The advent of nanosciences calls for the development of local structural probes, in particular to characterize ill-ordered or heterogeneous materials. Furthermore, because materials properties are often related to their heterogeneity and the hierarchical arrangement of their structure, different structural probes covering a wide range of scales are required. X-ray diffraction is one of the prime structural methods but suffers from a relatively poor detection limit, whereas transmission electron analysis involves destructive sample preparation. Here we show the potential of coupling pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The demonstration is carried out on a high-pressure pellet containing several carbon phases and on a heterogeneous powder containing chalcedony and iron pigments. The present method enables a non-invasive structural refinement with a weight sensitivity of one part per thousand. It enables the extraction of the scattering patterns of amorphous and crystalline compounds with similar atomic densities and compositions. Furthermore, such a diffraction-tomography experiment can be carried out simultaneously with X-ray fluorescence, Compton and absorption tomographies, enabling a multimodal analysis of prime importance in materials science, chemistry, geology, environmental science, medical science, palaeontology and cultural heritage.
The development of technologies for nuclear reactors
based on molten salts has seen a big resurgence. The success of thermodynamic
models for these hinges in part on our ability to predict at the atomistic
level the behavior of pure salts and their mixtures under a range
of conditions. In this letter, we present high-energy X-ray scattering
experiments and molecular dynamics simulations that describe the molten
structure of mixtures of MgCl2 and KCl. As one would expect,
KCl is a prototypical salt in which structure is governed by simple
charge alternation. In contrast, MgCl2 and its mixtures
with KCl display more complex correlations including intermediate-range
order and the formation of Cl–-decorated Mg2+ chains. A thorough computational analysis suggests that
intermediate-range order beyond charge alternation may be traced to
correlations between these chains. An analysis of the coordination
structure for Mg2+ ions paints a more complex picture than
previously understood, with multiple accessible states of distinct
geometries.
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