Miloš Kopecký

Location of Mn sites in ferromagnetic Ga1-xMnxAs studied by means of X-ray diffuse scattering holography

A three-dimensional image of the local neighbourhood of Mn atoms in a Ga 1-x Mn x As (x = 0.02) layer has been obtained by using X-ray diffuse scattering holography. The first and second nearest neighbours of the Mn atoms correspond to the local structure around Ga atoms in the zinc-blende GaAs structure. Accordingly, the Mn atoms are situated in substitutional positions.

X-ray diffuse scattering holography

It is shown that anomalous X-ray diffuse scattering can be treated as a hologram providing information on the local environment of the anomalous scatterer. In contrast with standard holography with atomic resolution, holographic oscillations of several percent of the total measured signal can be achieved by choosing suitable photon energies. The problem of virtual images can be solved very easily in the case of both centrosymmetric and non-centrosymmetric structures. Moreover, these holograms are not overlapped by dense and strong Kossel line patterns.

X-ray diffuse scattering in SrTiO3 and model of atomic displacements

The scattering intensity of synchrotron radiation in a compact three-dimensional volume of the reciprocal space (sinθmax/λ = 0.6283 A−1) of an SrTiO3 single crystal has been measured at temperatures of 115, 130 and 293 K. The curvilinear features of the diffuse pattern were modelled according to the displacements of both cations correlated along the axis [001]cub (where the subscript cub denotes the cubic structure). The results show that Sr2+ is displaced from the ideal positions along the crystallographic axis [001]cub, forming a short chain composed of about three Sr2+ ions along [001]cub, while the Ti4+ ions are moved in the directions [111]cub, or alternatively [101]cub or [011]cub. The Ti4+ displacements are also correlated along [001]cub, forming a similarly short chain.

A simple method of shielding area detectors from unwanted Bragg diffraction

Shielding of unwanted Bragg diffraction is performed by tiny permanent magnets made from SmCo5. The magnets are situated on a non-absorbing foil that is fixed tightly in a metal frame situated between the sample and the detector.

Modelling of cation displacements in SrTiO3 by means of multi-energy anomalous X-ray diffuse scattering

X-ray diffuse scattering of SrTiO3 has been measured at two photon energies, the first just below the absorption edge and the second far from the K absorption edge of strontium, in order to vary the atomic scattering factor of the strontium cations. It is shown that two different models of cation displacement comply with the single-energy diffuse scattering patterns, because single-energy diffuse scattering provides only ambiguous information on the directions of displacement of the Sr2+ and Ti4+ cations. However, the application of multi-energy anomalous diffuse scattering determines unambiguously that the Sr2+ cations are moved from their ideal positions in the [100] direction and the Ti4+ cations are shifted in {111} directions.

Imaging of interstitial atoms in Ga1−xMnxAs layers by means of X-ray diffuse scattering

The local atomic structure of a Ga1−xMnxAs (x = 0.07) layer during the annealing process was studied by means of X-ray diffuse scattering. The difference between the pair-distribution functions before and after annealing indicated the fraction of atoms that changed concentration and identified them to be exclusively interstitial atoms at the centres of gallium and/or arsenic tetrahedra in the GaMnAs unit cell.

X-ray diffuse scattering observations for SrxBa1−xNb2O6 single crystals with x=0.35 and 0.81

ABSTRACT In an attempt to contribute to understanding of structure–property relationship in ferroelectric relaxor SrBaNbO, we studied X-ray diffuse scattering for compositions with x = 0.35 and x = 0.81 at room temperature. Within signal-rich reciprocal-space images, features related to (i) local deformation due to chemical inhomogeneity, (ii) incommensurate modulation and (iii) fluctuation of polarization have been identified. There are substantial differences between the two compositions, some of which can be straightforwardly attributed to different states of ferroelectric ordering in these materials. In the Sr-rich composition, we have observed additional features in the form of -oriented streaks on planes. They indicate that transverse correlation of uniaxial polarization is anisotropic. At the same time, the fact that streaks appear also on the superlattice planes raises the question whether the modulation and polarization order parameter are interrelated.

Aperiodic atomic ordering in PMN recovered from the anomalous x-ray scattering

Various imaging techniques with atomic resolutions have been developed to study 3D arrangement of atoms in sub-nanometer and nanometer scale. The method presented is based on the measurement of the anomalous portion of x-ray scattering in large volume of the reciprocal space. It can be shown that, by analogy to x-ray holography with atomic resolution, the anomalous diffuse scattering provides information on both the amplitude and the phase of the scattered wave. The sample was PbMg1/3Nb2/3O3 (PMN), which belongs to relaxor ferroelectrics. Although the short and intermediate range chemical ordering is responsible for its relaxor behaviour, the real arrangement of cations remains unclear so far. The intensities of x-ray scattering in a large 3D volume of the reciprocal space were measured using an intense x-ray radiation generated by the synchrotron facility. Anomalous x-ray scattering has been obtained as a difference of intensity maps collected at two energies. The energy E1 just below the Nb K absorption edge was chosen in order to get a strong anomalous component in the scattering from Nb atoms. The data acquired at the energy E2 far from the absorption edge were used as the reference. The measured 3D pattern of anomalous scattering, including both discrete diffraction peaks and continuous x-ray diffuse scattering, allows us to reconstruct numerically an average environment around Nb atoms. The reconstructed real-space images clearly show a systematic local ordering of Mg2+ and Nb5+ cations. This ordering is more significant close to the central reference Nb atom but it is still apparent at the distance of about 3 nm giving thus an idea on the average size of ordered regions. More distant sites are occupied randomly with probabilities of occurrence of Mg2+ and Nb5+ cations given by their relative fraction in the sample. It is possible to determine conditioned probabilities of site occupancies by the given type of atom provided that the central site is occupied by Nb and to get information on the structure model of the ordered regions in the crystal.

When organic and metal–organic molecules cease to crystallize?

Structural Analysis of Hexa-4-methylimidazole nickel II 5aminoisopthalate tetrahydrate. Ersin Temel,a Alper Tolga Çolak,b Okan Zafer Yeşilel,c Handan Günay,c Orhan Büyükgüngör,a aDepartament of Physics, Faculty of Arts and Sciences, Ondokuz Mayýs University, 55139, Kurupelit/ Samsun, (Turkey). bDepartment of Chemistry, Faculty of Arts and Sciences, Dumlupýnar University, Kütahya, (Turkey). cDepartment of Chemistry, Faculty of Arts and Sciences, Eskişehir Osmangazi University, Eskişehir, (Turkey). E-mail: etemel@omu.edu.tr

Modulation in Tl2SeO4 in the temperature range 298–90 K

Synchrotron experiments have revealed a structural modulation in Thallium selenate, Tl2SeO4, in the temperature range 298–90 K. The modulation is manifested by the presence of the first-order satellites. In difference to the majority of β-K2SO4 compounds where the modulation takes place along the axis a (Pnma setting), the incommensurate modulation in Tl2SeO4 takes place along the b setting emphasizing exceptionality of Tl2SeO4 in this structural family. The modulation vector q = (0.0, 0.397(9), 0.0); the space group Pnma(0b0)000.