Functional materials — such as superconductors, magnets, piezoelectrics, and photovoltaics — play a crucially-important role in our everyday lives. As structural chemists, we are used to thinking about the ways in which structure and function are related. What is becoming increasingly apparent is that many of these important functionalities depend as much on instantaneous deviations in local structure — i.e. atomic fluctuations that exist over the nanoscale — as they do on the average structure of the material in question. Whilst these subtle structural distortions are measured as part of a conventional crystallographic experiment, any information on atom-atom correlations is lost as the data are averaged over the bulk crystal structure.
We have recently acquired a Ag-source powder diffractometer, specifically designed for the measurement of total scattering data, which can be transformed to yield a pair distribution function (PDF). The PDF contains the local structure information that is lost in a typical crystallographic experiment. Our diffractometer is one of very few in the world that is capable of making these measurements in-house and we aim to collect data on a variety of disordered materials in order to understand better their structure-property relationships.
An additional aspect to the project will be to model these data using, for example,a reverse Monte Carlo algorithm. The method is straightforward: by taking a large box of atoms (typically thousands) and calculating the total scattering patterns they would produce, each atom can then be moved at random until the the best fit against the real experimental data is obtained. The end result is an atomic configuration that contains both the short-range lattice distortions and the long-range average structure, as seen by a crystallographic experiment.
Some background reading:
- Young C. A. and Goodwin A. L., J. Mater. Chem. 21, 6464–6476 (2011)
- Dykhne T., Taylor R., Florence A. and Billinge S. J. L., Pharm. Res. 28, 1041–1048 (2011)
- Dixon E., Hadermann J., Ramos S., Goodwin A. L. and Hayward M. A., J. Am. Chem. Soc. 133, 18397–18405 (2011)