Chairpersons: Stas Sinogeikin (ssinogeikin@carnegiescience.edu) and Guoyin Shen (gshen@carnegiescience.edu)

The study of matter at extreme conditions represents a forefront area of research activity across the multidisciplinary sciences. Application of pressure to 1 Mbar or beyond produces fundamental changes in the nature of a material. These megabar regions are terra incognito and are ripe for new scientific discoveries of new materials with unique and unexpected structures and properties.

While single crystal diffraction is the only way to solve and refine new crystal structures and determination of electron density topology, traditional high-resolution single crystal diffraction at megabar pressures remains challenging. Except for some specific cases (e.g. Na and Li with melting depression) it is practically impossible to obtain sufficiently large strain-free single crystals at megabar pressures for majority of materials. The materials which transform to high-pressure phases typically form polycrystalline aggregates with (sub-)micron grains which can be annealed to stress-free state with e.g. IR laser heating. Single crystals of materials stable into multimegabar pressures are always heavily strained and subsequent laser annealing usually results in breaking down the sample into smaller crystallites. Therefore multigrain crystallography is perhaps the only way to refine structures and assess electron density topology at such extreme conditions.

In addition to intrinsic challenges of multigrain crystallography defined by overlap and limited signal to noise ratio of reflections, some specific challenges are introduced by high pressure instruments. Because of the nature of high-pressure research the majority of the studied samples in diamond anvil cells are extremely small – on the order of hundreds of cubic micrometers or smaller, and the sample size progressively decreases with increasing pressure, often reaching a size of only a few microns at (multi)megabar pressures. These tiny samples are surrounded by relatively massive diamond anvils which are usually 2-3 orders of magnitude thicker than the sample itself and produce multiple adverse effects such as extremely strong diamonds diffraction and Campton scattering, absorption of incident x-ray beam and of weak useful signal from the sample. In addition to strong diffraction and background signals from the diamonds, the diffraction image can also be contaminated by parasitic signals produces by the gasket, pressure medium, pressure markers, and unreacted material or accompanying materials with different composition and structure. In order to generate extreme pressures, the diamonds anvils must be properly supported by backing plates (which are typically opaque to x-rays), which puts significant restrictions on available 4-thetta range (typically to less than 70 degrees at megabar pressures). While increasing x-ray energy will increase the Q-range, it also results in lower resolution and lower quality data due to relatively (to Q) higher the x-ray beam divergence because of focusing requirements, effectively lower detector resolution (e.g. due to point spread function) and sensitivity (lower quantum efficiency), and weaker diffraction signal (higher transmission). Thus multigrain diffraction at megabar pressures requires careful considerations of these limiting factors and thoughtful optimization of the experimental conditions.

Reliable structural analysis requires crystal chemical information which may not be available for samples at Mbar pressures. At such extreme conditions chemical nature of materials can be fundamentally different with respect to ambient pressure and such parameters as coordination numbers, bond lengths, shape of coordination polyhedrons and others may look ‘weird’ from the point of view of conventional crystal chemistry. Theoretical modelling and other experimental techniques like spectroscopies may be implemented to prove crystal chemically unusual structures at Mbar pressure.

The objective of the discussion session-1 on challenges, limitations, and opportunities for multigrain crystallography at megabar pressures would be to review its potential areas of application and the most important experimental factors for optimizing the quality of multigrain diffraction data. The discussion topics will include areas of application of high pressure multigrain crystallography, the optimal DAC geometry and x-ray opening/energy relations, the optimal x-ray focus size and sample grain size (number of grains in scattering volume), acceptable x-ray beam divergence, optimum detectors, distinguishing the diffraction spectra from background scattering and parasitic diffractions peaks (especially for unknown phases), improving signal to noise ratio and other relevant topics.