Absolute refinement of crystal structures by X-ray phase measurements

Abstract

A pair of enantiomer crystals is used to demonstrate how X-ray phase measurements provide reliable information for absolute identification and improvement of atomic model structures. Reliable phase measurements are possible thanks to the existence of intervals of phase values that are clearly distinguishable beyond instrumental effects. Because of the high susceptibility of phase values to structural details, accurate model structures were necessary for succeeding with this demonstration. It shows a route for exploiting physical phase measurements in the crystallography of more complex crystals.

Phonon scattering mechanism in thermoelectric materials revised via resonant x-ray dynamical diffraction

“…The information extracted from the phases of the structure factors is then used to validate or select feasible model structures for the crystal. [37][38][39] The minimum crystal size for this type of experiment is, in general, ten times larger than the usual grain size found in powder samples. [40] Structure factor phases are susceptible to the differences between the vibration amplitudes of the atoms-root mean square (RMS) atomic displacements.…”

“…Reflections with phase susceptible to changes in ionic charges, resonance amplitudes, atomic positions, and occupation and DW factors have been identified by means of model structures. [37][38][39][40] For the CeFe 4 P 12 structure, the atomic planes of Ce and Fe are interleaved along [001] type directions, Fig. 1(c).…”

“…In previous applications of phase measurements, only one X-ray energy was used in each case since simple evaluation of just a few peak asymmetries allowed the validation of the proposed structural models, [37][38][39][40] as in the case of detecting hydrogen bonds between amino acid molecules. [39] By properly choosing the reference reflection with graphical help of the two-dimensional (2D) representation of Bragg cones (Supplementary Material), [54] one azimuthal scan at no particular X-ray energy had enough information to evidence the electron charges at the hydrogen bonds.…”

X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological moleculesviaphysical phase measurements

“…In crystals with small unit cells, dynamical diffraction regime is achieved in much smaller domains than in crystals with large cells such as protein crystals. Fact that has allowed phase measurements to reveal structural details-inaccessible by other techniques-in magnetic materials 16 , optical crystals with dopant ions 15,17 , and to solve chirality in crystals with no resonant atoms [18][19][20] .…”

“…if the MD intensity profile has lower/higher (L|H) or higher/lower (H|L) shoulders, is a reliable information even in crystals with some mosaicity 28,[35][36][37][38] . Very recently, it has been proposed that this fact leads to a window of accuracy in phase measurements, implying in new strategies on how to look at these asymmetries, and opening opportunities for high resolution studies of crystal structures 20 .…”