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The structural properties of liquid GeO2 are investigated by means of molecular dynamics simulation using a pairwise potential. The simulations were performed in the microcanonical ensemble on systems with up to 576 particles prepared at 21 different densities, corresponding to pressures from -2 to 30 GPa, and temperatures of 1500 K and 3000 K. The pair correlation function, coordination number, angular distribution, and both the neutron and x-ray static structure factors are obtained and compared with those of liquid silica. The analysis of these results for the system at zero pressure indicates that in the liquid state the short range order is dominated by the presence of slightly distorted Ge(O(1/2))(4) tetrahedra. These tetrahedra are linked to each other mainly through the corners, with a Ge-O-Ge angle of approximately 130 degrees, similar to the amorphous phase. Beyond the basic tetrahedron some order persists, but to less extent than in liquid silica. Simulation of systems at higher densities shows a volume collapse in the pressure-volume curve in the range of 4-8 GPa, suggesting the possibility that a liquid-liquid phase transition occurs, as the one observed in the amorphous phase.
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PMID:Structure of liquid GeO2 from a computer simulation model. 1508 74

The structure of several network-forming liquids and glasses is considered, where a focus is placed on the detailed information that is made available by using the method of neutron diffraction with isotope substitution (NDIS). In the case of binary network glass-forming materials with the MX2 stoichiometry (e.g. GeO2, GeSe2, ZnCl2), two different length scales at distances greater than the nearest-neighbour distance manifest themselves by peaks in the measured diffraction patterns. The network properties are influenced by a competition between the ordering on these "intermediate" and "extended" length scales, which can be manipulated by changing the chemical identity of the atomic constituents or by varying state parameters such as the temperature and pressure. The extended-range ordering, which describes the decay of the pair-correlation functions at large-r, can be represented by making a pole analysis of the Ornstein-Zernike equations, an approach that can also be used to describe the large-r behaviour of the pair-correlation functions for liquid and amorphous metals where packing constraints are important. The first applications are then described of the NDIS method to measure the detailed structure of aerodynamically-levitated laser-heated droplets of "fragile" glass-forming liquid oxides (CaAl2O4 and CaSiO3) at high-temperatures (~2000 K) and the structure of a "strong" network-forming glass (GeO2) under pressures ranging from ambient to ~8 GPa. The high-temperature experiments show structural changes on multiple length scales when the oxides are vitrified. The high-pressure experiment offers insight into the density-driven mechanisms of network collapse in GeO2 glass, and parallels are drawn with the high-pressure behaviour of silica glass. Finally, the hydrogen-bonded network of water is considered, where the first application of the method of oxygen NDIS is used to measure the structures of light versus heavy water and a difference of approximately equal to 0.5% is found between the O-D and O-H intra-molecular bond lengths. The experimental data are best matched by using path integral molecular dynamics simulations with a flexible anharmonic water model, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions.
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PMID:Identifying and characterising the different structural length scales in liquids and glasses: an experimental approach. 2393 52

The pressure-driven collapse in the structure of network-forming materials will be considered in the gigapascal (GPa) regime, where the development of in situ high-pressure neutron diffraction has enabled this technique to obtain new structural information. The improvements to the neutron diffraction methodology are discussed, and the complementary nature of the results is illustrated by considering the pressure-driven structural transformations for several key network-forming materials that have also been investigated by using other experimental techniques such as x-ray diffraction, inelastic x-ray scattering, x-ray absorption spectroscopy and Raman spectroscopy. A starting point is provided by the pressure-driven network collapse of the prototypical network-forming oxide glasses B2O3, SiO2 and GeO2. Here, the combined results help to show that the coordination number of network-forming structural motifs in a wide range of glassy and liquid oxide materials can be rationalised in terms of the oxygen-packing fraction over an extensive pressure and temperature range. The pressure-driven network collapse of the prototypical chalcogenide glass GeSe2 is also considered where, as for the case of glassy GeO2, site-specific structural information is now available from the method of in situ high-pressure neutron diffraction with isotope substitution. The application of in situ high-pressure neutron diffraction to other structurally disordered network-forming materials is also summarised. In all of this work a key theme concerns the rich diversity in the mechanisms of network collapse, which drive the changes in physico-chemical properties of these materials. A more complete picture of the mechanisms is provided by molecular dynamics simulations using theoretical schemes that give a good account of the experimental results.
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PMID:Networks under pressure: the development of in situ high-pressure neutron diffraction for glassy and liquid materials. 2574 15