David Keen joined ISIS in 1989, having already been a user on the LAD diffractometer, now replaced by the GEM instrument
, during his DPhil in Physics at the University of Oxford. The technique of crystallography, pioneered by the Bragg father and son team over one hundred years ago, was by then well established as a method of studying materials, provided they formed in nice, orderly crystals. X-ray crystallography enabled a huge range of technological advances through the understanding of materials, from the Braggs' minerals to Dorothy Hodgkin's studies on insulin and vitamin B12. In the 1940s, Ernest O. Wollan and Clifford Shull used the Graphite Reactor at Oak Ridge laboratory in the US to show that the same technique could be applied to neutrons, for which Shull was later awarded the Nobel Prize, with Bert Brockhouse.
However, the focus during this time was on understanding the crystal structure formed by the atoms within ordered materials. Alongside this, the technique of total scattering was used to study less ordered materials, including liquids and gases, and amorphous solids like glass. But in the late 1980s total scattering was 'rediscovered' as a way to uncover disorder within crystal structures where apparently ordered crystals were not so neat after all.
This, along with developments in instrumentation such the powder diffractometers at ISIS, led to a renewed interest in the technique of total scattering as a way of understanding the disorder, rather than order, in materials and what this means. By the time David started coming to ISIS the total scattering technique was used relatively widely but all but a few studies, Rosalind Franklin's work on coal-like carbons being a notable exception, applied it to non-crystalline materials. David was interested in what we could learn about how disorder within the crystal structure might affect the materials' properties.
“Essentially I wanted to understand why some crystals don't behave the way they are expected to," he says. “For example, most materials expand when heated, but a few contract. Understanding the disorder within the crystals might give us the answer, and that's where total scattering comes in."
David started with the structure of silver bromide, commonly used in photographic film, to understand how it melted. To achieve this he used neutron total scattering but there were still plenty of challenges. “The key is the ability to analyse and understand the data," he says, and this led him and Robert McGreevy, now the director of ISIS, to develop the reverse Monte Carlo method. “This allowed us to compare experimental data with disordered structural models of the material, and then tweak the models to give the best match to the data."
The reverse Monte Carlo method can be applied to both X-rays and neutron data, and it soon became apparent that using both techniques combined gives a more complete picture of the material. David says, “Essentially neutron data provide very high resolution information, but it's a complex and rather slow process. X-rays provide a little less detail, but in a much quicker, more convenient way."
This has meant that over the years David has found himself using multiple facilities, including the ESRF and ILL in Grenoble, and Diamond Light Source, and studying a wide range of materials using total scattering, “I have become a world expert in the technique, as applied to both neutrons and X-rays," he says, “Which makes me the go-to person for people wanting to understand disorder in a wide range of crystalline materials whether they are mineralogical, engineering materials or biological samples. This is what makes it really interesting for me – no two samples are the same!"
Not only that, as neutron and X-ray facilities continue to evolve, this opens up new materials for probing with total scattering. “X-rays are a fantastic tool, but even the fastest X-ray synchrotron measurements are still too slow for some applications," he explains. Now, new opportunities are opening up with X-ray Free Electron Lasers, or X-FELs, and David is an early user on the European X-FEL in Germany. “X-FELs allow us to study materials that change very rapidly," he says. “They can provide data on processes that happen in very short timescales – on the order of femto-seconds – such as ultrafast chemical reactions, or how materials respond to laser shock waves." It's not all plain sailing though. “The downside is, it's far from non-destructive – it's often referred to as the “diffract and destroy" approach!"
The next step for David is to bring together a group of scientists to exploit total scattering at X-FELs using a wide range of materials, to provide new insights into what happens at the atomic level, with smaller samples at incredible detail. “We really don't know what we might find out," he says. “And that's what keeps it interesting!"