedrally and octahedrally co-ordinated networ k formers as well as in systems where molecular dimer units pre-exist melti ng or are believed to form across melting. I first briefly review studies o f simple microscopic models showing (i) how the FSDP grows together with a network-type structure in trichloride melts on decreasing the metal-ion rad ius at constant density; and (ii) how it is modified oh quenching the struc ture of the halogen component. I then report on progress in developing pola risable-ion models, in parallel with a priori density functional calculatio ns, for the structure of molecular monomers and dimers in both neutral and ionised states, with a view to obtaining the essential input needed for cla ssical computer simulation studies of halide and oxide melts, (C) 1999 Else vier Science B.V. All rights reserved.