Journal of Applied Crystallography最新文献

Bismuth nanostructures are quite attractive in the area of colorimetry and semiconductor nanoelectronics because of their multi-coloured luminescence and quantum confinement. In this work, detailed studies of the crystalline structure and surface morphology of bismuth nanostructures grown on a planar CaF₂/Si(111) surface have been carried out. The growth was performed using molecular beam epitaxy. The different surface morphologies of Bi on CaF₂ are demonstrated. With an increase in temperature from room temperature to 200°C, Bi undergoes a morphology change from planar to island-like, with a tendency to enlargement. At temperatures of 100–125°C, two types of nanowires with different geometric parameters are observed. Bi nano-island nucleation on a grooved and ridged CaF₂/Si(001) surface was also studied. X-ray diffraction analysis shows an ordered multiple-domain growth character in all cases. Precise analysis of nanowire and island faceting based on a combination of scanning and transmission electron microscopy and X-ray diffraction simulations has been carried out.

New mechanochemical preparations of three multicomponent crystals (MCCs) of the form MCl:urea·xH₂O (M = Li, Na and Cs) are reported. Their structures were determined by an NMR crystallography approach, combining Rietveld refinement of synchrotron powder X-ray diffraction data (PXRD), multinuclear (³⁵Cl, ⁷Li, ²³Na and ¹³³Cs) solid-state NMR (SSNMR) spectroscopy and thermal analysis. The mechanochemical syntheses of the three MCCs, two of which are novel, were optimized for maximum yield and efficiency. ³⁵Cl SSNMR is well suited for the structural characterization of these MCCs since it is sensitive to subtle differences and/or changes in chloride ion environments, providing a powerful means of examining H…Cl⁻ bonding environments. Alkali metal NMR is beneficial for identifying the number of unique magnetically and crystallographically distinct sites and enables facile detection of educts and/or impurities. In the case of NaCl:urea·H₂O, ²³Na magic-angle spinning NMR spectra are key, both for identifying residual NaCl educt and for monitoring NaCl:urea·H₂O degradation, which appears to proceed via an autocatalytic decomposition process driven by water (with a rate constant of k = 1.22 × 10⁻³ s⁻¹). SSNMR and PXRD were used to inform the initial structural models. Following Rietveld refinement, the models were subjected to dispersion-corrected plane-wave density functional theory geometry optimizations and subsequent calculations of the ³⁵Cl electric field gradient tensors, which enable the refinement of hydrogen-atom positions, as well as the exploration of their relationships to the local hydrogen-bonding environments of the chloride ions and crystallographic symmetry elements.

Dark-field X-ray microscopy (DFXM) is a novel X-ray imaging technique developed at synchrotrons to image along the diffracted beam with a real-space resolution of ∼100 nm and a reciprocal-space resolution of ∼10⁻⁴ radians. Recent implementations of DFXM at X-ray free electron lasers have demonstrated DFXM's ability to visualize the real-time evolution of coherent gigahertz phonons produced by ultrafast laser excitation of metal transducers. Combining this with DFXM's ability to visualize strain fields due to dislocations makes it possible to study the interaction of gigahertz coherent phonons with the strain fields of dislocations and damping of coherent phonons due to interactions with thermal phonons. For advanced analysis of phonon–dislocation interactions and phonon damping, a formalism is required to relate phonon dynamics to the strains measured by DFXM. Here, kinematic diffraction theory is used to simulate DFXM images of the specific coherent phonons in diamond that are generated by the ultrafast laser excitation of a metal transducer. This formalism is also extended to describe imaging of incoherent phonons of sufficiently high frequency to be relevant for thermal transport, offering future opportunities for DFXM to image signals produced by thermal diffuse scattering. For both coherent and incoherent phonons, opportunities are discussed for optimized sampling of reciprocal space and time for deterministic measurements through advances in the optics and excitation geometry.

The Ramachandran steric map of torsion angles (ϕ, ψ) introduced in 1963 has been widely used for protein structure validation and model building. Many developments in the field have made it essential to develop a utility to plot assorted types of maps for the following specific reasons: (i) to investigate different types (Gly, Val/Ile, pre/trans/cis-Pro and general) of 2D and 3D maps, addressing the diverse steric environments and frequency distribution of conformations, (ii) to examine polypeptides containing non-standard residues, (iii) for better visualization and analysis of conformational excursions and transitions in simulation, and (iv) to analyse torsion angles across three rotatable bonds such as preferred backbone-dependent rotamers. The utility RamPlot is accessible online (https://www.ramplot.in) and offline (via GitHub, https://github.com/mayank2801/ramplot and PyPI repository). It serves as a unique tool to draw and interpret a great variety of Ramachandran maps for natural and non-standard residues, which is otherwise unfeasible using existing tools and servers.

Dark-field X-ray microscopy (DFXM) is a novel diffraction-based imaging technique that non-destructively maps the local deformation from crystalline defects in bulk materials. While studies have demonstrated that DFXM can spatially map 3D defect geometries, it is still challenging to interpret DFXM images of the high-dislocation-density systems relevant to macroscopic crystal plasticity. This work develops a scalable forward model to calculate virtual DFXM images for complex discrete dislocation structure(s) (DDS) obtained from atomistic simulations. Our new DDS-DFXM model integrates a non-singular formulation for calculating the local strain from the DDS and an efficient geometrical optics algorithm for computing the DFXM image from the strain field. We apply the model to complex DDS obtained from a large-scale molecular dynamics simulation of compressive loading on single-crystal silicon. Simulated DFXM images exhibit prominent contrast for dislocation features between the multiple slip systems, demonstrating the potential of DFXM to resolve features from dislocation multiplication. The integrated DDS-DFXM model provides a toolbox for DFXM experimental design and image interpretation in the context of bulk crystal plasticity for a range of measurements across shock plasticity and the broader materials science community.