High-pressure studies of crystals

Research Group: 
Centre for Condensed Matter and Material Physics
Full-time Project: 
QM Scholarship
Project Description: 
One of the challenges in modern condensed matter physics is to understand how materials respond to pressure. A lot of progress in recent years has gone into developing better experimental methods, together with supporting atom-scale simulation methods. When subjected to high pressure, a material may respond in various ways, most dramatically when there is a large change in the atomic arrangement in what we call a phase transition. The advances in techniques now enable more complex systems to be studied as a function of pressure, and we find that many of these materials do undergo phase transitions. The aim in this project is to study a new class of complex materials, known as metal-organic framework materials, in which the crystal structures consist of metal cations held together by their bonding to organic molecular anions in an infinite three-dimensional framework. Many of these materials have analogues to oxides, which frequently have been much more widely studied. Studies of the response of the material under pressure, and particularly when pressure induces phase transitions, give a lot of insight into the processes associated with the physics and chemistry of crystal structures and their properties.

The experimental approach is to use a combination of x-ray and neutron diffraction studies to measure the crystal structures of metal-organic frameworks as a function of pressure. X-ray diffraction studies will be performed both in the laboratories of the university and at synchrotron radiation sources. Neutron diffraction experiments will be performed at ISIS, the UK’s national neutron scattering facility. The experiments will be supported by simulations of the equilibrium crystal structure using quantum mechanical methods. In some cases we will use these simulation techniques to help understand the behaviour under pressure, and in other cases they will be used to predict new structures. There is a huge range of materials to work with, but the starting point will be a suite of materials that form structures analogous to the oxide perovskite structure, where the metal cations have octahedral coordination via a relatively small molecular anion, with a second organic anion lying in the centre of cube-shaped clusters of eight metal-organic linkages. A rich diversity of behaviour follows by changing both organic ions, or by using different metal cations with different electronic structure.

The student will gain a good foundation in understanding how to determine the atomic structure of materials through diffraction methods, together with good practical skills. In particular, a lot of training will be carried out using the local x-ray diffraction facilities in which pressure is applied using a device based on diamonds as the pressure anvils. The student will also learn how to perform state-of-the art atomistic simulations based on electronic structure, particularly with the emerging techniques associated with structure prediction. He/she will also be responsible for sample preparation and characterisation, with the scope to develop further new skills.
The student will need to be comfortable in taking a multiple-method approach, including both performing experiments, performing crystal structure analysis, and in running atomistic simulations.
SPA Academics: 
Anthony Phillips
Prof Martin Dove