Getting More from Powder Diffraction Experiment: Modulation-Enhanced Diffraction

Introducing a periodic perturbation of structural parameters of a given frequency results in a modulated diffraction response that may have a complex frequency spectrum. Frequency analysis of the diffraction signal allows untangling contributions from the average and varying part of the scattering density as well as interference between them; both model and real proof-of principal experiments will be discussed. An analysis of advantages and drawbacks of the modulation approach will be given together with new opportunities for structure determination and refinement offered by MED for powder diffraction experiments with synchrotron radiation.

Phase separation in AyFe_2-xSe₂ (A= K, Rb, Cs) superconductors

The interplay between superconductivity, magnetism and crystal structure in iron-based superconductors has attracted a great interest in the recent years as it is considered to be the key for understanding the mechanisms responsible for high temperature superconductivity. Alkali metal intercalated iron chalcogenide superconductors (A122) exhibit unique behavior which is not observed in other iron-based superconducting materials such as antiferromagnetic ordering above room temperature and iron vacancies ordering. These materials have complex crystal structures with several phase transitions and are mixtures of phases even in the form usually described as a single crystal. A pronounced reversible phase separation revealed in A122 single crystals, as well as controversies regarding the origin of superconductivity and the stoichiometry and symmetry of the superconducting phase are still in the forefront of scientific activity. Here we will present a diffraction study of the crystal structures, antiferromagnetic ordering and intrinsic phase separation in alkali-metal iron chalcogenides [1]. The complementary scanning electron microscope study, including high-resolution electron back-scatter diffraction mapping will be also presented [2].

Chemical Selectivity in Diffraction by Statistical Analysis of in situ XRPD Data

X-ray diffraction methods in general allow only a limited chemical selectivity. Structural information on a subset of atoms can be obtained by a modulation enhanced diffraction (MED) experiment, using a periodic stimulus supplied in situ on a crystal, while diffraction data are collected several times within a stimulus period. The data are then treated by statistical methods such as phase sensitive detection (PSD) and Principal component analysis (PCA) techniques. The application of PSD to diffraction has been proposed as a tool to extract crystallographic information on a subset of atoms [1], thus allowing to introduce selectivity in diffraction. Simulated and experimental PSD-MED powder data were produced by using a TS-1 zeolite as spectator, in which Xe, acting as active species, is adsorbed and desorbed in a periodically modulated mode. By first demodulating these data, MED allowed to obtain the powder diffraction pattern of the active subset, i.e. to obtain selectively the crystallographic information on Xe, by solving the crystal structure of the active species out of the zeolite framework. The "real world" experiments indicated that the PSD-MED approach has some limitations related to its theoretical assumptions. PCA is widely used in spectroscopic analyses and was recently applied to XRPD data by some of us [2]. PCA was exploited to evaluate the in situ XRPD data quality, to speed up the data analysis and data pre-treatment required by PSD and improve the extraction of the substructure information from MED data. It resulted that the first two components obtained by PCA are related to the 1- and 2-omega patterns from PSD. The two approaches (PCA and PSD) are finally compared from the viewpoint of their capacity of gathering information on the Xe substructure inside the zeolite channels and used in a synergic way to obtain the optimal data analysis efficiency.

Structural and Magnetic Chirality of Cu₂OSeO₃

We determine the chirality of the magnetic and crystal structures, respectively, for the magnetoelectric insulator Cu₂OSeO₃ using small-angle diffraction of polarized neutrons and resonant contribution to X-ray single crystal diffraction of synchrotron radiation. This compound crystallizes in the P2₁3 space group similar to other chiral but metallic magnets, such as MnSi, MnGe, MnSi_1+xGe_x, Fe_1+xCo_xSi, Mn_1+xFe_xSi, Mn_1+xCo_xSi, FeGe, Mn_1+xFe_xGe. It has recently been shown that the structural and magnetic chiralities for metallic helimagnets are linked to each other [1], also in the so-called skyrmion phase [2]. Here we measure the spin chirality by comparing neutron scattering maps from Cu₂OSeO₃ with the reference MnSi, which has left-handed magnetic spiral and absolute crystal structure denoted as left-handed [1]. Similar to the reference MnSi system, the crystallographic chirality of Cu₂OSeO₃ is fixed on the basis of absolute structure determination taking into account the refinement of the Flack parameter. We find that the crystal and magnetic structures of Cu₂OSeO₃ have the same chirality. The similar relationship is found for MnSi, Mn_1+xFe_xSi, MnGe, while FeGe and Fe_1+xCo_xSi always show the opposite chiral correlation between magnetic and crystal structures. Notably, the relationship between two chiralities for Cu₂OSeO₃ found in the experiment is opposite to that proposed from recent theoretical calculations [3], thus calling for a revision of the theory of possible microscopic mechanisms contributing to the phenomenological antisymmetric magneto-lattice coupling.