Jason P. Hodges

Conceptual design of a macromolecular neutron diffractometer (MaNDi) for the SNS

This paper describes the design criteria, calculations and simulations for a high-resolution macromolecular neutron diffractometer (MaNDi) for the Spallation Neutron Source (SNS). MaNDi is optimized to achieve 1.5 A resolution from crystals of 0.1–1 mm3 with lattice repeats in the range of 150 A. It was determined that locating MaNDi on a decoupled hydrogen moderator beamline with a curved guide will provide data of higher resolution and higher signal-to-noise than a coupled hydrogen moderator at the SNS. In addition, for an instrument with an initial flight path of 24 m at the 60 Hz source and a wavelength bandwidth of Δλ ≃ 2.7 A, bandwidth selection disk choppers can shift the wavelength range higher or lower for different experiments. With a wavelength range of 1.5–4.2 A and dmin = 2.0 A, simulations predict experiment duration times of 1–7 d, which is expected to revolutionize neutron macromolecular crystallography (NMC) for applications in the fields of structural biology, enzymology and computational chemistry.

Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

The layered perovskite NdBaCo2O5+δ (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577–852 °C and in 10−1 to 10−4 atm oxygen. The best fit to the data was obtained in the tetragonal (P4/mmm) space group. No oxygen atom vacancy ordering was observed that warranted a lowering of the symmetry to orthorhombic (Pmmm). Two P4/mmm structural models were investigated: Model 1 (no split sites) and Model 2 (split Nd and O2 sites). Transport of oxygen through the material via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen atom sites in the Nd layer. Total oxygen stoichiometry values were in the range 5.51 ≤ δ ≤ 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO2 at a given temperature due to increasing vacancy concentration in the Nd layer.

Sol–gel synthesis of the magnetically frustrated oxides Sr2FeSbO6 and SrLaFeSnO6

The use of the sol–gel synthesis method has been extended to include the preparation of stannates and antimonates from a citrate precursor. Samples of SrLaFeSnO6 and Sr2FeSbO6 have been prepared in a few hours. The structural and magnetic properties of the former are compared with those of a sample prepared over a period of several days by a solid-state route. The properties of Sr2FeSbO6 have been investigated by a combination of X-ray diffraction Mossbauer spectroscopy and SQUID magnetometry. The monoclinic perovskite structure has a partially (84%) ordered distribution of Fe and Sb cations over the six-coordinate sites; a transition to a magnetically frustrated phase occurs at 36 K.

Fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y (Ln=La, Nd; A=Ca, Sr)

The low-temperature (200–350 °C) fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y(Ln=La, Nd; A=Ca, Sr) using F2 gas, CuF2 , and NH4F is reported. The incorporation of large levels of fluoride ions is observed for each of these fluorinating agents. The general characteristics of each method are discussed, and it is shown that for this system, fluorination mainly occurs by insertion of fluorine for reaction with F2 , substitution of fluorine for oxygen for NH4F, and a mixture of the two processes for CuF2 . For A=Sr, it is assumed that fluorine inserts mainly between the two CuO2 layers, since large expansions of the unit cell along the c direction are observed. No evidence for bulk superconductivity has so far been observed after fluorination.

Strategies for solving neighboring-element problems: a case study using resonant X-ray diffraction and pulsed neutron diffraction to examine Sr8Ga16Ge30

The distribution of gallium and germanium over the available framework sites in the type-I clathrate Sr 8 Ga 16 Ge 30 (Pm3n) has been determined by powder diffraction using several different combinations of resonant scattering data sets, collected at energies close to both the Ga and Ge K-edges, and time-of-flight (TOF) neutron diffraction data. Based on a combined refinement using three X-ray data sets and a composition restraint, the fractional occupancies of the 6c, 16i and 24k sites by gallium are estimated to be 0.705 (5), 0.181 (3) and 0.376 (2), respectively. The required resonant scattering factors were determined by Kramers-Kronig transformation from X-ray absorption spectra. The results from refinements using single data sets and various combinations of data sets are compared. The high degree of scattering contrast that resonant diffraction can provide leads to very precise site occupancies. However, systematic errors in the resonant diffraction intensity data can considerably degrade the accuracy of the results. The use of a carefully chosen multiple-data-set strategy can minimize bias in the refinement results by reducing the correlations between site occupancies, atomic displacement parameters and histogram scale factors. The effect of errors in the resonant scattering factors on the refinement results was also examined.

Polar and Magnetic Layered A-Site and Rock Salt B-Site-Ordered NaLnFeWO6 (Ln = La, Nd) Perovskites

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T(N) ≈ 25 K for NaLaFeWO6 and at ∼21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = ((1/2) 0 (1/2)) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

POWGEN: a third-generation high resolution high-throughput powder diffraction instrument at the Spallation Neutron Source

Powgen represents a departure from previous designs for a time-of-flight powder diffractometer at a spallation neutron source and may be considered a third-generation design. The instrument is optimized for both parametric studies of materials under a wide range of conditions (T, P, H, flowing gases, etc) and ab-initio crystal structure determinations of complex solid-state materials with asymmetric unit-cells of the order {approx}1500 {angstrom}{sup 3}. The geometric design of the instrument allows for all detected scattered neutrons to be focused onto a single diffraction profile yielding high count rate while preserving good resolution {Delta}d/d = 0.0015 at a d = 1 {angstrom}.

Helical magnetism and structural anomalies in triangular lattice α-SrCr2O4

α-SrCr(2)O(4) has a triangular planar lattice of d(3) Cr(3+) made from edge sharing CrO(6) octahedra; the plane shows a very small orthorhombic distortion from hexagonal symmetry. With a Weiss temperature of - 596 K and a three-dimensional magnetic ordering temperature of 43 K, the magnetic system is quasi-two-dimensional and frustrated. Neutron powder diffraction shows that the ordered state is an incommensurate helical magnet, with an in-plane propagation vector of k = (0, 0.3217(8), 0). Temperature dependent synchrotron powder diffraction characterization of the structure shows an increase in the inter-plane spacing on cooling below 100 K and an inflection in the cell parameters at the magnetic ordering temperature. These anomalies indicate the presence of a moderate degree of magnetostructural coupling.

Investigation of the high-temperature redox chemistry of Sr2Fe1.5Mo0.5O6−δvia in situ neutron diffraction

Crystallographic structural changes were investigated for Sr2Fe1.5Mo0.5O6−δ, an electrode material for symmetric solid oxide fuel cells. The samples of this material were heated and cooled in wet hydrogen and wet oxygen atmospheres, to simulate the reducing and oxidizing conditions experienced under actual fuel cell operating conditions, and their structures and oxygen contents were determined using in situ powder neutron diffraction. The existence of a reversible tetragonal to cubic phase transition was established to occur between room temperature and 400 °C, both on heating and cooling in either oxygen or hydrogen. The oxygen content reaches a low value of 5.50(2) at 850 °C in wet hydrogen. Excellent correlations are observed between the oxygen content of the structure and the conductivities reported in the literature.

Magnetic and Structural Studies of the Multifunctional Material SrFe0.75Mo0.25O3−δ

SrFe0.75Mo0.25O3-δ has been recently discovered as an extremely efficient electrode for intermediate temperature solid oxide fuel cells (IT-SOFCs). We have performed structural and magnetic studies to fully characterize this multifunctional material. We have observed by powder neutron diffraction (PND) and transmission electron microscopy (TEM) that its crystal symmetry is better explained with a tetragonal symmetry (I4/mcm space group) than with the previously reported orthorhombic symmetry (Pnma space group). The temperature dependent magnetic properties indicate an exceptionally high magnetic ordering temperature (TN ∼ 750 K), well above room temperature. The ordered magnetic structure at low temperature was determined by PND to be an antiferromagnetic coupling of the Fe cations. Mössbauer spectroscopy corroborated the PND results. A detailed study, with X-ray absorption spectroscopy (XAS), in agreement with the Mössbauer results, confirmed the formal oxidation states of the cations to be mixed valence Fe(3+/4+) and Mo(6+).