Local Structure and the Atomic Pair Distribution Function (PDF): What, Why, How?

The properties of materials depend sensitively on the arrangements of the atoms, the so-called structure-property relation. But what length-scale of the structural order is important? Do the properties depend on the local bonding or the long-range order? 

The answer to this question is somewhat complex, but for many properties the local coordination of atoms is the strongest determinant. For example, biological enzymes operate within a single macromolecule to do their business, and the actual chemistry takes place at a small active site within that molecule. It doesn't get much more local than that. For >100 years we have been able to find the long-range ordered structure using crystallography. It is often harder work to get the local structure directly. 

However, very robust experimental approaches such as the atomic pair distribution function (PDF) analysis of powder and single crystal diffraction data, can give the local structure directly. For many years this was a somewhat esoteric method that was carried out almost exclusively at powerful synchrotron and neutron sources around the world. As computational and experimental capabilities developed it has been possible to apply it more quantitatively to a wider and wider array of scientific problems from catalysis, to battery materials, thermoelectrics, quantum materials as well as cultural heritage problems. 

In situ and time resolved experiments are also possible now. In the coming years, a second revolution looks set to take place with the method as laboratory instrument manufacturers have developed their capabilities for this class of measurement, and now high quality PDFs can be obtained rapidly on the latest laboratory instruments. 

In this talk I will briefly introduce the method for those who are not familiar with it (what), show some of the most exciting cutting edge scientific applications (why) and briefly mention some of the aspects when doing this in the laboratory (how).