Organic crystal structures, solved and refined from powder data, may be fully wrong, even if they are chemically sensible and give a good fit to the powder patterns.

Two examples are shown.

In both cases, the atomic positions and the molecular packing were completely wrong.

Example 1:

The crystal structure of the commercial organic hydrazone Pigment Red 52:1, Ca²⁺(C₁₈H₁₁ClN₂O₆S)^2-*H₂O was determined from powder data in the usual way by indexing, structure solution by real-space methods, and Rietveld refinement. The resulting structure was chemically sensible and gave a good fit to the powder data. By chance, a single-crystal of poor quality was obtained, and the correct structure was determined by a combination of single-crystal structure analysis and Rietveld refinement. The structure initially determined from powder data turned out to be completely wrong. The correct and the wrong structures differ in the position and coordination of the Ca²⁺ ions, as well as the position and mutual arrangement of the anions (see Figure).

Example 2:

The crystal structure of 4,11-difluoroquinacridone, C₂₀H₁₀F₂N₂O₂, was solved from unindexed powder data by a global fit of millions of random structures to the powder pattern using the FIDEL method [1,2], which uses cross-correlation functions for the comparison of experimental and simulated powder patterns. The structures were subsequently refined by the Rietveld method. Four completely different structures (different space groups, different molecular packings, different H bond topolgies) were obtained. All four structures were chemically sensible, had a good fit to the powder data, gave a good fit to the pair-distribution function and good lattice energies. One is correct, the other were wrong [3].

[1] S. Habermehl, P. Mörschel, P. Eisenbrandt, S.M. Hammer, M.U. Schmidt, Acta Cryst. B70 (2014), 347-359.

[2] S. Habermehl, C. Schlesinger, M.U. Schmidt, in preparation.

[3] C. Schlesinger, A. Fitterer, C. Buchsbaum, S. Habermehl, M.U. Schmidt, in preparation.

Next generation materials of nearly every kind rely on chemical, electronic, and/or magnetic heterogeneity for creating, harnessing, and controlling functionality. Exploration of these phenomena increasingly involve multiple length-scale scattering probes and require sophisticated modeling approaches to characterize and understand them. Total scattering methods, including both Bragg and diffuse scattering signals, are providing key insights into how long-range, nanoscale, and local atomic structure motifs differ in materials and cooperate to deliver their unique properties. The nuances of capturing nanoscale heterogeneities, including correlated defects, chemical short-range order, and stacking fault distributions, represent a modern frontier in the field of crystallography. We will explore this theme through detailed investigation of two distinct materials classes. First, we will present the operando study of nanostructured fluorite catalysts. We will specifically follow the nature of correlated oxygen vacancies at elevated temperatures, including their behavior under acid-gas exposure. Second, we will present structure-property characteristics of new pyrochlore and perovskite high entropy oxides (HEOs). HEOs exhibit a single-phase crystal structure containing five or more different metal cations of the same amount on single crystallographic lattice sites; their compositional and configurational disorder and associated structural diversity offer great potential for unique material characteristics. We will highlight contemporary challenges and opportunities in the quest to extract crystal structure models from experimental data with the detail needed to guide and validate solid state theories, and design new and improved functional materials.

Most of the natural or fabricated fibrous materials exhibit multiscale structures, which critically influence their mechanical, optical, and electronic properties. Therefore, knowing the structure is important to steer the properties or design novel fibrous material. This requires multiscale structural characterization to enrich their structure-properties relationship. State-of-the-art small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques are extremely powerful to characterize such materials from the nanometer to the Ångström scale [1].

In this contribution, multiscale structural insights of different fibrous materials such as electrospun nanofiber scaffolds [1], thermal protective fabrics [2], and fibrous biocomposite tissues would be presented with emphasis on their structure-properties relationship primarily using SAXS and WAXD methods. The schematic of the multiscale structure of the electrospun nanofiber scaffolds is shown in figure 1 as an example. Furthermore, the application of gained structural knowledge to steer the properties of polymeric nanofibers and the design of novel humid responsive nanofibrous scaffolds would be discussed.

[1] A.K. Maurya, L. Weidenbacher, F. Spano, G. Fortunato, R.M. Rossi, M. Frenz, A. Dommann, A. Neels, A. Sadeghpour, Structural insights into semicrystalline states of electrospun nanofibers: a multiscale analytical approach, Nanoscale 11(15) (2019) 7176-7187.

[2] A.K. Maurya, S. Mandal, D.E. Wheeldon, J. Schoeller, M. Schmid, S. Annaheim, M. Camenzind, G. Fortunato, A. Dommann, A. Neels, A. Sadeghpour, R.M. Rossi, Effect of radiant heat exposure on structure and mechanical properties of thermal protective fabrics, Polymer 222 (2021) 123634.

Orientation and conformation in nanoscale amorphous regions often dominate the properties of soft materials such as composites and semicrystalline polymers. Robust correlations between between structure in these amorphous regions and important properties are not well developed due to a lack of measurements with high spatial resolution and a sensitivity to molecular orientation. I will describe our approach to solving this issue using polarized resonant soft X-ray scattering (P-RSoXS), which combines principles of soft X-ray spectroscopy, small-angle scattering, real-space imaging, and molecular simulation to produce a molecular scale structure measurement for soft materials and complex fluids.

Because P-RSoXS is relatively new to the scattering community, I will first cover the basics of the measurement. The fundamental principles of P-RSoXS and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, the spectroscopic basis for P-RSoXS, will be reviewed. The P-RSoXS experiment will be discussed including sample preparation and constraints, which differ considerably from analogous scattering techniques such as conventional small-angle X-ray scattering (SAXS) and small angle neutron scattering (SANS). I will also cover approaches for including gases or liquids in the experiment, and describe available measurement facilities. Data collection best practices will be reviewed.

I will then describe polarized resonant soft X-ray scattering (P-RSoXS) measurements of model systems including polymer-grafted nanoparticles. Analysis will focus on quantitative extraction of orientation details from nanoscale glassy regions. This work is now accelerated by a powerful analysis framework using parallel computation across graphics processing units (GPUs) for the forward-simulation of P-RSoXS patterns. In polymer-grafted nanoparticles, we can apply this framework to fit quantitative and detailed descriptions of amorphous chain orientation with ≈ 2 nm resolution.

Characterising the supramolecular organisation of macromolecules in the presence of varying degrees of disorder remains one of the challenges of structural research. Discotic liquid crystals (DLCs) are an ideal model system for understanding the role of disorder on multiple length scales. Consisting of rigid aromatic cores with flexible alkyl fringes, they can be considered as one-dimensional fluids along the stacking direction and they have attracted attention as molecular wires in organic electronic components and photovoltaic devices [1].

With its roots in single-particle imaging, fluctuation x-ray scattering (FXS) [2] is a method that breaks free of the requirement for periodic order. However, the interpretation of FXS data has been limited by difficulties in analysing intensity correlations in reciprocal space [3]. Recent work has shown that these correlations can be translated into a three-and four-body distribution in real space called the pair-angle distribution function (PADF) – an extension of the familiar pair distribution function into a three-dimensional volume [4]. The analytical power of this technique has already been demonstrated in studies of disordered porous carbons and self-assembled lipid phases [5,6].

Here we report on the investigation of order-disorder transitions in liquid crystal materials utilising the PADF technique and the development of facilities for FXS measurements at the Australian Synchrotron

X-Ray Free Electron Lasers provide a means of conducting crystallography experiments with remarkable time and spatial resolution. These methods can directly recover the electron density of materials. However, there are stringent requirements such as crystal size, number density per exposure, and the crystal order which are required for reconstruction. Membrane proteins, which do not readily crystallise or meet these requirements [1], are particularly interesting to study as they comprise up to 50% of drug targets [2], but less than 10% of the protein structures in the Protein Data Bank [3].

The Pair Angle Distribution Function (PADF) describes the three and four body correlations of the electron density in a sample, and can be recovered from X-ray cross-correlation analysis (XCCA) [4]. Although PADF analysis does not recover the electron density directly, it still contains significant information about the local three dimensional structure of the material. PADF analysis also has the potential to relax the stringent crystal requirements of current single crystal experiments.

We discuss the sensitivity of the PADF to different protein structures [5], and the correlations generated at different length scales; from atomic bonding to tertiary structure. Our aim is to further develop PADF analysis to recover crystal structure factors using X-ray cross-correlation analysis.

[1] Johansson, L.C.; Arnlund, D.; White, T.A.; Katona, G.; DePonte, D.P.; Weierstall, U.; Doak, R.B.; 
Shoeman, R.L.; Lomb, L.; Malmerberg, E.; et al. Lipidic phase membrane protein serial femtosecond 
crystallography. Nat. Methods 2012, 9, 263–265.

[2] Cournia, Z.; Allen, T.W.; Andricioaei, I.; Antonny, B.; Baum, D.; Brannigan, G.; Buchete, N.V.; Deckman, J.T.; Delemotte, L.; del Val, C.; et al. Membrane protein structure, function, and dynamics: A perspective from experiments and theory. J. Membr. Biol. 2015, 248, 611–640.


[3] Berman, H.M.; Battistuz, T.; Bhat, T.N.; Bluhm, W.F.; Bourne, P.E.; Burkhardt, K.; Feng, Z.; Gilliland, G.L.; Iype, L.; Jain, S.; et al. The Protein Data Bank. Acta Crystallogr. Sect. D Biol. Crystallogr. 2002, 58, 899–907.

[4] Martin, A.V. Orientational order of liquids and glasses via fluctuation diffraction. IUCrJ 2017, 4, 24–36.

[5] Adams, Patrick, et al. "The Sensitivity of the Pair-Angle Distribution Function to Protein Structure." Crystals 10.9 (2020): 724.