Fluctuations in crystals by neutron and x-ray scattering

Research Group: 
Centre for Condensed Matter and Material Physics
Full-time Project: 
QM Scholarship
Project Description: 
Although we often learn about the atomic structure of materials through the abstract representation of crystals as atomic spheres held in place by rigid bonds, in reality the atoms inside a crystal are vibrating with quite large amplitude. These atomic motions are responsible for many properties, including the thermodynamic properties (temperature, heat capacity), physical properties (thermal conductivity, thermoelectricity), the variation of the material and its properties with temperature (pyroelectricity, thermal expansion), and for large scale changes associated with phase transitions (ferroelectricity, superconductivity). Whilst we do have a good picture of the atomic-scale dynamics of materials within certain approximations, when the amplitudes are large theory will be inadequate. The project is to explore a new approach to understanding the atomic-scale dynamics of materials based on an approach using modelling driven by experimental neutron and x-ray scattering data.

We have had a lot of success in recent years using an approach called the Reverse Monte Carlo (RMC) method. The basic idea is the atoms within a large configuration that represents the crystal structure are moved around using a statistic algorithm until the predictions based on the configuration are in best agreement with data. The best type of data for this are neutron and x-ray scattering data, measured to the highest possible scattering vector to ensure sub-atomic scale resolution for the configuration. This project is to develop a new approach whereby instead of using the atomic positions as the basic variables we use the phonons themselves as the dynamic variables, projecting the phonons onto the atomic positions within the configuration of atoms. We have previously demonstrated that it is possible to go from the atomic configuration to reconstruct the phonons; this project is to develop the methodology to go in the opposite direction. The motivation for this is that we believe such an approach can yield more accurate analysis (mostly through a reduction in statistical noise), and that it immediately gives us the variable of greatest interest. This approach will be used to study materials where large-scale atomic vibrations are clearly present, including materials that undergo phase transitions, materials that have negative thermal expansion, and materials where it is known that thermal conductivity is anomalously low. The main task of the project, therefore, is to develop, implement, test and exploit a version of the RMC method based on phonon variables, together with carrying out experiments at the UK synchrotron and neutron radiation beam facilities. The resultant code will be incorporated within our RMCprofile suite, which has international impact.

There are three components of this project. First is to develop the phonon-based methodology both in theory and then as a set of algorithms. Second is a considerable programming task, and the student will need to develop modern programming skills. Third is to carry out experiments using the world-leading UK’s national neutron and synchrotron radiation facilities (ISIS and Diamond respectively), and to use the new methods to analyse data. The student will learn skills in all these areas through what will be a very broad-based training. This will include learning to write proposals for experiments at the radiation beam facilities, and of course modern methods in computer programming.

1. Phonons from powder diffraction: A quantitative model-independent evaluation. AL Goodwin, MG Tucker, MT Dove, and DA Keen. Physical Review Letters 93, 075502 (4 pp), 2004. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.075502
2. RMCProfile: Reverse Monte Carlo for polycrystalline materials. MG Tucker, DA Keen, MT Dove, AL Goodwin and Q Hui. Journal of Physics: Condensed Matter 19, 335218 (16 pp), 2007. http://iopscience.iop.org/0953-8984/19/33/335218/
3. Flexibility of zeolitic imidazolate framework structures studied by neutron total scattering and the reverse Monte Carlo method. EOR Beake, MT Dove, AE Phillips, DA Keen, MG Tucker, AL Goodwin, TD Bennett and AK Cheetham. Journal of Physics: Condensed Matter 25, 395403 (9 pp), 2013. http://iopscience.iop.org/0953-8984/25/39/395403/
The student will need to be comfortable in using computers, and will be expected to develop a high level of programming skills.
SPA Academics: 
Anthony Phillips
Prof Martin Dove