Journal of Applied Crystallography最新文献

A new, accurate method for fast and precise optical alignment of crystals in a diamond anvil cell (DAC) on a diffractometer has been developed. It enables highly accurate crystal alignment within instruments with a Boehler-Almax DAC design, achieving precision better than 0.02 mm easily. Other advantages of this method are simplicity, speed and instant visual feedback when aligning the crystal. This method employs Snell's law, which relates the angles of incidence and refraction of light passing through different media to estimate the crystal position within the DAC by measuring the apparent transverse dis-placement of the crystal image at various viewing angles after rotating the DAC by 180°. This information allows for fine-tuning of the crystal alignment within the DAC, ensuring optimal conditions for high-pressure diffraction experiments.

Atomistic interface structures compatible with periodic boundary conditions for the strain-induced subsolidus martensitic transition between quartz and coesite have been investigated. We identified layers of atoms that remained unchanged in terms of neighbor interactions throughout the transformation. Our analysis revealed that the orientation relationships between quartz and coesite, namely (1011)_Qz||(010)_Coe and (1321)_Qz||(010)_Coe, are consistent with experimental observations. Using density-functional-theory-based tight-binding model cal-culations, we determined an interface energy of approximately 660 mJ m^-2 for these interfaces and strain energies of 196 (6) and 2760 (160) J mol^-1 atom^-1 for the (1321)_Qz||(010)_Coe and (1011)_Qz||(010)_Coe oriented interfaces, respectively. To visualize these interface structures and facilitate their identification in experiments, we simulated high-resolution transmission electron microscopy images and electron diffraction patterns.

A crystallogenesis study of human transthyretin using the counter-diffusion method is described as an alternative to conventional convective vapour diffusion and batch approaches for protein crystallization. The X-ray diffraction results show systematic trends that exhibit unique patterns of crystallization and high crystal quality as well as a remarkable degree of coherence within extended crystal rods that wholly fill the capillaries used. Preliminary neutron diffraction data have been recorded from a number of these samples, validating the feasibility of this methodology for neutron crystallography.

Fast quenching dynamics in confined laser-induced microexplosions have been shown to lead to localized shockwaves that can create nanometre-scale domains in novel high-pressure crystalline phases. In the case of silicon, new silicon polymorphs such as bt8-Si and st12-Si have been recently observed, which are predicted to have bandgaps desirable for photovoltaic applications. Identification of these phases has been previously achieved by analysis of selected-area electron diffraction (SAED) patterns taken from laser-shock-affected areas. However, this analysis was complicated by pattern overlap from the many crystallites in the selected area, and many spots were found to agree with multiple potential phases. To overcome this ambiguity and enable the identification of the phase of Bragg spots observed in SAED patterns from polymorphic nanomaterials, we developed a new algorithm that we termed poly. This method is based on maximizing the magnitude and angular correlation between observed diffraction spots and those values derived from a known potential phase. We present the performance of this algorithm on simulated electron diffraction patterns as well as experimental SAED patterns measured from laser-shock-affected silicon samples. We find that the most abundant phases in the affected areas are t32-Si and t32*-Si and report on their relaxation into other high-pressure silicon phases over the course of 90 days after the laser-induced confined microexplosion.

Contrast-variation small-angle neutron scattering (CV-SANS) is a powerful tool to evaluate the structure of multi-component systems by decomposing the scattering intensities I measured with different scattering contrasts into partial scattering functions S of self- and cross-correlations between components. The measured I contains a measurement error ΔI, and ΔI results in an uncertainty in the partial scattering functions ΔS. However, the error propagation from ΔI to ΔS has not been quantitatively clarified. In this work, we have established deterministic and statistical approaches to determine ΔS from ΔI. We have applied the two methods to (i) computational data for a core-shell sphere, and experimental CV-SANS data of (ii) clay/polyethylene glycol aqueous solutions and (iii) polyrotaxane solutions, and have successfully estimated the errors in S. The quantitative error estimation in S offers a strategy to optimize the combination of scattering contrasts to minimize error propagation.

Diffuse scattering is a component of the powder pattern bearing information on the local atomic structure and disorder of crystalline materials. It is visible in the X-ray diffraction patterns of binary structures like Ag₂O, which has a large mean squared displacement for its constituent elements. Pair distribution function (PDF) analysis is widely employed to extract this local structural information, embedded in the widths of PDF peaks. However, obtaining the PDF from experimental data requires a Fourier transform, which introduces aberrations in the transformed data due to instrument resolution, complicating the distinction between its static and dynamic components. In this work, the analysis of thermal diffuse scattering is performed directly on the X-ray powder pattern, using the traditional Rietveld method integrated with a correlated displacement model for atomic pairs. The Ag₂O case study data were collected using synchrotron radiation at room temperature, supplemented by laboratory experiments up to 200°C. An Einstein model was used to obtain the harmonic and anharmonic force constants of the system. The force constants were also obtained via density functional theory and ab initio molecular dynamics simulations and showed similar values to the experiments. The analysis reveals the complex dynamic structure of Ag₂O, characterized by high anisotropy in phonon dispersion relations and the presence of soft phonon modes, which explain the significant displacement parameters observed. The proposed approach can be easily employed for other binary or more complex systems to understand the dynamics of local forces through X-ray diffraction analysis.

It is reported how the popular SHELXL-like weighting scheme leads to bias in residuals when the standard uncertainty (s.u.) values of the observed intensities are underestimated, which appears to be the case very frequently. An underestimation of the s.u. values by a factor of 5 destroys the symmetry of the residuals with respect to zero and leads to an average increase in U _equiv of 3.35% in a simulation.

Grazing-incidence X-ray diffraction (GIXD) is the technique of choice for obtaining crystallographic information from thin films. An essential step in the evaluation of GIXD data is the extraction of peak intensities, as they are directly linked to the positions of individual atoms within the crystal unit cell. In order to obtain reliable intensities independent of the experimental setup, a variety of correction factors need to be applied to measured GIXD raw data. These include the polarization of the incident beam, solid-angle variations, absorption effects, the transmission coefficient and the Lorentz correction. The aim of this work is to provide a systematic compilation of these intensity corrections required for state-of-the-art GIXD setups with static area detectors. In a first step, analytical formulae are derived on the basis of theoretical considerations. The obtained intensity corrections are then applied to measured GIXD raw data from samples with different textures, including a single crystal and thin films containing either randomly distributed or oriented crystallites. By taking advantage of the symmetries inherent in the different types of textures, integrated peak intensities are determined, and these are compared with intensities calculated from single-crystal diffraction data from the literature. Accurate intensity corrections promise an improved quality of crystal structure solution from thin films and contribute to achieving accurate phase and texture quantifications from GIXD measurements.

The newest eight members of the Editorial Board of Journal of Applied Crystallography are introduced.

If discrete or low-dimensional structure models are placed in large supercells in space group P1, the intrinsic periodicity of the Rietveld method is disrupted and their structural sites are scanned by millions of hkls. This allows a Rietveld-compatible calculation of diffuse scattering and small-angle scattering which is demonstrated here for a benzene molecule, a PbS quantum dot, a hydroxy-apatite nano-fibril and turbostratic carbon. Total scattering patterns are compared with the Debye scattering equation and accompanied by composite pair distribution function modelling using the same models.