Journal of Applied Crystallography最新文献

Natural optical activity (NOA), the ability of a material to rotate the plane of polarized light, has traditionally been associated with structural chirality. However, this relationship has often been oversimplified, leading to conceptual misunderstandings, particularly when attempts are made to correlate structural handedness directly with optical rotatory power. In reality, the relationship between chirality and NOA is more nuanced: optical activity can arise in both chiral and achiral crystal structures, and the sign of the rotation cannot necessarily be inferred from the handedness of the space group. In this work, we conduct a first-principles investigation of NOA in SiO₂ and AlPO₄ crystals, focusing on their enantiomorphic structural phase transition from high-symmetry hexagonal (P6₄22 or P6₂22) to low-symmetry trigonal (P3₁21 or P3₂21) space groups. This transition, driven by the condensation of a zone-centre Γ₃ phonon mode, reverses the screw-axis type given by the space-group symbol while leaving the sign of the optical activity unchanged. By following the evolution of the structure and the optical response along the transition pathway, we clarify the microscopic origin of this behaviour. We demonstrate that the sense of optical rotation is determined not by the nominal handedness of the screw axis given in the space-group symbol but by the atomic-scale handedness of the most polarizable atoms of the structure.

The conditions for the formation of a sequence of rotationally ordered phase states characteristic of the rare earth perovskites ABO₃ have been studied: Pm3m (C phase) → R3c (Rh phase) → Pnma (O phase). A necessary condition is the presence of crystal instabilities with respect to in-phase and antiphase rotations of the BO₆ octahedra, described by the order parameters ψ and ϕ, respectively. Both types of instability are caused by the tendency of the linear configuration of the B—O—B bonds to bend. Therefore, the degree of crystal instability with respect to the order parameters, ϕ and ψ, is not significantly different, and the order parameters themselves are competitors, where the occurrence of one prevents the occurrence of the other. Considering the results of phenomenological theory and analysis of phase transitions in rare earth aluminates, a scenario for the realization of this sequence has been developed and the factors responsible for this process have been identified. It is shown that the degree of deformation of the A—O and B—O bonds has a significant effect on the temperatures of the phase transitions. The important role of cation A in the realization of the Rh–O phase transition is noted. The process of formation of phase O during the Rh–O phase transition has also been studied.

A reverse Monte Carlo (RMC)/pair distribution function (PDF) method was used to reconstruct the disordered 3D atomic structure of simulated AuPd nanoparticles with composition profiles corresponding to increasing degrees of diffusional mixing. The quality of the reconstructions was assessed by comparing the PDFs calculated from the reconstructed and target structures. For each stage of diffusional mixing, weighted profile R factors between 6% and 7% were obtained, demonstrating that the diffusion process could be followed in considerable detail. These results suggest that RMC methods may offer a useful approach to the study of solid-state diffusion in core/shell nanoparticles and, more generally, of any nanostructured material that exhibits a continuously variable composition profile.

Ammonium dihydrogen phosphate (ADP) crystals of large dimensions (80 × 80 × 20 mm), to be used as X-ray beam expanders in the BEaTriX facility at INAF-OABrera, have been characterized on the ESRF BM05 beamline using the rocking curve imaging technique to test the planarity of the lattice planes. The difference in Bragg angles between the Si(111) monochromator and ADP(008) strongly affects the angular position of the diffraction peak as a function of sample position due to wavelength dispersion, which must be corrected in order to measure lattice curvature accurately. It is found that the diffracted image size exhibits a 4.5% elongation parallel to the scattering plane, an effect which has not been previously accounted for and which may lead to an incorrect wavelength dispersion correction. This contribution is critical for accurate measurement of resolution and lattice plane curvatures.