Network structures: from fundamentals to functionality

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.

Our proposal aims at understanding the fundamentals of the structure-function relationship for key network forming materials. The scope of the proposal is multi-disciplinary and the results will impact on areas that include materials, physics, chemistry and geology. For example, amorphous oxides play an essential role in most scientific and technological disciplines where examples include the glasses used for lasers and optical communications, the insulating oxide layers in silicon-based electronic devices, and molten silicates in planetary science. Rare-earth doped aluminosilicate glasses are at the heart of materials used as lasers and optical amplifiers for all-fibre optical systems. Chalcogenide glasses have wide application in infra-red optics and in re-writable optical disc technology where structural transformations govern the optical reflectivity and hence the functionality of the materials. Researchers from Corning Glass Inc. have a broad interest in chalcogenide materials as recovered from extreme conditions and many of their publications in recent years have been devoted to chalcogenide glasses.

Water and its adaptations when salt is added are of fundamental importance and have multiple applications as solvents in chemical and geological processes. For example, CO2 is an important greenhouse gas and several routes have been proposed for its sequestration by e.g. solubility trapping in deep aquifers and subsequent mineral trapping in carbonates. Key to these processes is the structure of so-called geological fluids e.g. the water network as modified by salts such as NaCl under geological (high pressure P and temperature T) conditions. The structure of geological fluids is also important for enhanced oil recovery, mass transfer within the Earth's crust by hydrothermal transport, and magma generation at convergent tectonic plate margins. Fluids under high P and T conditions can also be used to advantage in promoting the dissolution of reactants that are otherwise insoluble, leading to synthesis routes for new materials.

It is therefore plausible that our research will, in the medium to long term, impact on: -

(i) Commercialisation and exploitation of scientific knowledge - through the provision of new chalcogenide materials with novel functional properties, through the elucidation of the phenomenon of rare-earth clustering in photonic materials, and through the ability to make new materials via the principle of rational design.

(ii) Environmental sustainability, protection and impact reduction - through an enhanced understanding of geological fluids and their impact on issues like the sequestration of greenhouse gases such as CO2.

(iii) Evidence based policy making - through the creation of enhanced thermodynamic models of geological fluids to predict the fate of CO2 injected in deep aquifers in both the short- and long-term.

(iv) Increased public awareness and understanding of science, economic and societal issues - by exploiting preliminary contacts with local groups such as the Cafe Scientifique and the Bath Literary and Scientific Institution, which have a long standing tradition of hosting regular public discussions of topical scientific issues.

(v) Training of skilled people for non-academic professions - the Researcher Co-Investigator will gain the generic transferrable skills associated with the planning and realisation of a multi-faceted interdisciplinary project at all of its stages, from conception to conclusions.