Characterization of materials using Pair Distribution Function (PDF) is usually coupled with high-energy beamlines at synchrotron sources. The efforts to transfer the method to laboratories have already yielded results, and nowadays it is possible to collect reasonably good PDF data using in house equipment. Mirijam Zobel, junior professor at the University of Bayreuth, took the data collection one step further. She presented an innovative, yet pragmatic solution to collect high-resolution PDF data in the laboratory and won the DECTRIS award.
“Prof. Zobel and her team came up with a solution that allows for measurements of high-quality data over a wide angular range. Equally important, they have used proven technologies to create a dedicated diffractometer that can be used with a variety of sample environments. With this, the potential of in-house PDF analysis is unlocked”, says Clemens Schulze-Briese, CSO of DECTRIS.
Mirijam Zobel will present her work at the European Crystallographic Meeting in Vienna in August 2019. Winning the DECTRIS Award will help with that as it contributes 500 CHF towards attending the conference, and additional 500 CHF to be used freely. We are looking forward to meeting Mirijam Zobel in Vienna to hand over the award. Congratulations!
Below, Mirijam Zobel describes her award-winning work:
In its simplest definition, Pair Distribution Function (PDF) is a histogram over all interatomic distances in a sample. As it poses no restrictions on the order or periodicity of the atoms in the material, PDF can be used to characterize materials that exhibit chemical short-range order, such as non-crystalline materials, disordered nanomaterials, liquids and glasses. Already a century ago, the PDF analysis was used for the structural characterization of liquid alcohols, and the very first PDF data was collected in a laboratory. However, the method was forgotten for long due to long measurement times, lack of X-ray intensities and suitable detectors. The evolution of synchrotron sources, detection technologies and computing power has changed that, and led to the explosive development in PDF studies at high-energy beamlines. Nowadays, the laboratory PDF is mainly regarded as a preparative tool to screen samples prior to beamtimes rather than a technique useful for collecting data that can be used for decent structural refinements.
The work of Mirijam Zobel also started at synchrotron sources. She and her group successfully tackled problems, such as the structural evolution during nanoparticle formation and crystallization, the restructuring of interfaces, and the chemistry of heterogeneous catalysts [1-3]. Prof. Zobel’s experience in collecting data at several high-energy beamlines and her knowledge about the technology behind it led her to the next step.
“Can’t we design a laboratory PDF diffractometer that can provide high-quality laboratory PDF data which is good enough for structural refinements?” asked Mirijam Zobel.
The essential requirements for PDF measurements are easily summarized. A diffractometer should feature transmission geometry, a reasonably high X-ray energy and a suitable detector setup to cover a large angular range. However, creating a truly useful instrument is not so easy. To deliver high quality data, the instrument has to feature monochromatic radiation and minimal air scatter, and to be used by the whole group (and beyond), it needs to be user-friendly and low-maintenance.
In collaboration with STOE & Cie GmbH, Prof. Zobel optimized the existing STOE STADI P diffractometer by equipping it with Ag source and four MYTHEN2 R 1K modules that are controlled by one detector control system (DCS4). The modules are placed on the diffractometer arm to build the MYTHEN2 R 4K system that can cover 140° in 2θ of data by moving the detector only once. Each module features a 1000 µm thick silicon sensor, what yields 50 % detection efficiency for Ag X-rays. This combination of X-ray source and detector size, geometry and efficiency reduces the measurement time and the temporal inconsistencies between low and high 2θ angles. For example, it takes only 6 hours to measure complete powder data sets extending beyond 20 Å. However, not only the Q-range but also the angular resolution is extremely high. One more time, the marriage between the monochromator and MYTHEN2’ 50 micron strip proves to be a success. In this case, pure Ag Kα1 radiation was achieved by the Ge(111) crystal monochromator, while the detector threshold was kept at the 50% of the incoming radiation. The last step of the STADI P optimization tackled the background problem. Namely air scattering causes high background in the collected data, what only complicates the data processing. The problem was solved by small modifications: implementing extended collimators and optimizing the beam stop position.
“With this setup, we are able to measure and refine laboratory PDF data with an unprecedented data quality over several tens of Angströms in real space and with high fit qualities”, concludes Mirijam Zobel.
If you are curious about the first results, stay tuned! The work was recently published in Rev. Sci. Instr. 
Congratulations Mirijam and the team!
Photo: Christian Wißler, University Bayreuth
 M. Zobel et al. (2016) The evolution of crystalline ordering for ligand-ornamented zinc oxide nanoparticles, CrystEngComm 18 2163-2172. DOI: 10.1039/C5CE02099A
 S.L.J. Thomä, S. W. Krauss, M. Eckardt, P. Chater, M. Zobel (2019) Atomic insight into hydration shells around facetted nanoparticles, Nat. Commun. DOI: 10.1038/s41467-019-09007-1
 M. Ertl. et al (2018) Oxygen Revolution Catalysis with Mössbauerite - A Trivalent Iron-Only Layered Double Hydroxide, Chemistry: A European Journal 24, 36, 9004-9008, DOI: 10.1002/chem.201801938
 S.L.J. Thomä, N. Prinz, T. Hartmann, M. Teck, S. Correll, M. Zobel (2019) Rev. Sci. Instr., 90, 043905 DOI: 10.1063/1.5093714