Chairpersons: Dmitry Popov (dpopov@carnegiescience.edu) and Robert Suter (suter@andrew.cmu.edu)

Spatially and time resolved microstructural information is crucial for understanding mechanisms of pressure induced processes such as phase transitions, chemical reactions, deformation, melting, recrystallization, and crystal growth. This discussion will be focused on recent developments of high pressure X-ray microscopy based on Laue and high energy X-ray diffraction.

High energy X-ray microscopy can provide structural information along with positions, orientations and strain of thousands of crystals. High energies allow penetration through complex sample containers such as diamond anvils cells in either normal incidence or “through the gasket” geometries. The high energy diffraction microscopy (HEDM) methods allow for parallel data collection from crystals distributed over millimeter diameter regions with either a line focused (few micron high) or a larger beam. In the line focused case, grain cross-sections are probed for orientation variations, location and shape information but the high resolution imaging detector needs to be as close as possible to the sample. The “far-field” measurement is less constraining with respect to detector positioning and can be used to map grain centers of mass, orientations, and lattice strain states. These methods can also be combined with high energy tomography or simple radiography to locate few micron sized objects within the field of view. Optimization of these techniques for experiments in diamond anvil cells will be discussed.

Polychromatic beam diffraction (Laue method) is a powerful tool to get spatially and time resolved information on crystal morphology, lattice rotation, deviatoric strain and texture. Laue diffraction has obvious advantages over the use of monochromatic beam: comparable number of reflections can be recorded multiple orders faster, because there is no need to rotate the sample. Much better spatial resolution can be obtained with X-ray beams focused down to submicron level. Collecting consequential sets of 2D scans across a sample in high pressure cell with polychromatic beam, one can get spatially and time resolved information simultaneously and a translational scan with submicron resolution can be collected in hours. On the other hand despite polychromatic beam diffraction is widely implemented to study materials at ambient pressures its applications in high pressure area so far are very limited. The scientific opportunities with the high pressure Laue diffraction will be important part of this discussion.

The major challenge of Laue diffraction is that absolute unit cell parameters are not available. This problem can be solved by using monochromators which can be moved ‘in’ and ‘out’ of the beam and provide X-ray monochromatic beam switchable with the white beam in order to measure d-values of sufficient number of reflections for unit cell parameters refinement. Tunable monochromators can be also used for mapping of reciprocal space. Another way to measure energies of reflections is using of filter’s absorption edges. Optimization of all these procedures will be discussed.

High pressure cells introduce some specific challenges to Laue diffraction experiments due to high background scattering from diamonds and limited 2θ angles. At ambient pressure Laue measurements are usually done in 90° geometry because it provides much more reflections than in transmitted geometry. Application of 90° geometry at high pressure requires panoramic high pressure cells with X-ray transparent gasket materials. Because of diamonds differential aperture needed for 3D mapping cannot be placed close enough to the sample, so currently only 2D mapping can be conducted. All these challenges and limitations will be discussed in details together with instrumentation related problems including mechanical stability, X-ray focusing systems, fast translational stages, and area detectors.

Analysis of high pressure Laue diffraction data will be discussed including indexing of reflections, mapping of crystal morphology and strain, precise strain refinement.

Useful links:

High pressure Laue diffraction in HPCAT:

http://scitation.aip.org/content/aip/journal/rsi/86/7/10.1063/1.4926894

X-ray microscopy using polychromatic X-ray beam:

http://science.sciencemag.org/content/334/6060/1234.full