Disordered networks are at the heart of a multitude of materials with functional properties where examples range from the glasses used in optical communications technology to the role of water in geological processes. Establishing the network structure, and its relation to a system's physico-chemical and opto-electronic properties, is a prerequisite for making new materials through the principle of rational design. Here we tackle this issue by using an integrated approach to investigate the fundamentals of basic networks, using pressure to manipulate the bonding and network topology. Oxide and chalcogenide glasses along with water will be investigated, the systems chosen to be exemplars of network forming materials with different bonding mechanisms. The contrasting bonding schemes confer the networks with different characteristics and have the potential for making modified materials with tailored functional and structural properties. Applications include the recovery to ambient conditions of materials with novel characteristics, sequestration of the green house gas CO2 by geological fluids, and the effect of rare-earth clustering on the photonic properties of glass. The inherent disorder of liquid and glassy network structures is a blessing, in delivering materials of unique scientific and technological importance, but is also a curse, in providing complexity on the atomic scale. The method of neutron diffraction with isotope substitution (NDIS) has played a pivotal role in unravelling the mysteries of disordered materials since it allows access to the so-called partial structure factors i.e. to the maximum information that can be extracted from a diffraction experiment. Over the last 3 years, Bath has led an initiative to develop the techniques for measuring accurate neutron diffraction patterns for glasses and liquids at high pressures using the Paris-Edinburgh press. Thus, the time is now ideal to exploit the NDIS method to make in situ high pressure and temperature investigations of structurally disordered materials. We intend to investigate the mechanisms of structural collapse in three classes of system with different bonding schemes and concomitant network properties, namely oxide glasses (GeO2), chalcogenide glasses (e.g. GeSe2, As2Se3, AsSe) and water. These particular systems are chosen because they are archetypical materials for the study of disordered networks e.g. they either show or are anticipated to show polyamorphic phase transformations in which there is an abrupt change in their structure and physical properties with change of pressure and/or temperature. In the case of the chalcogenide glasses, the large structural variability leads to the possibility of recovering new materials with novel functional properties to ambient conditions. The structure of two types of adapted networks will also be considered, namely salty water and rare-earth alumino silicate glasses. In the former, the experiments will be made under the high pressure and temperature conditions relevant for geological fluids where applications include the sequestration of CO2. In the latter, the phenomenon of rare-earth clustering will be investigated with a view to controlling the separation of nearest-neighbour ions and hence the optical properties of these materials. Complementary information will be provided, where applicable, by NMR (Warwick), high energy x-ray diffraction, EXAFS spectroscopy and other experimental techniques. The NMR work will include well established nuclei (27Al and 29Si for the alumino silicates) but will extend the boundaries of the method by using 17O, 73Ge and 77Se. A combination of isotopic enrichment and NMR enhancement schemes will maximise the amount of structural information that can be extracted by using these nuclei as probes. Importantly, the experimental work will be enriched and complemented by molecular dynamics simulations made in collaboration with groups in Oxford, Cambridge and Strasbourg.
|Effective start/end date||5/06/12 → 4/10/15|
rare earth elements
nuclear magnetic resonance
x ray diffraction