G. Heger

Laboratory X-ray powder diffraction: a comparison of different geometries with special attention to the usage of the Cu Kα doublet

In a laboratory X-ray powder diffraction study, the evaluation of the patterns of three Bragg–Brentano powder diffractometers with different monochromator geometries has been undertaken. For the measurements on each diffractometer, the standard reference material SRM 640 (silicon) and the corundum samples SRM 674a and SRM 1976 have been used. In each case, the peak profiles were fitted with a split Pearson VII function and the FWHM (full width at half-maximum) parameters and exponent m were determined for the left (lower 2θ) and the right (higher 2θ) sides of the Bragg peaks. It was found that there is a strong dependence of both the FWHM and the exponent m on the diffraction angle for the two configurations that included monochromators, whereas nearly constant values of m were found for the geometrically simplest diffractometer working without a monochromator. Finally, the two components of the Cu Kα doublet show systematically different peak profiles. There is a clear difference not only concerning the FWHM, which becomes more obvious at higher 2θ values, but also in the course of m with respect to the diffraction angle for the left and the right tails of the powder reflections. This is the main reason for the difficulties in Kα2 stripping and also in single-line-profile analysis when using the Kα doublet. Therefore, it is not surprising that this phenomenon, which can be explained by Heisenbergs uncertainty principle, does affect the reliability (represented by standard R values) of structure refinement by the Rietveld method.

Orientationa Disorder, the Orientational Density Distribution and the Rotational Potential in C60

Bragg intensities from neutron and X-ray diffraction data of C 60 single crystals were used to determine the nuclear- and electron-density distributions of C 60 at room temperature. The anisotropic density distribution is reconstructed by the maximum-entropy method and evaluated in terms of symmetry-adapted spherical harmonics. From this analysis, the orientational probability density function f(ω) has been calculated and the rotational potential V(ω) that is experienced by a C 60 molecule in the cubic surrounding at 295K has been obtained. f(ω) shows strong deviations from the uniform orientational probability density function that would result from isotropic rotation. Accordingly, V(ω) exhibits well developed minima. The absolute potential minimum is found at an Euler-angle set ω 1 and a second set of minima at slightly higher energy at ω 2 . The potential difference between V(ω 1 ) and V(ω 2 ) is 313 K, whereas the overall rotational potential barrier height amounts to 522 K. ω 1 and ω 2 are comparable with the major and minor orientations that are adopted by the molecules in their low-temperature arrangement. The angles ω 1 and ω 2 are fixed by the intrinsic geometry of the Euler-angle space (α, β, γ) under the combined action of the cubic site and the icosahedral molecular point group.

Structural phase transition and hydrogen ordering of TlH2PO4 at low temperature.

The crystal structure of TlH(2)PO(4) (TDP) has been studied at low temperature. The lattice parameters were derived from high-resolution X-ray powder diffraction in the temperature range between 8 and 300 K. A detailed crystal structure analysis of the antiferroelectric low-temperature phase TDP-III has been performed based on neutron diffraction data measured at 210 K on a twinned crystal consisting of two domain states. The structure model in the triclinic space group P\bar 1 is characterized by a complete ordering of all the H atoms in the asymmetric O-H...O hydrogen bonds. The phase transition from the ferroelastic TDP-II to the antiferroelectric TDP-III phase at 229.5 +/- 0.5 K is only slightly of first order and shows no detectable hysteresis effects. Its mechanism is driven by the hydrogen ordering between the partially ordered TDP-II state and the completely ordered TDP-III state. The polymorphism of TDP and the fully deuterated TlD(2)PO(4) (DTDP) is presented in the form of group-subgroup relations between the different space groups.

Crystal Structure of LiH2PO4 Studied by Single-Crystal Neutron Diffraction

Structure of a LiH 2 PO 4 (LDP) single crystal has been studied at room temperature and at 100 K by means of neutron diffraction. The LiO 4 tetrahedra in LDP are linked by vertices forming chains along the a -axis, and a three-dimensional network is formed by PO 4 groups connected by two different hydrogen bonds. The Li + ions in this structure were found to be quite immobile in spite of their very light mass; The very high proton conductivity in the system may have directly to do with the relatively weak hydrogen bonds and the relatively long interoxygen distances facilitating reorientation of the proton acceptor complexes, rather than with the Li + ionic motions.

Scientific Review: HEiDi: Single Crystal Diffractometer at the Hot Source of the FRM II

HEiDi is one of the two single crystal diffractometers at the neutron source Heinz Maier-Leibnitz (FRM II). The instrument is placed at the beam tube SR9b in the experimental hall. It was developed in collaboration between the RWTH Aachen (Institut für Kristallographie) and the TU München (FRM II) to cover a broad range of scientific cases in the area of structural research on single crystals.

Isotope Effect in TlH2PO4 and TlD2PO4

The crystal structure of the antiferroelectric phase of TlD 2 PO 4 , deuterated thallium dihydrogenphosphate, has been determined from single-crystal neutron diffraction data collected at room temperature. The para-antiferroelectric transition (T d c = 353K) of TlD 2 PO 4 is analysed from a structural point of view and compared with the phase transition of TlH 2 PO 4 at T 1 = 357 K, already characterized. The distinct phase sequences observed in the two compounds when decreasing temperature from that of the high-temperature prototype phase (prototype phase/room-temperature phase/low-temperature phase) are discussed and associated with the different ordering of the two crystallographically inequivalent H (D) atoms existing in the prototype phase.

Study of the structural phase transition in Gd2−xCexCuO4

Abstract The structural phase transition in Gd 2− x Ce x CuO 4 ( x = 0, 0.12) characterized by the rotation of the CuO 4 -squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd 2 CuO 4 and below 707.2(7) K for Gd 1.88 Ce .12 CuO 4 with an unusual temperature dependence. The rotation of the CuO 4 -squares is coupled to a spontaneous strain along the c -axis. Structure analysis of the undistorted high temperature T′-phase in Gd 2 CuO 4 indicates persisting local distortions.

Development of compact magnetostatic cavities for 3He spin filter cells

In order to preserve the 3He polarization in a spin filter cell used as a polarizer or analyser on a neutron beam line, the cell should be situated in a homogeneous magnetic guide field. Mu-metal magnetic shields are used to assure the stability of the guide field in the presence of moderate magnetic stray fields. Very compact (shoe box dimension) magnetostatic cavities made of mu-metal and permanent magnets have recently been constructed and optimized for rather large (13 cm long, 6 cm diameter) spin filter cells for use with hot neutrons. The relaxation time due to the magnetic gradients measured in the cells, situated in these compact boxes, is more than 500 h at 1 bar 3He pressure. Here we present the construction of such cavities and discuss the possibilities of further improvement in performance.

Oxygen defect structure in La2NiO4+δ

Abstract The structure of La2NiO4.13 has been studied as a function of temperature (20–673 K) with single crystal neutron diffraction. In order to achieve a high resolution, data sets were measured up to 1.05 A −1 . Structure refinements were carried out in this space group F4/mmm. Due to the incorporated oxygen on interstitial lattice sites the apical oxygen becomes disordered and the NiO6 octahedra are partly tilted around the [110] direction. The disorder is still present at temperatures above the high temperature orthorhombic/tetragonal phase transition of stoichiometric La2NiO4.00 at 670 K.

3He spin filters for spherical neutron polarimetry at the hot neutrons single crystal diffractometer POLI-HEiDi

3D vector polarisation analysis called also SNP (Spherical Neutron Polarimetry) is a powerful method for the detailed investigation of complex magnetic structures. The precise control of the incoming and scattered neutron polarisations is essential for this technique. Here we show an instrumental setup, that was recently implemented on the new single crystal diffractometer POLI-HEiDi at the FRM II for performing SNP experiments using two 3He spin filters for the production and for the analysis of the neutron polarisation. The design and optimisation procedure for the used spin filter cells are presented. Methods for in-situ measurements of the incoming polarisation as well as the particularities of the using two spin filters and corrections for the time dependent relaxation are discussed. Statistical precision of 1% has been achieved for the measurements of the polarisation matrix under the real experimental conditions using described cells and applying proposed correction method for the data.