Journal of Applied Crystallography最新文献

Time-of-flight neutron transmission and diffraction measurements were performed on four solid beryllium metal grades using the HIPPO neutron time-of-flight diffractometer equipped with an energy-resolved neutron imaging detector at Los Alamos National Laboratory. Analysis of the extracted total neutron cross sections from the transmission measurements revealed a reduction in the coherent elastic scattering component compared with the values predicted for an ideal polycrystalline material. This reduction could be explained by a lattice-plane-dependent extinction model, which underscores the need to account for extinction effects in neutron transmission measurements. The diffraction measurements rule out texture as the cause of the discrepancies between the calculated and observed cross sections. This study highlights neutron time-of-flight transmission as a sensitive method for investigating extinction phenomena.

We study X-ray diffraction in smectic liquid crystal multilayers. Such systems are fabricated as freely suspended films and have a unique layered structure. As such, they can be described as organic Bragg mirrors with sub-nanometre roughness. However, an interesting peculiarity arises in the diffraction from these structures: the characteristic shape of diffraction peaks associated with dynamical scattering effects is not observed. Instead, the diffraction can be well described kinematically, which is atypical for Bragg mirrors. In this article we investigate the transition between the kinematical and dynamical regimes of diffraction. For this purpose, we analyse the reflection of synchrotron radiation from a real liquid crystal sample with both kinematical and dynamical theories. On the basis of these theories, we derive a quantitative criterion for the transition from the kinematical to the dynamical regime. This, in turn, allows us to explain the peculiar diffraction behaviour in smectic films with thicknesses exceeding thousands of molecular layers.

Epitaxial β-Ga₂O₃ films grown on (001) diamond substrates with a β-(Al/Ga)₂O₃ buffer layer exhibit strong texture with multiple rotational domain variants sharing a common growth axis. This texture is attributed to two interrelated structural and geometrical factors: (1) pseudo-symmetry about the growth direction due to the high symmetry of the oxygen sublattice which results in close interplanar spacings of different lattice planes that determine the in-plane lattice mismatch, and hence two different crystallographic relationships and types of domain variants; (2) the lack of higher-order symmetry in the C2/m monoclinic structure of β-Ga₂O₃ and the higher symmetry of the diamond substrate that leads to various subvariants. The microstructure of the films consists predominantly of domain clusters, with domains rotated by either ∼60° or 120° (±10°) relative to each other about the growth axis ∼[102]. Some domain boundaries (DBs), visible near edge-on in cross-sectional and plan-view projections, exhibit a high degree of coherency. These highly coherent DBs with small lattice rotation and/or distortion near the DB are observed where small in-plane and off-plane DB lattice mismatch is expected. Larger lattice mismatch between domains is accommodated by relatively large lattice rotation and/or distortion near the DB, as well as changes in DB structure and shape. Understanding the origin of texture and the characteristics of common planar defects in β-Ga₂O₃ will offer insights into their impact on thermal and electrical transport properties and enable effective microstructural optimization for successful integration into future power electronics.