Most nanocrystals are expected to show deviations from a perfect crystal lattice inside the grains, which is referred to as modulation waves[1], because of their significant surface effects, leading to exceptional physical and chemical properties. Conventional X-ray diffraction fails to reveal modulation waves owing to the assumption of the periodic structure, whereas electron diffraction from a single grain is one of the most powerful probes to distinguish the core structure from the surface structure on the atomic level. It is, however, still challenging to investigate modulation waves from the core to the surface, which is the atomic-level core-shell structure. In this study, we have demonstrated that synchrotron X-ray total scattering makes it possible to visualize the core-shell structure on the picometer level in Pd nanocrystals.

X-ray total scattering provides a potential for visualization of modulation waves[2]; nevertheless its applications have been very limited because the approach is extremely demanding of experimental data. We have developed the high-resolution and high-accuracy total scattering measurement system, OHGI (Overlapped High-Grade Intelligencer), at SPring-8[3,4] to overcome the limitations. Recent studies have demonstrated that our total scattering data are of the highest quality in terms of both Bragg and diffuse scattering[5-7]. With this system, Pd nanocrystals were measured under hydrogen pressure. The total scattering data were converted into atomic pair distribution functions (PDF) on the basis of the principle of maximum entropy[8]. The resulting PDFs were virtually free from spurious ripples at no expense of real-space resolution. We have attempted to model modulation waves from the PDFs on the basis of an fcc Pd lattice. The model suggests that the interatomic distances between Pd atoms in the shell region are longer than those in the core by a few picometers. In addition, we found that the core-shell structure undergoes significant changes by hydrogenation. The picometer-level core-shell structure can explain that implied by neutron diffraction, where both tetrahedral and octahedral sites are occupied by hydrogen atoms in the surface[9]. In this presentation, I will discuss the relationship between the modified core-shell structure and hydrogen-storage kinetics in Pd nanocrystals.

[1] Hudry, D., Howard, I. A., Popescu, R., Gerthsen, D. & Richards, B. S. (2019). Adv. Mater. 31, 1900623.

[2] Palosz, B., Grzanka, E., Gierlotka, S. & Stelmakh, S. (2010). Z. Kristallogr. 225, 588.

[3] Kato, K., Tanaka, Y., Yamauchi, M., Ohara, K. & Hatsui, T. (2019). J. Synchrotron Rad. 26, 762.

[4] Kato, K. & Shigeta, K. (2020). J. Synchrotron Rad. 27, 1172.

[5] Svane, B., Tolborg, K., Kato, K. & Iversen, B. B. (2021). Acta Cryst. A77, 85.

[6] Pinkerton, A. (2021). Acta Cryst. A77, 83.

[7] Beyer, J., Kato, K. & Iversen, B. B. (2021). IUCrJ 8, 387.

[8] Kato, K. et al., submitted.

[9] Akiba, H., Kofu, M., Kobayashi, H., Kitagawa, H., Ikeda, K., Otomo, T. & Yamamuro, O. (2016). J. Am. Chem. Soc. 138, 10238.

Permanent magnets are key enablers driving the 21^st century’s diverse electromagnetic technologies [1]. Ceramic magnets based on iron oxides are corrosion resistant and cost-effective permanent magnetic materials with minimal ecological footprint, compared to their lanthanide-based counterparts (such as SmCo₅; Nd₂Fe₁₄B etc.) [1,2]. Among the ceramic magnets, the hexaferrites (AFe₁₂O₁₉; A = Ba/Sr) constitute the bulk of industrially produced permanent magnets [3,4]. A permanent magnet’s properties emerge hierarchically and are influenced by the structure on various length scales. From atomic interactions in the unit cell (~ 1–10 Å), through the interactions between crystallites/domains (~10 nm–1 µm), to the consolidated macroscopic forms (~ 0.1 mm–0.1 m), magnetism evolves over length scales spanning 10 orders of magnitude [5]! For maximal magnetic performance, the structure of a permanent magnet must be optimized at all levels. Control over the structure is obtained by controlling the synthesis and processing parameters. Additionally, these fabrication methods need to be economical, effective and compatible with industrial scale synthesis processes.

Recent work from our group has demonstrated the fabrication of high-performance nanostructured SrFe₁₂O₁₉ hexaferrite permanent magnets from solid-state processing of ferrihydrite – a poorly crystalline, structurally disordered nanomaterial [6,7]. This fabrication method is low-cost, scalable and compatible with industry processes. In the presence of Sr²⁺ ions, ferrihydrite is seen to act as a building block for the SrFe₁₂O₁₉ hexaferrite, irrespective of synthesis technique (hydrothermal/microwave/solid-state synthesis) [7–9]. While the transformation of ferrihydrite to hexaferrite has been documented in previous studies, a detailed understanding of the conversion process in situ is lacking. The fundamental question that remains to be answered: how does long-range crystalline order in the hexaferrite magnet arise from the nanoscale-disordered ferrihydrite precursor?

Here, we report on our efforts towards addressing this question using in­ situ­ synchrotron X-ray scattering studies to investigate the ferrihydrite-hexaferrite transformation. Ferrihydrite samples (with Sr²⁺) were heated while scattering data was collected in situ at the P02.1 beamline at PETRA III. The P02.1 beamline allows for 2 operating modes separately optimized for X-ray Bragg scattering and total scattering[10]. In situ X-ray scattering experiments were performed in both modes. Combining Rietveld modelling of Bragg scattering data and Pair Distribution Function (PDF) modelling of total scattering data provided detailed insight into the real-time structural evolution of disordered ferrihydrite to crystalline hexaferrite. PDF analysis shed light on the evolution of the short- and intermediate-range structural features during the transformation. Rietveld analysis helped ascertain the long-range order, nanoscale morphology and crystallite growth mechanism. This clear picture of the ferrihydrite-hexaferrite transformation over multiple relevant length scales is expected to aid future efforts in engineering better permanent magnets. Meanwhile, beyond permanent magnetic materials, these insights are also expected to contribute toward a broader understanding of the evolution of crystalline order from nanoscale disorder.

[1] Gutfleisch, O., Willard, M. A., Brück, E., Chen, C. H., Sankar, S. G., & Liu, J. P. (2011) Adv. Mater. 23, 821.

[2] Sugimoto, M. (2010) J. Am. Ceram. Soc. 82, 269.

[3] Pullar, R. C. (2012) Prog. Mater. Sci. 57, 1191.

[4] de Julian Fernandez, C., Sangregorio, C., de la Figuera, J., Belec, B., Makovec, D., & Quesada, A. (2020) J. Phys. D. Appl. Phys. 54, 153001.

[5] Skomski, R. (2003) J. Phys. Condens. Matter. 15, R841.

[6] Christensen, M. & Vijayan, H. Enhanced magnetic properties through alignment of non-magnetic constituents (2020) European Provisional Patent P5698EP00-CLI 2020.

[7] Vijayan, H., Knudsen, C. G., Mørch, M. I., & Christensen, M. (2021) Mater. Chem. Front. 5, 3699.

[8] Granados-Miralles, C., Saura-Múzquiz, M., Bøjesen, E. D., Jensen, K. M. Ø., Andersen, H. L., & Christensen, M. (2016) J. Mater. Chem. C. 4, 10903.

[9] Grindi, B., BenAli, A., Magen, C., & Viau, G. (2018) J. Solid State Chem. 264, 124.

[10] Dippel, A.-C., Liermann, H.-P., Delitz, J. T., Walter, P., Schulte-Schrepping, H., Seeck, O. H., et al. (2015) J. Synchrotron Radiat. 22, 675.

While simple associations exist between piezoelectric properties and processing history, there is considerable scope for design of materials based on a more detailed molecular understanding of the re-arrangements which underpin the piezoelectric phenomenon in silk fibroin.[1] Crystallinity, and the two-phase model of semi-crystalline model of polymers, are often used to understand the properties of protein based materials where there is considerable thermodynamic drive to short range ordering of polymer chains, folding, which is not present in melt processed thermoplastics. Our investigations aim to probe the relationship between structure and dynamics in silk fibroin based materials and correlate these with the piezo-electric signal.

Recently, we have used a triggered and summative data acquisition scheme to synchronise of X-ray scattering data with a piezoelectric cycle of a compressed electro-spun fibroin mat.[2] This mode, provided a steady perturbed state can be sampled, the summation of multiple similar stages to provide superior statistics than would be possible by sampling a single cycle. The setup is shown in Figure 1A. At rest this poorly ordered system exhibits a limited number of very broad peaks but quite a high degree of chain folding.[3] With compression there is marked increase in the scattered intensity, both in the small (SAXS) and the wide (WAXS) angle regimes, as well a shift and reduction in broadness of the WAXS peaks (Figure 1B). We interpret the increase in the SAXS signal as an increase in scattering from grain boundaries and the WAXS as the formation of new crystalline domains. However, the limited number of very broad diffraction peaks make these data unsuitable for structural determination.

In order to provide an alternative, but complementary, perspective on the structural dynamics and the nature of the potential surface along which the polymer folds, during the piezo cycle we have turned to a computation approach. Density functional theory (DFT) calculations were performed within the framework of the projector augmented wave potentials parametrised by Perdew et al., [3] and the Tkatchenko-Scheffler correction [4] with a self-consistent screening to account for the weak correlations. Full structural optimisation of orthorhombic C₂₀O₈N₈H₃₂ was performed. The calculated lattice parameters (a = 9.409 Å, b = 6.984 Å, c = 9.221 Å) and the corresponding diffraction pattern (black vertical lines) are compared with the experimental data in Figure 1C.

Keywords: piezoelectricity; biopolymer; synchrotron wide angle X-ray scattering; millisecond resolved diffraction

The authors acknowledge beamtime on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at National Supercomputer Centre (NSC) in Linköping, Sweden.

[1] Rockwood, D. N., Preda, R. C., Yücel, T., Wang, X., Lovett, M. L. & Kaplan, D. L. (2011). Nature Protocols 6, 1612.

[2] Sencadas, V., Garvey, C., Mudie, S., Kirkensgaard, J. J. K., Gouadec, G. & Hauser, S. (2019). Nano Energy 66, 104106.

[3] Perdew, J. P., Burke, K. & Ernzerhof, M. (1996). Physical Review Letters 77, 3865-3868.

[4] Tkatchenko, A. & Scheffler, M. (2009). Physical Review Letters 102, 073005.

High-throughput synthesis using inkjet printing allows the deposition of hundreds of nanoparticle compositions on a single substrate. It also introduces the challenge to characterize the phases and structures of these nanoparticles in an automated high-throughput way. Here, we develop a method to screen hundreds of nanoparticle combinations, collect the x-ray diffraction image, transfer the data to pair distribution functions and analyze the phase information. It has been successfully applied to characterize the phase compositions and atomic structures in an iron, nickel, cobalt nanoparticle array. Combining this method with inkjet printing and optical screening, it is possible to achieve the fully automated high-throughput searching for the optimal combinatorial nanoparticle catalysts.

Functional nanomaterials are frequently synthesized by the facile sol-gel method. In a broader sense, the process can be described as the conversion of molecular precursors in solution into inorganic solids via hydrolysis, condensation and aggregation [1]. It allows the targeted control of structural characteristics e.g. particle/crystallite size and polymorphism of a material. This is of particular importance for quantum materials, where the charge, spin, orbital and lattice are intrinsically coupled due to strong electronic interactions [1]. Since the sol-gel method is a non-equlibrium process, the synthesis of pure nanocrystalline samples is challenging if various stable and metastable phases exist, often leading to co-crystallization. Subtle changes in the synthesis parameters, such as temperature, pH and complexing agent, can strongly influence the resulting structural and physical properties of the materials. Despite this knowledge and the popularity of this synthesis method, studies on the parameters driving the crystallization process are rare and a deep understanding of the formation mechanisms is usually lacking.

The Bi₂O₃-Fe₂O₃ system is known to be challenging from a synthetic point of view, as sillenite-type Bi₂₅FeO₃₉, mullite-type Bi₂Fe₄O₉ and perovskite-type BiFeO₃ have a strong tendency to co-crystallize [3]. The target compound Bi₂Fe₄O₉ shows multiferroic behaviour close to room-temperature [4] and a spin liquid state just above the transition [5]. Its exotic magnetism materialises due to five competing magnetic exchange interactions involving two distinct Fe-sites, which drive antiferromagnetic coupling in the ab–plane and non-collinear ferromagnetic ordering along the c-axis [6]. Below a critical size of ~120 nm, size-dependent properties can be observed due to significant changes in the structural lattice [7].

In this study, we investigate how the synthesis parameters in a sol-gel approach affect the crystallization pathways and kinetics of Bi₂Fe₄O₉. We follow the transformation of molecular precursors into the fully crystalline structures using in situ total scattering and Pair Distribution Function (PDF) analysis with a second-scale time resolution. In total, five different precursors were synthesized using the respective metal nitrates and meso-erythritol as the complexing agent. The phases qualitatively appearing during crystallization as well as their transition and growth kinetics can be controlled by the synthesis medium and ratio of metal nitrate to complexing agent. More specifically, we observe multiple crystallization pathways including the initial formation of rhombohedral BiFeO₃ and subsequent transition into orthorhombic Bi₂Fe₄O₉, co-crystallization of BiFeO₃ and Bi₂Fe₄O₉, or the direct formation of Bi₂Fe₄O₉ from the precursor. During crystal growth, the lattice parameter b decreases significantly, although Bi₂Fe₄O₉ is known to exhibit positive thermal expansion [8] highlighting the influence of the crystallite size on the lattice. In addition, the overall crystallization process is predetermined very early in the synthesis process and mainly governed by the gel structures formed during evaporation of the solvent and organic components, as suggested by ex situ PDF analysis.

[1] Niederberger, M. (2007). Acc. Chem. Res. 40, 793. [2] Samarth, N. (2017). Nat. Mater. 16, 1068.

[3] Carvalho, T. T. & Tavares, P.B. (2008). Mater. Lett. 62, 3984.

[4] Singh, A. K., Kaushik, S. D, Kumar, B., Mishra, P. K., Venimadhav, A., Siruguri, V. & Patnaik, S. (2008). Appl. Phys. Lett. 92, 132910.

[5] Beauvois, K., Simonet, V., Petit, S., Robert, J., Bourdarot, F., Gospodinov, M., Mukhin, A. A., Ballou, R., Skumryev, V. & Ressouche, E. (2020). Phys. Rev. Lett. 124, 127202.

[6] Ressouche, E., Simonet, V., Canals, B., Gospodinov, M. & Skumryev, V. (2009). Phys. Rev. Lett. 103, 267204.

[7] Kirsch, A., Murshed, M. M., Litterst, F. J. & Gesing, Th. M.(2019). J. Phys. Chem. C 123, 3161.

[8] Murshed, M. M., Nénert, G., Burianek, M., Robben, L., Mühlberg, M., Schneider, H., Fischer, R. X. & Gesing, Th. M. (2013). J. Solid State Chem. 197, 370.

Keywords: Crystallization; Pair Distribution Function analysis; Phase transition; In situ total scattering; X-ray diffraction; Synthesis

“Funded by the Deutsche Forschungsgemeinschaft (DFG – German Research Foundation), project number 429360100”

The local structures of four citric acid cross linked cyclodextrin nanosponges (CD); goethite (CDG), hydroxyapatite (CDHA) and goethite/hydroxyapatite (CDGHA) functionalized cyclodextrin nanosponges, prepared by ultrasound-assisted polycondensation polymerization, was studied the using atomic pair distribution function (PDF) technique. The PDFs were analyzed by comparing experimentally determined PDFs from samples under study and those from known control samples and the overall structural information extracted through visual inspections of PDFs. The samples do show the feature of cyclodextrin network linked by citric acid, with strong sharp peaks in the low-r region and weak broader peaks in the mediate and high-r region. CD and CDHA show the feature of polymer cyclodextrin, since there is only noise and density modulation after 13 Å in their PDFs, while the CDG and CDGHA show the feature of crystallinity (signal even after 13 Å), which is approximately the largest atomic distance in cyclodextrin. The short-range order, which is the spacing of neighbor glucose and glucose connected by citric acid networks, is similar for all samples despite their difference in crystallization. CD and CDHA have quite similar cyclodextrin network according to their similarity in PDF while CDG and CDGHA also have cyclodextrin network but contains crystalline goethite.