The defect structure of oxygenated and reduced Nd2CuO~(y =4) was investigated by single-crystal neutron diffraction. Structural refinements indicate the presence of interstitial oxygen atoms in the "apical" O(3) position, directly above or below the copper atoms. The occupancy of the apical oxygen site is -0.10 (per formula unit) for the oxygenated sample and -0.04 for the reduced one. Both the in-plane

No abstract

Two powder samples of electrochemically oxidized La2Cu04+z (nominally 5=0.08 and 0.12) and one single crystal (5=0.1), with superconducting critical temperatures of 32, 42, and 40 K, respectively, were studied using neutron diffraction. All samples appear to be single phase, both at room temperature and at low temperature (10 -18 K), as evidenced by sharp Bragg peaks, indicating that these samples have compositions beyond the phase-separated region of the phase diagram. A detailed analysis of the Bragg rejections demonstrated that the basic crystallographic structure of all samples has Fmmm symmetry, with the excess oxygen located between adjacent LaO layers. However, a number of low-intensity peaks in the powder data suggested the existence of a very large superstructure. The satellites could be clearly identified in the single-crystal data, allowing the propagation vectors of the modulation to be determined. Rietveld refinements of the average structure, based on the main Bragg peaks, are presented here for samples prepared with this technique.

Multitemperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are reported. For the framework atoms reciprocal space structural refinements give total occupancies in the unit cell of Ga/Si=3.8/2.2, 1.8/14.2, 10.2/13.8 for the 6c, 16i, and 24k sites respectively, thus showing that Ga avoids the tetrahedral 16i positions. The guest atom displacement parameters obtained from structure factor fitting are analyzed with semianharmonic Einstein models giving Einstein temperatures (ΘE) of 69(1), 98(7), and 124(2) K for Ba(2)(100), Ba(2)[100], and Ba(1), respectively. The analysis furthermore suggests that all guest atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-guest interaction at high temperatures. The structure factors are subsequently used in the maximum entropy method calculations to obtain direct space nuclear densities. These are modeled with anharmonic one-particle potential models to fourth order. Even at elevated temperatures anharmonicity is limited indicating that the low thermal conductivity of the clathrate has a different origin.

Structural parameters derived from 9(1)K X-ray diffraction data and 13(1)K time-of-flight neutron diffraction data on perdeuterated tetraamminedinitronickel(II), Ni(ND3)4(NO2)2, are compared. It is shown that excellent agreement can be obtained for both positional and thermal parameters derived separately from the two experiments, provided that great care is taken in all steps of the process, including data collection, data reduction, and nuclear and electronic structure refinement. The mean difference in the thermal parameters, , is as low as 0.00034/~ 2 and <(AUij/cr)2> I/2 = 1.92, showing that, even without any form of scaling between the parameters, the same values can be obtained. This, compared with other such studies, indicates that time-of-flight neutron diffraction data can give structural information of a quality comparable to monochromatic neutron diffraction. The excellent correspondence between the thermal parameters derived separately from X-ray and neutron diffraction data gives confidence in the deconvolution of the thermal motion from the X-ray diffraction data, which is necessary for any study of a static electron density distribution.

Several tetraphenylethenes 2a, 4a–i with lipophilic side chains were synthesized and their mesomorphic properties were investigated. The most promising candidates turned out to be tetrakis[4‐(trisalkyloxybenzoyloxy)phenyl]ethenes 4e–i with chain lengths between C7 and C12. Compounds 4 were prepared by a convergent strategy employing a McMurry coupling of 4,4′‐dimethoxybenzophenone 1c, followed by demethylation and subsequent esterification of 2c with either 4‐decyloxybenzoic acid 3a, 3,4‐didecyloxybenzoic acid 3b, or O‐alkylated gallic acids 3c–i. Despite the twisting of the central tetraphenylethene moiety compounds 4c–i display hexagonal columnar mesophases according to differential scanning calorimetry (DSC), polarizing microscopy, and X‐ray diffraction studies.

The wavelength dependence of the neutron absorption cross section of hydrogen in [ReH3(C2H4) 2-{P(i-Pr)2Ph}2] was investigated by recording the decrease in Bragg peak intensity from a standard NaC1 crystal caused by attenuation of the incident beam by the Re sample. Data were collected on the Single Crystal Diffractometer (SCD) at the Intense Pulsed Neutron Source (IPNS, Argonne National Laboratory) which enabled concurrent measurements at a selection of wavelengths using the time-of-flight technique.To calculate the optimal absorption correction in a single-crystal neutron diffraction experiment, the linear absorption coefficient p of a sample should be determined directly by transmission measurements. In practice, however, it is common to calculate # from tabulated values of atomic absorption cross sections and knowledge of the size and contents of the unit cell.In this context hydrogen poses a problem, since its contribution to the attenuation of intensity is appreciable and not well characterized, although its cross section for true absorption is relatively small. This attenuation effect depends on the chemical environment of the bound hydrogen atom. It is hoped that the new results will be an improvement on the estimates currently used, and thereby be useful for future accurate structure determinations in areas such as electron density deformation experiments, where accurate corrections are of paramount importance (Coppens & Hall, 1983).The linear absorption coefficient is usually calculated from the expressionwhere #(2) is the linear absorption coefficient, V is the volume of the unit cell, n~ is the number of the ith type of atom in the unit cell and a,(2) is the effective absorption cross section (the sum of the true absorption and the incoherent scattering cross sections). The *And Chemistry and Materials Science and Technology Divisions, Argonne National Laboratory, Argonne, Illinois 60439, USA. summation runs over all elements present in the sample. The effective absorption cross section of hydrogen can be removed from this summation to givei~H where nn is the number of hydrogen atoms in the unit cell and a n is the effective absorption cross section of hydrogen. The summation now runs over all non-H elements, whose absorption cross sections are tabulated for a fixed wavelength and taken to be proportional to wavelength (Bacon, 1975) if the incoherent scattering cross sections are small, as is the case here.Hence (2) can be rewritten asi~H where ai (20) is the tabulated absorption cross section for the ith element at a wavelength 2o. Equation (3) is of the form where a = V/n n,i¢H These a and b are constants for a given compound and can be evaluated from crystallographic information and tabulated absorption cross sections.In this experiment p(2) is calculated from the measured attenuation through a known thickness of material using the Beer-Lambert law:where I is the path length in the sample. This principle has been used previously (Koetzle & McMullan, 1980) for crystals of organic molecules ...

No abstract