Polyamorphism and the Universal Liquid-Liquid Critical Point in the Supercooled State
Neville Greaves, Aberystwyth
Abstract
The physics of critical phenomena is well-established in systems as diverse as molecular fluids, crystalline alloys, and magnetic materials. As the critical point is approached the susceptibility increases anomalously and fluctuations give rise to dramatic opalescence. Evidence has recently emerged for the existence of a second critical point in the liquid state at supercooled temperatures, below which phases coexist differing in density but sharing the same composition – polyamorphism. Whilst much attention has been paid to supercooled water, polyamorphic phases have been observed in many elemental and oxide liquids potentially offering routes to low entropy glasses. Our recent direct observation of a polyamorphic phase transition in levitated molten yttria-alumina offers the first opportunity to study the associated critical point in a real supercooled system. In situ small angle X-ray scattering records sharp rises in the average correlation length of density fluctuations and in the compressibility as the transition is approached. Both increases approximate to the universal power law relations predicted by the 3D Ising model in common with all critical point phenomena. The observation brings the second critical point predicted in liquids into line with other critical phenomena. In so doing the experimental approach promises to unify polyamorphism in systems as chemically different as water, elemental liquids, silica and complex molten refractory oxides.
Targeted synthesis of inorganic materials for energy application
Matthew J. Rosseinsky* Department of Chemistry, University of Liverpool Abstract
Abstract
New inorganic materials are needed for a range of applications, particularly in the energy area. This presentation addresses the synthesis of new ionic and mixed conductors, and the broader synthetic challenges that targeting the synthesis of materials with specific properties poses. These challenges are addressed by both classical solid state methods and unit cell by unit cell approaches. Materials with high oxide ion mobilities are of importance in solid oxide fuel cells and ceramic gas separation membranes. We have recently identified oxygen interstitial insertion mechanisms into polyhedral partially condensed networks and shown that these defects have high mobility at low temperature.1 This talk will address the structural chemistry and physical properties of these materials, and defect ordering mechanisms at high interstitial loadings. The generation of high mobilities in mixed conductors is discussed in the context of recent work on the targeted synthesis of modular oxide structures2,3. Synchrotron radiation is a key characterising tool in these studies.
Ultrafast magnetic X-ray scattering
Gerhard Grübel, DESY
Abstract
The structure and dynamics of magnetic nanosystems is of both, fundamental and technological interest. Ideally, one would like to probe magnetisation dynamics on a time scale of 100 femtoseconds (fs), with nanometre spatial resolution, while being able to do the measurements element-specifically in order to account for the complex composition of actual magnetic media. Storage ring sources provide us with the necessary structural information while ultrafast magnetic scattering experiments require flashes of resonantly tuned soft X-rays that can be anticipated given the current construction of X-ray free-electron lasers (FEL) in Stanford, CA, Hyogo, Japan and Hamburg, Germany. The FLASH facility in Hamburg already provides uniquely intense coherent short pulses in the EUV energy range with the shortest fundamental wavelength of 6.1 nm. Using the fundamental wavelength of 7.97 nm it was possible to detect the fifth harmonic at 1.59 nm with an average energy of 3.5 nJ. This wavelength corresponds to the Co L3 absorption edge and enabled us to perform the first resonant magnetic scattering experiment using FEL pulses of about 20 fs duration.