LnHal2Hn ? Neue Phasen in den ternären Systemen Ln/Hal/H. I. Strukturen

LnHal₂H_n — New Phases in the Ternary System Ln/Hal/H. I. Structures Rare earth hydride halides of the compositions LaBr₂(H,D)_0.90, CeBr₂H_0.87, Cel₂H_0.90, Cel₂D_0.96, and GdI₂H_0.97 have been synthesised by heating either the dihalides, LnHal₂, or mixtures of LnHal₃ and LnH_x at 700°C ?800°C in hydrogen atmosphere. The heavy atom partial structures, as determined by X-ray scattering, correspond to the various sequences found in MoS₂ and NbS₂. From neutron scattering experiments the hydrogen atoms were located in the positions centering the anionic prisms and the cationic triangles.     

Metal fractionation of atmospheric aerosols via sequential chemical extraction: a review

This review surveys schemes used to sequentially chemically fractionate metals and metalloids present in airborne particulate matter. It focuses mainly on sequential chemical fractionation schemes published over the last 15 years. These schemes have been classified into five main categories: (1) based on Tessiers procedure, (2) based on Chesters procedure, (3) based on Zatkas procedure, (4) based on BCR procedure, and (5) other procedures. The operational characteristics as well as the state of the art in metal fractionation of airborne particulate matter, fly ashes and workroom aerosols, in terms of applications, optimizations and innovations, are also described. Many references to other works in this area are provided.     

Crystal refinement and magnetic structure of KNi4(PO4)3: a novel example of three interacting magnetic sub-lattices

KNi(4)(PO(4))(3) has been synthesised following a method previously reported by some of us and studied on the basis of magnetization and neutron powder diffraction (NPD) data. Magnetization measurements suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions: magnetization versus magnetic field curves present a remanent magnetization of around 2.15 micro(B) at T=2 K. The magnetic structure of the KNi(4)(PO(4))(3) has been determined at low temperature from the NPD data. These measurements show that there are three magnetic sub-lattices of Ni(2+) ions, which interact through common oxygen or phosphate groups, giving rise to FM and AFM couplings. The resulting interactions are FM in nature. Such a complex behaviour could provide an interesting model to analyse magnetic interactions in more condensed systems, such in mixed metal oxides.     

Ammonothermal Synthesis,X-Ray and Time-of-Flight Neutron Crystal-Structure Determination,and Vibrational Properties of Barium Guanidinate,Ba(CN3H4)2

We report the crystal structure of Ba(CN₃H₄)₂ as synthesized from liquid ammonia. Structure solution based on X-ray diffraction data suffers from a severe pseudo-tetragonal problem due to extreme scattering contrast, so the true monoclinic symmetry is detectable only from neutron powder diffraction patterns, and structure solution and refinement was greatly aided by density-functional theory. The symmetry lowering is due to slight deviations of the guanidinate anion from the mirror plane in space group P b2, a necessity of hydrogen bonding. At 300 K, barium guanidinate crystallizes in P2₁/c with a=6.26439(2) Å, b=16.58527(5) Å, c=6.25960(2) Å, and a monoclinic angle of β=90.000(1)°. To improve the data-to-parameter ratio, anisotropic displacement parameters from first-principles theory were incorporated in the neutron refinement. Given the correct structural model, the positional parameters of the heavy atoms were also refinable from X-ray diffraction of a twinned crystal. The two independent guanidinate anions adopt the all-trans- and the anti-shape. The Ba cation is coordinated by eight imino nitrogens in a square antiprism with Ba−N contacts between 2.81 and 3.04 Å. The IR and Raman spectra of barium guanidinate were compared with DFT-calculated phonon spectra to identify the vibrational modes.     

An Electron Diffraction and XRPD Study of Superlattice Ordering in the Elpasolite-Related Oxyfluoride K3MoO3F3

A detailed electron diffraction and XRPD study has been made of the room-temperature α polymorph of K₃MoO₃F₃. It is shown that the true symmetry of this polymorph is neither tetragonal, trigonal, nor triclinic as previously reported but rather monoclinic I1a1, a=2a_p−c_p, b=4b_p, c=a_p+2c_p when expressed in terms of the underlying elpasolite (ordered perovskite) parent structure type. A highly structured, three-dimensional, continuous diffuse intensity distribution (presumably arising from local O/F ordering and associated structural relaxation) is shown to coexist with the sharp satellite reflections characteristic of the monoclinic supercell.     

Investigation of antibody structures by scattering techniques

Summary Different experimental methods - hydrodynamical measurements, electronmicroscopy, X-ray crystallography, Yray and neutron small-angle scattering — provide data on the shape and dimensions of immunoglobulin molecules. Comparison of these data using model computation shows that the shape ofIgG immunoglobulin in dissolved state differs from that in crystalline state. The same statement is valid for the shape ofFab andFc fragments. Immunoglobulins of different origin have different dimensions. Correlation between the number of interchain disulphide bonds and the values of the radius of gyration may be observed. The value of the radius of gyration decreases when theIgG molecule binds haptens while the shape of the molecule seems to remain unchanged. This effect has a nonlinear dependence on the saturation of binding sites and may be connected with the effector function ofIgG molecule.     

Crystal structure analysis of β-tricalcium phosphate Ca3(PO4)2 by neutron powder diffraction

The crystal structure of sintered β-tricalcium phosphate, Ca₃(PO₄)₂, was refined using a high-resolution neutron powder diffraction data and the Rietveld method. This material was confirmed to have a rhombohedral structure (space group R3c, Z=21). Unit-cell parameters with higher precision (a=b=10.4352(2) Å, c=37.4029(5) Å, α=β=90°, and γ=120° in the hexagonal setting) and positional parameters for oxygen with equal precision were obtained by the neutron powder diffraction technique, compared with the single-crystal X-ray diffraction data by Dickens et al. (J. Solid State Chem. 10 (1974) 232). The site Ca(4) with atomic coordinates [0.0, 0.0, −0.0851(6)] was confirmed to be very different from the other four Ca sites: The position Ca(4) is three-fold coordinated with oxygen atoms, and has lower occupancy factor of 0.43(4), and a higher isotropic thermal parameter. On the contrary, each of the Ca(1), Ca(2), Ca(3), and Ca(5) is fully occupied by one Ca atom and these positions are coordinated with seven, eight, eight, and six oxygen atoms, respectively. The bond valence sums of Ca(4) and Ca(5) are lower (0.7) and higher (2.7), respectively, than the others (1.8-2.1).     

Polypyrrole - Semiconductor photovoltaic devices

Abstract

A new approach is given for obtaining fast electro-optical response and good angular-viewing characteristics in a liquid-crystal variable-retardation device. The improved characteristics make the new device well suited for use in field-sequential color systems.     

Raman Scattering and X-Ray Diffraction Studies on Alkali-Metal Intercalated Clays

Abstract

Vermiculite and fluor-tetrasilicic mica were intercalated with alkali metals (Na, K and Rb) by vapor transport. X-ray and Raman scattering experiments were conducted to study the structure and lattice dynamics of the new clay compounds. When Rb and K atoms were intercalated to the host materials, superlattices were formed. In Na intercalated compounds, however, no superlattice x-ray diffraction peaks were found. Raman spectra exhibited drastic changes after alkali-metal intercalation, reflecting the structural changes found by our x-ray diffraction.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001