Density functional theory is the established theoretical method to describe materials properties. In the computer there are no practical limits to the pressures and temperatures which can be calculated. So simulation can predict the properties of materials under extreme conditions, either guiding experimentalists to find new materials, or calculating properties on materials which may be difficult to measure experimentally. Typical applications have involved calculating crystal structures, optical and conduction properties, and phase transitions. With the latest supercomputers, such as the national HECToR facility at EPCC, it is now becoming possible to calculate melting lines and the structure of a melt. Projects in this area can be either purely computational, or combined theory-experiment in one of the areas listed elsewhere in CSEC.
Pressures above 1 million atmospheres (100GPa) can be generated routinely using a diamond anvil pressure cell. The current maximum pressure obtained with such a device is 400GPa, but pressures well beyond that might be generated by using nano-fabrication techniques to miniaturise both the diamond anvils and sample. In this project, focused ion beam technology will be used to "sculpt" diamond anvils to micron dimensions to achieve pressures beyond those currently achievable. The new technology will be used to study simple systems e.g. H2, Li, Na, N2, and O2 to explore phenomena at extreme densities.
Hydrogen is the simplest element which makes up about 74% of the visible (baryonic) matter. Its phase diagram is of the great importance not only to the fundamental physics but also to understanding of the planetary and star formation. Due to hydrogen light mass, its behaviour is governed by the strong quantum effects even at the high compressions (high densities). By comparing the behaviour of H2 and D2 (the heavier isotope having quite different quantum properties) one can gain the insights on the quantum processes which are taking place at high densities. In this project, you will use diamond anvil cells to generate pressures at low temperatures combined with the optical spectroscopy (CSEC) and x-ray diffraction (ESRF, France; Diamond, UK).
The behaviour of hydrogen, the most abundant element in the Universe, at high densities has been widely explored in recent years both experimentally and theoretically. These studies have yielded a wealth of information on the material, but detailed static compression experiments have generally been limited to low-temperatures (<300 K) and maximum pressures of ~300GPa. However, there are now numerous questions regarding the behaviour of hydrogen at high pressures and temperatures, the answers to which will have important implications for both fundamental physics and planetary science. In this project, optical spectroscopy will be used to constrain the behaviour of H2 (D2) at extreme compressions and elevated temperatures, searching for the predicted transition to a metallic superfluid state. In this project, you will use diamond anvil cells and external heating techniques combined with the optical spectroscopy (CSEC).
The fascination with extreme conditions is also fuelled by the possibility of producing potentially important industrial materials with properties unprecedented in materials produced under ambient conditions. Further, large numbers of modern materials are used in hostile environments that place stringent limits on the performance of mechanical, chemical or biological properties. Extreme conditions provide a different dimension to the synthesis of new materials, acting as an initiator and catalyst for chemical processes that would not happen otherwise. High-pressure synthesis offers a general route to extremely hard materials by bringing the atoms closer together and forcing the short bonds to participate in chemical reaction forming very dense structures. In this project, you will use diamond anvil cells combined with the laser heating, optical spectroscopy (CSEC) and x-ray diffraction (ESRF, France; Diamond, UK) to synthesise and characterise novel materials.
Igneous petrology, the part of Earth sciences which objects are magmas, has experienced a dramatic turn with the recognition of the importance plaid by liquid immiscibility in the genesis of magmatic rocks. Melt unmixing may hold a key to a number of major, long-standing problems including the genesis of lunar basalts, the formation of the Earth and asteroids. This project aims at investigating experimentally the effect of pressure on magmas immiscibility by measuring the physical properties (density, structure) of these liquids at extreme conditions.
Thermoelectric materials generate an electric voltage when subjected to a temperature gradient. Such materials allow to build reliable and compact electric power generators without moving parts that can operate on various sources of heat. In particular, they can be used to generate electrical power from waste heat of other power generators such as the combustion engines in cars. The reverse process can be used for direct electric refrigeration, e.g. for spot-cooling electronic components. However, thermoelectric devices are presently used only in niche applications, because the conversion efficiency of the known thermoelectric materials is not high enough to make them commercially viable for general use. The application of high pressure has been shown to enhance the thermoelectric properties of several materials. In this project, you will use experimental and/or computational techniques to study the properties of thermoelectric materials at high pressure to pave the way for improved materials of the future.
Neutron diffraction will be is used to investigate a proposed new form of ice at very high pressure and temperature, and what form of water it melts to under these extreme conditions. Experiments will be done on the UK neutron source in Oxfordshire, and possibly also on the European neutron facility in Grenoble, France. Results will be relevant both to understanding ice and water in a fundamental way and also to the physics of ice in planets and satellites.
Hydrogen is the simplest of elements and one of the few systems where quantum behaviour has strong effects on the crystal structure. These quantum effects are of considerable interest to theoretical modellers since they provide a rigorous test of modelling techniques where both the electrons and nuclei are treated as quantum objects. However, the crystal structure of hydrogen remains very poorly characterised. We aim to use neutron diffraction (the only technique able to determine the crystal structure of hydrogen accurately) to explore the crytal structure of hydrogen at low temperatures and high pressure. Experiments will be done on the UK neutron source in Oxfordshire, and possibly also on the European neutron facility in Grenoble, France.
Under modest pressures many simple gases form solid hydrates when mixed with water. As one example, a third of the Earth's methane is found at the bottom of the oceans in the form of methane hydrate. The gas hydrates are often important for models of the outer planets and their satellites and may have potential technological applications for carbon sequestration and gas transport. They are also models for the study of hydrophobic interactions which are important in the understanding of protein folding. This project aims to explore the structural systematics of gas hydrates at high pressure and to understand the factors determining hydrate stability. Experiments will be done on neutron and x-ray sources in Oxfordshire and in Grenoble, France.
In the context of the outer solar system, the planet forming 'minerals' are hydrogen, helium, water, ammonia and methane. The high pressure properties of these systems and their mixtures is thus crucial for models of the evolution of planets such as Uranus and Neptune and large satellites such as Ganymede, Titan and Triton. This project will explore the high pressure structures of systems such as methane, ammonia-water, ammonia, etc using x-rays and neutrons. Experiments will be done on neutron and x-ray sources in Oxfordshire and in Grenoble, France.
At ambient conditions, the group I elements (Li, Na, K, Rb Cs) are regarded as "simple metals" whose single valence electrons have only a weak interaction with the atomic core. Under pressure, however, the same simple metals are found to undergo transitions to very complex forms, which calculations suggest may be semi-metallic, or even semi-conducting. In this project, you will use nano-fabrication techniques to make "designer diamond" anvils, which will be used to measure the resistivity of alkali metals to extreme pressures. Changes in resistivity will be correlated with changes in crystal structure, determined using x-ray diffraction at the synchrotron radiation sources Diamond and the ESRF.
The upper pressure limit of the diamond anvil cell is some 4 million atmospheres (4 bars or 400 Pa). Pressures above this can be accesses only by dynamic compression methods, where extremely intense pulsed laser beams are used to generate a compression wave that then compresses the sample. Such techniques can generate pressures above 1 TPa (10 Mbars) and perhaps to 5 TPa or more. In this project, you will use powerful laser sources - OMEGA and JANUS, and perhaps also the National Ignition Facility (NIF), the most powerful laser in the world - to compress samples to above 1 TPa, and determine their crystal structure using nanosecond x-ray diffraction. The project will involve simulations of target designs, and will be conducted in collaboration with researchers from Lawrence Livermore National Laboratory and AWE Aldermaston.
The physical properties of the lanthanide (4f) and actinide (5f) elements (Ce-Lu and Th-Lr) vary dramatically as one traverses the groups from low-Z to high-Z. The properties of any one metal can be changed in a similar manner by the application of pressure, and many of these elements undergo a sequence of structural phase transitions accompanied by changes in electronic structure. We have recently made significant progress in unravelling the complex structural sequences seen in Pr and Eu under pressure at room temperature (both of which were PhD projects), and now plan to extend these studies to higher pressures and temperatures. The project will involve experiments at synchrotron sources in the UK, France and Germany, and may also involve dynamic-compression studies at laser sources in the UK and US. This project will be conducted in collaboration with AWE Aldermaston, and the project may attract additional CASE studentship support.
A study of silicate liquids at high pressure, aims at experimentally simulate the formation of rocky planets (Mercury, Venus, Earth and Mars). Planets were indeed born molten and silicates account for 2/3 of their mass. Only a limited amount of information exists about changes in silicate melts properties with T or P or both, and none come from the application of modern ‘in situ’experimental studies at high P and T. The objectives here are to measure the physical and chemical properties of silicate melts to understand the formation of planetary reservoirs and map the final destination of the elements, i.e. whether they will end up in the atmosphere, mantle or core. Experiments will be conducted at X-ray synchrotron facilities using a range of apparatus (Paris-Edinburgh press, multi-anvil press, diamond-anvil cell).
Our atmosphere essentially derives from ingassing and degassing of the molten early Earth. Nowadays, the importance of volatile in melts is most obvious for volcanic eruptions as the injection of these gases such as halogens or CO2 into the atmosphere can have large environmental impacts. Degassing of melts is also of interest in materials sciences and technical processes. The crucial missing informations to understand and model magma degassing is how volatile elements are incorporated in magmas vs fluids at depth, and how that affect their density. The get these informations, this project will combine the use of medium-range order probes (Raman spectroscopy, synchrotron X-ray diffraction) with chemically sensitive probes (e.g. X-ray absorption spectroscopy) at extreme P-T conditions.
