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Local structure

Traditionally studies of crystalline materials emphasise the role of the periodic arrangement of the atoms. This periodicity, and its underlying symmetry, has effects on the formal description of properties such as the spectra of electron energies and vibrational frequencies. On the other hand, studies of liquids and glasses can only focus on the arrangements of neighbouring groups of atoms, which we call local structure. Indeed, analysis at this level often yields surprising insights into the structure of even crystalline materials. Thus local structure is important for a wide range of condensed matter and materials science. Examples include negative thermal expansion materials, amorphous materials, pharmaceutical materials, optoelectronic materials, electrically or magnetically ordered materials including multiferroics, and shape memory alloys.

One important method for determining local structure is by total scattering analysis. Our methods for studying the structures of both crystalline and non-crystalline materials involve measuring the intensities of beams of radiation (X-rays or neutrons) as they are scattered from the material, but they differ in the way the data are analysed. For crystalline materials, much of the scattering is in the form of intense peaks at specific scattering angles - called Bragg peaks - and these contain information about the arrangements of atoms within the repeating structure via a Fourier transform. For non-crystalline materials, we need to perform a Fourier transform of the entire spectrum of scattered intensity, and this provides a histogram of all interatomic distances. Recently it has been recognised that in many interesting cases the properties of crystalline materials are to a large extent determined by short-range fluctuations of the crystal structure, and the local structure seen by the atoms can differ from that suggested by the periodic arrangement. In these cases, using the methods associated with local structure in conjunction with traditional crystallographic approaches can give entirely new insights into the properties and behaviour of materials. For more information, please contact Prof. Dove or Dr Phillips.

Another important method is muon spin resonance (┬ÁSR), which relies on implanting positively charged muons within a sample. These particles precess in the local magnetic field before decaying into a positron (and two neutrinos). Detecting the emitted positrons thus affords insight into the local magnetic structure or spin density at the muon site. For more information, please contact Dr Drew.

X-ray absorption spectroscopy, a blanket term which covers both EXAFS and XANES, refers to experiments in which X-rays are used to excite core electrons from within a sample. Oscillations just above the absorption edge reveal the local structure around the specific element targeted, which can include coordination number, bond lengths, and oxidation state. For more information, please contact Dr Sapelkin.

Surface scanning microscopy is another important local probe. For more information, please contact Dr Baxendale.

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