Sharma, B.K.; (2010) Intermediate-range order in molten network-forming systems. Doctoral thesis , UCL (University College London).

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Abstract

Molecular Dynamics simulations using the polarisable ion model (PIM), which accounts for many-body ion polarisation in addition to short-range repulsion and simple Coulombic interaction between ions, are undertaken in a study of the structure of molten network-forming liquids. The primary focus is the investigation of the structural origin of intermediate range order (IRO), the ordering of atoms beyond the nearest-neighbour in liquids and glasses often highlighted by the presence of a first sharp diffraction peak (FSDP) in total and partial structure factors. Two primary modelling approaches are applied. In the first, specific systems of MX2 stoichiometry are targeted (ZnCl2, MgCl2 and GeSe2) allowing for direct comparison with the results from scattering experiments. An ionic description for GeSe2 represents a stern test as this system is often described as having considerable metallic character. In the second approach, key system parameters are systematically varied in order to control the network topology and examine the evolution of IRO. A key structural property, the presence of a FSDP in the concentrationconcentration structure factor, SCC(kFSDP), is observed and structure factors, “coloured” according to network connectivity, show its presence to be dependent on the percolation of edge-sharing units disturbing the corner-sharing tetrahedral network. The effect on the network topology and IRO of varying both temperature and pressure, properties often difficult to obtain experimentally, are observed. The inherent structure of the systems studied shows the presence of newly resolved features, including a distortion of constituent polyhedra. Homopolar bonds are induced by combining Morse and Born- Mayer potential and their effect on the underlying IRO examined.

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