BCC-Cu nanoparticles: from a transient to a stable allotrope by tuning size and reaction conditions.

Metallic copper generally adopts an FCC structure. In this work, we detect highly unusual BCC-structured Cu nanoparticles as a transient intermediate during the H2 reduction of a CuI precursor, [Cu4OtBu4], grafted onto the surface of partially dehydroxylated silica. The Cu BCC structure, assigned by in situ Cu K-edge XANES and EXAFS, as well as in situ synchrotron PXRD, converts upon heating into the most commonly found FCC allotrope. DFT calculations show that the BCC-Cu phase is in fact predicted to be more stable for small particles, and that their stability increases at lower H2 concentrations. Using this knowledge, we show that it is possible to synthesize BCC-structured Cu nanoparticles as a stable allotrope by reduction of the same grafted precursor either in 10% H2 diluted in Ar or 100% H2 at low temperature.

Methane storage in flexible metal-organic frameworks with intrinsic thermal management.

As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Although activated carbons, zeolites, and metal-organic frameworks have been investigated extensively for CH4 storage, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal-organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp(2-) = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp 'step'. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.

Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots.

Size and shape tunability and low-cost solution processability make colloidal lead chalcogenide quantum dots (QDs) an emerging class of building blocks for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead chalcogenide QDs are known to crystallize in the rock-salt structure, although with very different atomic order and stoichiometry in the core and surface regions; however, there exists no convincing prior identification of how extreme downsizing and surface-induced ligand effects influence structural distortion. Using forefront X-ray scattering techniques and density functional theory calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe QDs undergo a lattice distortion with displacement of the Pb sublattice, driven by ligand-induced tensile strain. The resulting permanent electric dipoles may have implications on the oriented attachment of these QDs. Evidence is found for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral shape with nonpolar {110} facets. On varying the nature of the surface ligands, differences in lattice strains are found.

Nanostructured Drugs Embedded into a Polymeric Matrix: Vinpocetine/PVP Hybrids Investigated by Debye Function Analysis.

Microcrystalline vinpocetine, coground with cross-linked polyvinylpyrrolidone, affords hybrids containing nanosized drug nanocrystals, the size and size distributions of which depend on milling times and drug-to-polymer weight ratios. Using an innovative approach to microstructural characterization, we analyzed wide-angle X-ray total scattering data by the Debye function analysis and demonstrated the possibility to characterize pharmaceutical solid dispersions obtaining a reliable quantitative view of the physicochemical status of the drug dispersed in an amorphous carrier. The microstructural properties derived therefrom have been successfully employed in reconciling the enigmatic difference in behavior between in vitro and in vivo solubility tests performed on nanosized vinpocetine embedded in a polymeric matrix.

Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy Elucidate the CO2 Promotion on a Commercial Methanol Synthesis Catalyst.

Optimal amounts of CO2 are added to syngas to boost the methanol synthesis rate on Cu-ZnO-Al2 O3 in the industrial process. The reason for CO2 promotion is not sufficiently understood at the particle level due to the catalyst complexity and the high demands of characterization under true reaction conditions. Herein, we applied operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy on a commercial catalyst to gain insights into its morphology and surface chemistry. These studies unveiled that Cu and ZnO agglomerate and ZnO particles flatten under CO/H2 and/or CO2 /H2 . Under the optimal CO/CO2 /H2 mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H2 O byproduct, enhancing the water-gas shift activity and thus the methanol production. Our results provide a rationale to the CO2 promotion emphasizing the importance of advanced analytical methods to establish structure-performance relations in heterogeneous catalysis.

Site-occupancy factors in the Debye scattering equation. A theoretical discussion on significance and correctness.

The Debye scattering equation (DSE) [Debye (1915). Ann. Phys. 351, 809-823] is widely used for analyzing total scattering data of nanocrystalline materials in reciprocal space. In its modified form (MDSE) [Cervellino et al. (2010). J. Appl. Cryst. 43, 1543-1547], it includes contributions from uncorrelated thermal agitation terms and, for defective crystalline nanoparticles (NPs), average site-occupancy factors (s.o.f.'s). The s.o.f.'s were introduced heuristically and no theoretical demonstration was provided. This paper presents in detail such a demonstration, corrects a glitch present in the original MDSE, and discusses the s.o.f.'s physical significance. Three new MDSE expressions are given that refer to distinct defective NP ensembles characterized by: (i) vacant sites with uncorrelated constant site-occupancy probability; (ii) vacant sites with a fixed number of randomly distributed atoms; (iii) self-excluding (disordered) positional sites. For all these cases, beneficial aspects and shortcomings of introducing s.o.f.'s as free refinable parameters are demonstrated. The theoretical analysis is supported by numerical simulations performed by comparing the corrected MDSE profiles and the ones based on atomistic modeling of a large number of NPs, satisfying the structural conditions described in (i)-(iii).

Large-scale oxygen order phase transitions and fast ordering kinetics at moderate temperatures in Nd2NiO4+δ electrodes.

Non-stoichiometric 214-nickelates with Ruddlesden-Popper (RP) type frameworks emerged as potential candidates for mixed electronic/ionic conductors in the intermediate temperature range. In this work we investigated structural aspects of the oxygen ion mobility diffusion mechanisms in non-stoichiometric Nd2NiO4+δ nickelates by X-ray (laboratory and synchrotron) as well by neutron diffraction. Temperature dependent synchrotron powder diffraction revealed a phase diagram of unprecedented complexity, involving a series of highly organized, 3D modulated phases related to oxygen ordering below 800 K. All phase transitionsimply translational periodicities exceeding 100 Å, and are found to be of 1st order, together with fast ordering kinetics. These surprising structural correlations, induced by the presence of interstitial oxygen atoms, suggest a collective phason-like oxygen diffusion mechanism together with dynamical contributions from the aperiodical lattice creating shallow diffusion pathways down to room temperature.

Charge fluctuations in the intermediate-valence ground state of SmCoIn5.

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn5 and other Ce-based heavy fermion materials, depends strongly on the efficiency with which f electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce3+ (4f1, J = 5/2) with Sm3+ (4f5, J = 5/2), we show that a combination of the crystal electric field and on-site Coulomb repulsion causes SmCoIn5 to exhibit a Γ7 ground state similar to CeCoIn5 with multiple f electrons. We show that with this single-ion ground state, SmCoIn5 exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of f holes below a temperature scale set by the crystal field, Tv ≈ 60 K. Our result provides evidence that in the case of many f electrons, the crystal field remains the dominant tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of "115" materials.

Sifting fluctuation scattering from microtextured samples.

As the relationship of texture and microtexture to fluctuation X-ray scattering (FXS) has been clarified in detail, key progress is expected in the exploitation of FXS-based structural investigation of matter exhibiting complex order.

Diffraction from Nanocrystal Superlattices.

Diffraction from a lattice of periodically spaced crystals is a topic of current interest because of the great development of self-organised superlattices (SL) of nanocrystals (NC). The self-organisation of NC into SL has theoretical interest, but especially a rich application prospect, as the coherent organisation has large effects on a wide range of material properties. Diffraction is a key method to understand the type and quality of SL ordering. Hereby, the characteristic diffraction signature of an SL of NC-together with the characteristic types of disorder-are theoretically explored.

Frustration, strain and phase co-existence in the mixed valent hexagonal iridate Ba3NaIr2O9.

Using detailed synchrotron diffraction, magnetization, thermodynamic and transport measurements, we investigate the relationship between the mixed valence of Ir, lattice strain and the resultant structural and magnetic ground states in the geometrically frustrated triple perovskite iridate Ba3NaIr2O9. We observe a complex interplay between lattice strain and structural phase co-existence, which is typically not observed in this family of compounds. The low temperature magnetic ground state is characterized by the absence of long-range magnetic order, and points towards the condensation of a cluster glass state from an extended regime of short range magnetic correlations.

Effects of Structural and Microstructural Features on the Total Scattering Pattern of Nanocrystalline Materials.

Atomic- and nanometer-scale features of nanomaterials have a strong influence on their chemical and physical properties and a detailed description of these elements is a crucial step in their characterization. Total scattering methods, in real and reciprocal spaces, have been established as fundamental techniques to retrieve this information. Although the impact of microstructural features, such as defectiveness of different kinds, has been extensively studied in reciprocal space, disentangling these effects from size- and morphology-induced properties, upon downsizing, is not a trivial task. Additionally, once the experimental pattern is Fourier transformed to calculate the pair distribution function, the direct fingerprint of structural and microstructural features is severely lost and no modification of the histogram of interatomic distances derived therefrom is clearly discussed nor considered in the currently available protocols. Hereby, starting from atomistic models of a prototypical system (cadmium selenide), we simulate multiple effects on the atomic pair distribution function, obtained from reciprocal space patterns computed through the Debye scattering equation. Size and size dispersion effects, as well as different structures, morphologies, and their interplay with several kinds of planar defects, are explored, aiming at identifying the main (measurable and informative) fingerprints of these features on the total scattering pattern in real and reciprocal spaces, highlighting how, and how much, they become evident when comparing different cases. The results shown herein have general validity and, as such, can be further extended to other classes of nanomaterials.

Dynamic magnetic crossover at the origin of the hidden-order in van der Waals antiferromagnet CrSBr.

The van-der-Waals material CrSBr stands out as a promising two-dimensional magnet. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below TN ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers. Furthermore, we unravel the low-temperature hidden-order within the long-range magnetically-ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal field. These take place upon cooling below Ts ≈ 100 K, until a spin freezing process occurs at T* ≈ 40 K. We argue this complex behavior to reflect a crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by its mixed-anion character. Our findings reinforce CrSBr as an important candidate for devices in the emergent field of two-dimensional magnetic materials.

Size and shape dependence of the photocatalytic activity of TiO2 nanocrystals: a total scattering Debye function study.

Nanocrystalline TiO(2) samples, prepared for smart textiles applications by the sol-gel technique in acidic or basic media, have been characterized by synchrotron X-ray powder diffraction and total scattering methods based on a fast implementation of the Debye function and original algorithms for sampling interatomic distances. Compared to the popular and widely used Rietveld-based approaches, our method is able to simultaneously model both Bragg and diffuse contributions and to quantitatively extract either sizes and size distribution information from the experimental data. The photocatalytic activity of the investigated samples is here systematically correlated to the average sizes and size distributions of anisotropically shaped coherent domains, modeled according to bivariate populations of nanocrystals grown along two normal directions.

Signature of antiphase boundaries in iron oxide nanoparticles.

Iron oxide nanoparticles find a wide variety of applications, including targeted drug delivery and hyperthermia in advanced cancer treatment methods. An important property of these particles is their maximum net magnetization, which has been repeatedly reported to be drastically lower than the bulk reference value. Previous studies have shown that planar lattice defects known as antiphase boundaries (APBs) have an important influence on the particle magnetization. The influence of APBs on the atomic spin structure of nanoparticles with the γ-Fe2O3 composition is examined via Monte Carlo simulations, explicitly considering dipole-dipole interactions between the magnetic moments that have previously only been approximated. For a single APB passing through the particle centre, a reduction in the magnetization of 3.9% (for 9 nm particles) to 7.9% (for 5 nm particles) is found in saturation fields of 1.5 T compared with a particle without this defect. Additionally, on the basis of Debye scattering equation simulations, the influence of APBs on X-ray powder diffraction patterns is shown. The Fourier transform of the APB peak profile is developed to be used in a whole powder pattern modelling approach to determine the presence of APBs and quantify them by fits to powder diffraction patterns. This is demonstrated on experimental data, where it could be shown that the number of APBs is related to the observed reduction in magnetization.

Mechanism of magnetization reduction in iron oxide nanoparticles.

Iron oxide nanoparticles are presently considered as main work horses for various applications including targeted drug delivery and magnetic hyperthermia. Several questions remain unsolved regarding the effect of size onto their overall magnetic behavior. One aspect is the reduction of magnetization compared to bulk samples. A detailed understanding of the underlying mechanisms of this reduction could improve the particle performance in applications. Here we use a number of complementary experimental techniques including neutron scattering and synchrotron X-ray diffraction to arrive at a consistent conclusion. We confirm the observation from previous studies of a reduced saturation magnetization and argue that this reduction is mainly associated with the presence of antiphase boundaries, which are observed directly using high-resolution transmission electron microscopy and indirectly via an anisotropic peak broadening in X-ray diffraction patterns. Additionally small-angle neutron scattering with polarized neutrons revealed a small non-magnetic surface layer, that is, however, not sufficient to explain the observed loss in magnetization alone.

Unravelling Magnetic Nanochain Formation in Dispersion for In Vivo Applications.

Self-assembly of iron oxide nanoparticles (IONPs) into 1D chains is appealing, because of their biocompatibility and higher mobility compared to 2D/3D assemblies while traversing the circulatory passages and blood vessels for in vivo biomedical applications. In this work, parameters such as size, concentration, composition, and magnetic field, responsible for chain formation of IONPs in a dispersion as opposed to spatially confining substrates, are examined. In particular, the monodisperse 27 nm IONPs synthesized by an extended LaMer mechanism are shown to form chains at 4 mT, which are lengthened with applied field reaching 270 nm at 2.2 T. The chain lengths are completely reversible in field. Using a combination of scattering methods and reverse Monte Carlo simulations the formation of chains is directly visualized. The visualization of real-space IONPs assemblies formed in dispersions presents a novel tool for biomedical researchers. This allows for rapid exploration of the behavior of IONPs in solution in a broad parameter space and unambiguous extraction of ​the parameters of the equilibrium structures. Additionally, it can be extended to study novel assemblies formed by more complex geometries of IONPs.

The MYTHEN detector for X-ray powder diffraction experiments at the Swiss Light Source.

The MYTHEN single-photon-counting silicon microstrip detector has been developed at the Swiss Light Source for time-resolved powder diffraction experiments. An upgraded version of the detector has been installed at the SLS powder diffraction station allowing the acquisition of diffraction patterns over 120 degrees in 2theta in fractions of seconds. Thanks to the outstanding performance of the detector and to the calibration procedures developed, the quality of the data obtained is now comparable with that of traditional high-resolution point detectors in terms of FWHM resolution and peak profile shape, with the additional advantage of fast and simultaneous acquisition of the full diffraction pattern. MYTHEN is therefore optimal for time-resolved or dose-critical measurements. The characteristics of the MYTHEN detector together with the calibration procedures implemented for the optimization of the data are described in detail. The refinements of two known standard powders are discussed together with a remarkable application of MYTHEN to organic compounds in relation to the problem of radiation damage.

Texture corrections for total scattering functions.

Many functional materials are today synthesized in the form of nanoparticles displaying preferred orientation effects to some small or large extent. The analysis of diffraction data of such kinds of systems is best performed in the framework of the total scattering approach that prescinds from translation symmetry assumptions. Therefore modified expressions were derived for the most common total scattering functions, in particular the Debye scattering equation (DSE) which yields the texture-averaged differential cross section as a function of atomic coordinates and texture parameters. The modified DSE encodes higher-order even spherical Bessel functions which account for the texture effect. Selection rules arising from experimental geometries and symmetries are discussed. In addition the duality of the texture effect is introduced showing the effects of texture on both the I(Q) and {\cal{G}}(r). The paper includes several definitions and appendices which are meant to be useful for those involved in the development of crystallographic computing.

Structure, Morphology, and Faceting of TiO2 Photocatalysts by the Debye Scattering Equation Method. The P25 and P90 Cases of Study.

Characterization of functional nanocrystalline materials in terms of quantitative determination of size, size dispersion, type, and extension of exposed facets still remains a challenging task. This is particularly the case of anisotropically shaped nanocrystals (NCs) like the TiO2 photocatalysts. Here, commercially available P25 and P90 titania nanopowders have been characterized by wide-angle X-ray total scattering techniques. Synchrotron data were modelled by the reciprocal space-based Debye scattering equation (DSE) method using atomistic models of NC populations (simultaneously carrying atomic and nanoscale structural features) for both anatase and rutile phases. Statistically robust descriptors are provided of size, morphology, and {101} vs. {001} facet area of truncated tetragonal bipyramids for anatase, jointly to polymorph quantification. The effects of using the proper NC shape on the X-ray diffraction pattern are analyzed in depth through DSE simulations by considering variable bipyramid aspect ratios (resulting in different {101} vs. {001} surface) and relative dispersion in a bivariate manner. We demonstrate that using prismatic NCs having equal volume and aspect ratio as bipyramids provides reasonably accurate sizes and {101} and {001} surface areas of the parent morphology.