Martin Meven

Magnetic structure of EuFe2As2 determined by single-crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Magnetic structure ofEuFe2As2determined by single-crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Antiferromagnetic ordering and structural phase transition in Ba2Fe2As2 with Sn incorporated from the growth flux

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe2As2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Neel temperature T_N observed in the experiment.

The whole range of hydrogen bonds in one crystal structure: neutron diffraction and charge-density studies of N,N-dimethylbiguanidinium bis(hydrogensquarate).

N,N-Dimethylbiguanidinium bis(hydrogensquarate) features an impressive range of hydrogen bonds within the same crystal structure: neighbouring anions aggregate to a dianionic pair through two strong O-H···O interactions; one of these can be classified among the shortest hydrogen bonds ever studied. Cations and anions in this organic salt further interact via conventional N-H···O and nonclassical C-H···O contacts to an extended structure. As all these interactions occur in the same sample, the title compound is particularly suitable to monitor even subtle trends in hydrogen bonds. Neutron and high-resolution X-ray diffraction experiments have enabled us to determine the electron density precisely and to address its properties with an emphasis on the nature of the X-H···O interactions. Sensitive criteria such as the Laplacian of the electron density and energy densities in the bond-critical points reveal the incipient covalent character of the shortest O-H···O bond. These findings are in agreement with the precise geometry from neutron diffraction: the shortest hydrogen bond is also significantly more symmetric than the longer interactions.

Ab initio ORTEP drawings: a case study of N-based molecular crystals with different chemical nature

The thermal motion of atoms and functional groups is a key characteristic of any molecular crystal, and such motion derived from scattering experiments is conveniently visualised by means of thermal ellipsoids (the famous “ORTEP” drawings). Unfortunately, it is often impossible to obtain the underlying anisotropic displacement parameters (ADPs) for hydrogen atoms, due to their low X-ray scattering power, and sometimes no ADPs can be refined at all even for heavier atoms. In these cases, it would seem advantageous to estimate ADPs by first-principles techniques, and indeed such ab initio ORTEP plots have become available very recently. Here, we test this young method for a representative set of hydrogen-bonded molecular crystals: first, we study urea (CON2H4) as a well-known benchmark, then, its all-nitrogen analogue guanidine (CN3H5); finally, we move on to rubidium guanidinate (RbCN3H4) as a specimen with pronounced ionic interactions. For all three systems, ADPs have been obtained from density-functional theory (DFT) based phonon computations using the PHONOPY software. The results are compared with neutron-diffraction data as the experimental “benchmark” in this regard, and a critical discussion of experimental aspects is given. We observe excellent agreement between experiment and theory for the hydrogen-bonded systems urea and guanidine at low temperature, whereas high-temperature data for guanidine deviate visibly, and the more salt-like RbCN3H4 may suffer from a less-than-ideal description even at 12 K. Both are discussed in depth as there are possible solutions and directions for further research. Generally, the present results shine a favourable light on a future, more routine application of combined experimental/theoretical approaches in chemical crystallography.

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.

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.

Growth and characterization of large high quality brownmillerite CaFeO2.5 single crystals

The growth conditions of large and high quality single-crystals of CaFeO2.5 by the floating-zone technique in an image furnace are discussed. Structural characterization of the as grown single crystals have been carried out by neutron and X-ray diffraction as well as by HRTEM revealed the excellent quality in terms of composition homogeneity and crystalline quality. Magnetic measurements have been performed on oriented crystals by SQUID and neutron diffraction in the range of 5-700 K in order to clear up controversial discussions on possible magnetic phase transitions. The magnetic transitions reported elsewhere are discussed in terms of oxygen non-stoichiometries leading to multiphase CaFeO2.5+/-[small delta] related to the crystal growth conditions.

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.