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The General Structure Analysis System (GSAS-II) package provides materials and crystallographic analysis for all types of diffraction data. It was initially made available with very limited capabilities, but over much of the last decade the features have been expanded, so that GSAS-II is now a comprehensive tool for nearly all types of structural and materials characterization studies. The need to provide materials to teach use of GSAS-II, while the software has been undergoing constant revision and expansion, has required new approaches for documentation. This has included providing tutorials, as each major new capability has been added, and context-sensitive help for each section of the program. Comments in the code are also expanded into a software reference guide. Most recently, video versions of more than half of the tutorials were created and others were provided with animated graphics. All GSAS-II documentation is web-based.

The GSAS-II suite is a complete crystallographic analysis package for all types of X-ray and neutron diffraction data, suited for fitting models ranging from simple materials to proteins, and designed around an integrated graphical user interface. This article describes a new module within GSAS-II, GSASIIscriptable, which provides access to the GSAS-II data structures and an extensible mechanism to run a significant fraction of GSAS-II functionality, particularly for powder diffraction applications, from within Python scripts or directly from commands in a shell/batch script. This allows parallelization in a high-performance computing environment; near ideal speedup is seen with up to 240 simultaneous processes.

Several different approaches have traditionally been used for detection of X-ray powder diffraction patterns, including area detectors, point detectors and position-sensitive detectors. Each has advantages. This paper discusses use of a low-cost CCD detector attached to a diffractometer arm, where line-by-line readout of the CCD is coupled to continuous motion of the arm. When this type of detector is used and where X-ray optics are employed to focus the source image onto the detector plane both high-resolution and rapid measurements can be performed, with data collection over a complete 2θ range. This is particularly advantageous for synchrotron applications but valuable also for Guinier diffractometer laboratory instruments. Peak resolutions are shown to be moderately better than what can be obtained with a position-sensitive detector and significantly better than with an area detector. Many samples have intrinsically broadened peak shapes for which little improvement in data quality could be obtained with an analyzer-crystal detector. With comparable numbers of modules, these CCD data collection speeds can be close to those with position-sensitive detectors, but without the low-angle asymmetry seen in the latter.

The crystal structures of superconducting HgBa2CuO4+δ and HgBa2CaCu2O6+δ have been investigated with a pressure up to 0.6 GPa and HgBa2Ca2Cu3O8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c-axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a-axis. The one-layer compound, HgBa2CuO4+δ, shows the largest a-axis compressibility, while HgBa2Ca2Cu3O4+δ shows the smallest compressibility. The bond compressibilities of HgBa2CuO4+δ and HgBa2CaCu2O6+δ are significantly different from HgBa2Ca2Cu3O8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

8 pages; International audience; We used high-resolution neutron powder diffraction to accurately measure the atomic positions and bond lengths in biogenic and geological calcite. A special procedure for data analysis was developed in order to take into account the considerable amounts of magnesium present in all the investigated samples. As a result, in biogenic calcite we found some atomic bonds to have significantly different lengths as compared to those in geological calcite, after the contribution of magnesium is accounted for. The maximum effect (elongation up to 0.7%) was found for the C−O bonds. We also analyzed changes in frequencies and spectral widths of normal vibrations of carbonate groups in biogenic calcite (as compared to geological calcite) measured by Raman and Fourier transform IR techniques. Surprisingly, the frequency shifts after subtracting the magnesium contribution are close to zero. At the same time, substantial spectral broadening (up to 1.2%) in biogenic calcite as compared to geological samples was detected. Possible explanations for the experimental findings are discussed.

We used high-resolution neutron powder diffraction technique in order to accurately measure the atomic positions and bond lengths in calcium carbonates of biogenic (mollusk-made) and geological origin. As a result, in biogenic calcium carbonate we identified atomic bonds, first of all the C O bonds and some O O bonds, which obey significant modification (about 1%) with respect to those in geological calcium carbonate. Bond length changes are presumably due to the organic/inorganic interactions in natural bio-composites. Generally, the effect is more pronounced for aragonite, which is structurally more flexible (nine unfixed parameters in atomic positions) than calcite (one parameter of this kind only). The observed bond modifications can be a source of the reported changes in the frequencies of normal vibrations of the carbonate groups measured by Raman or Fourier-transform infrared spectroscopy (FTIR) techniques.

Time-of-flight neutron powder diffraction data for NIST Standard Reference Materials have been used to study the adequacy of the peak profile model obtained from a convolution of back-to-back exponentials with a pseudo-Voigt function that is widely used in Rietveld refinement. It is shown that, while the empirical models ford-spacing (wavelength) dependence of Gaussian and Lorentzian components of the pseudo-Voigt function and rise exponent are satisfactory, the behavior of the decay exponent and peak positions demonstrate significant deviations, which can be corrected by numerical methods. The practical side of this process as implemented inGSASandFULLPROFand the effect of the corrections on the Rietveld analysis results are discussed.

Atomic positions in geological aragonite and biogenic aragonite, obtained from several mollusk shells, were precisely measured by high-resolution neutron diffraction, which is more sensitive than X-ray diffraction to the positions of light atoms, C and O, in the CaCO3 structure. Comparable analysis of atomic positions revealed, for the first time, the changes in some bond lengths and atomic configurations in biogenic specimens with respect to geological ones. Most pronounced are modifications in the aplanarity of the carbonate groups. These modifications correlate with the observed shifts of the ν2 frequency in the IR absorption spectra. The changes in atomic positions as well as the changes in lattice parameters detected by high-resolution X-ray diffraction are attributed to the interaction between organic macromolecules and growing crystallites during biomineralization. The obtained results allow us deeper understanding of the specific routes used by nature for growing bio-composites with superior chara...

By using combinations of multiple protein powder diffraction patterns obtained from an image plate to ad-spacing of 2 Å, which differ by solvent-induced and radiation-damage-induced lattice strains, the powder overlap problem is partially resolved in a stereochemically restrained Rietveld refinement. The results for hen egg white lysozyme (HEWL) include, for the first time with powder data, placement of a substantial number of water molecules and structural results that approach the quality normally obtained by single-crystal methods. This study explores the lattice strains induced by changes in salt concentration, changes in solvent pH and the effect of low-dose radiation damage. For HEWL, lattice strains are not monotonic, so that with increasing NaCl concentration (0.25–1.25 M) theaaxis increases by ∼0.5%, while thecaxis decreases by ∼1.5%, and this variation is pH dependent. Low-dose radiation damage similarly induces non-monotonic lattice strains, similar but smaller than those arising from increased salt concentration. The effect of using these powder data in a combined Rietveld analysis is effectively to deconvolute the overlapping reflections by differing shifts in their relative positions.

We attempted to characterize by neutron powder diffraction the monoclinic α′ phase that is known to form at low temperatures in dilute Pu–Ga alloys. This attempt was unsuccessful, as we did not detect any transformation to the α′ phase, but instead observed a line-broadening effect in the fcc δ phase. This effect is large enough to be visible in the raw diffraction data and is highly anisotropic in crystal space. The onset temperature of the line broadening (150 K) coincides with previous observations of the δ–α′ transformation. Bulk α′ was not observed. We believe that the development of α′ nuclei creates a spatially inhomogeneous stress distribution in the δ matrix, which in turn exhibits an anisotropic response, governed by its elastic anisotropy. We have analysed this observation of anisotropic microstrains in terms of the fictive microstresses required to produce them by elastic deformation. During the course of this work, we found a pseudo-isotope effect in the room temperature lattice constants of ...

The binding of N-acetylglucosamine oligosaccharides (NAGn, n = 2-6) to hen egg-white lysozyme (HEWL; EC 3.2.1.17) was investigated by X-ray powder diffraction at room temperature. Each NAGn examined was found to bind to lysozyme in rapid-precipitation preparations in 1.0 M NaCl pH 6.0 buffer. The location of each NAGn was easily found from difference Fourier maps generated from structure factors extracted during preliminary Rietveld refinements. Full NAGn-protein structures were subjected to combined Rietveld and stereochemical restraint refinements (Rwp = 2.28-2.59%; Rp = 1.81-2.04%; RF2 = 3.91-5.80%) and revealed binding modes for NAGn that depended on the length of the NAG oligosaccharide. The NAG2 ligand was found in the BC sites in the cleft of HEWL, NAG3 was found to bind in both the ABC and BCD sites in the ratio 35:65 and NAG4 and NAG5 bound to the ABCD and ABCDE sites, respectively, while NAG6 only bound to sites ABCDE, leaving the F site empty with the remaining saccharide ring located in a solvent region adjacent to the A site. All protein powder diffraction patterns in this study consisted of extremely sharp Bragg peaks consistent with approximately 1 microm crystallites that were devoid of line-broadening defects. Details of the stereochemical restraints used in these refinements and their impact on structural validation are also discussed.

The co-deformation of Cu–Ag or Cu–Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic–plastic response, which is related to both magnet design and service life. The second derivative ∂2 σ/∂2 e of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. T...

We report on the temperature variation of the interatomic distances in isostructural U 2 Ni 2 In and U 2 Pd 2 Sn, both of which order antiferromagnetically at low temperatures. Both compounds exhibit complex non-collinear arrangements of the magnetic moments confined to the tetragonal basal plane, which is perpendicular to the shortest interuranium distance along the c -axis at low temperatures. The different temperature dependencies of the shortest interatomic links between uranium and the transition metal (Ni or Pd) provide evidence for the dual nature of 5f–d hybridization in these two compounds. We argue that magnetic ordering in U 2 Pd 2 Sn arises due to increased 5f–d hybridization (promoting stronger exchange) while the reduced hybridization in U 2 Ni 2 In allows for the formation of stable U magnetic moments.

Iron (III) complexes of the composition [FeX 2 (OPPh 3 ) 4 ][FeX 4 ], where X is Cl or Br were synthesised by several methods. It was found that each particular synthesis has yielded solids with slightly different powder patterns and also their comparison to those of calculated from single crystal data showed remarkable differences. A series of constrained Rietveld refinements has been performed on the title complexes to explain these differences. In a course of Rietveld refinement it was found that preferred orientation is not the only factor affecting the shape of the pattern and atoms parameters were included into refinement too. The results of Rietveld refinement allowed to conclude that the structures of compounds prepared by different methods are identical. In addition, it was shown that also in the case of rather complicated structure like the title complexes the powder diffraction method can lead to reasonable results.

The multiple-data-set capabilities of the Rietveld refinement program GSAS was used to extract a quantitative texture description from a simultaneous refinement of 52 time-of-flight neutron diffraction patterns taken from a NiTi polycrystalline sample held in a variety of orientations in the diffractometer. The patterns were analyzed using a generalized spherical-harmonic model for the preferred orientation. With this method the accuracy of the available crystal structure parameters of the monoclinic B 19’ phase of near stochiometric NiTi (50.14 atomic \% Ni) system at room temperature has been improved dramatically.

In an earlier paper (Lawson et al., 2000, Phil. Mug. B, 80, 53) we described the measurement and interpretation of Debye–Waller factors in a δ-phase Pu0.98Ga0.02 alloy. We concluded that the measurements could be described using a Debye model with a strongly temperature-dependent Debye-Waller temperature. In those experiments, we observed significant diffuse scattering in the neutron powder diffraction patterns. In this paper we describe a new technique for analysing diffuse scattering based on Rietveld analysis, and apply the technique to data for δ-phase Pu0.98Ga0.02 and for Pb. The results from the correlations among lattice vibrations show that the diffuse scattering is Warren–Borie scattering and that the diffuse scattering is in basic agreement with that expected from the Debye model except that the scattering from the nearest-neighbour correlations in Pu0.98Ga0.02 is anomalous at high temperatures. We also discuss the observation of significant anisotropic broadening of the diffraction pea...

High pressure Raman and neutron scattering study of carbon blacks and highly oriented pyrolytic graphite is reported. It is found that carbon black particles are composed of graphitic nanocrystallites and amorphous carbon. Pressure-induced order in inter-atomic distances within nanocrystallites is completely reversible. Relative concentration of amorphous carbon decreases slightly with increased pressure. This process differs from temperature induced transformation of amorphous carbon into ordered carbon. Post-production treatment at high temperatures results in lateral and vertical growth of graphitic crystallites, and at sufficiently high temperatures almost all amorphous carbon is transformed into graphitic structures. Within the pressure range under study, 5 GPa, only a small fraction of amorphous carbon is transformed into ordered structures. Pressure induced frequency shift of the E2g bands of various carbon blacks is explained in terms of a modified intermolecular potential.

By combining high-resolution X-ray powder diffraction data and stereochemical restraints, Rietveld refinement of protein crystal structures has been shown to be feasible. A refinement of the 1261-atom protein metmyoglobin was achieved by combining 5338 stereochemical restraints with a 4648-step (dmin= 3.3 Å) powder diffraction pattern to give the residualsRwp= 2.32%,Rp= 1.66%,R(F2) = 3.10%. The resulting tertiary structure of the protein is essentially identical to that obtained from previous single-crystal studies.

Theab initiostructure solution of sodiumpara-hydroxybenzoate from high-resolution X-ray powder diffraction data is reported. The compound is of interest with respect to understanding the mechanism of Kolbe–Schmitt type reactions. It crystallizes in space groupP21,Z= 2, with unit-cell parametersa= 16.0608 (3),b= 5.38291 (9),c= 3.63834 (6) Å, β = 92.8692 (5)° andV= 314.153 Å3. The compound consists of layers of distorted NaO6prisms perpendicular to theaaxis and phenol rings perpendicular to these layers pointing up and down. The molecular structure is held together by van der Waals forces between the phenyl groups of different layers and additional hydrogen-bridge bonding between the phenolate oxygen atoms. The sample showed powder peak widths which are not a smooth function of diffraction angle; a recently implemented phenomenological model was able to describe this effect sufficiently well to obtain excellent Rietveld fits to the data. The accuracy of modeling the data makes this one of the rare cases where the position of a hydrogen atom could be unambiguously determined by powder techniques.

This report discusses a new method to determine experimental pressure–volume–temperature (P–V–T) data and to correct misalignment of diffraction optics using thermoelastic equations of state of multiple phases. Analytical approaches as well as numerical approaches are presented for the technique. A number of phase combinations yield the best experimental result and the whole-pattern Rietveld and/or Le Bail refinement of diffraction patterns of multiple phases is a significant aspect of the technique. Specific examples indicate that the corrected pressure, temperature and diffraction optics are robust. Errors are estimated from the uncertainties of the measured cell dimensions. The error propagation for different experimental cases is discussed, along with the conditions and limitations of the method. This technique forP–V–Tdetermination and the correction of diffraction optics is crucial for experiments conducted under extreme conditions of high pressure, high temperature and dynamic reactions.

A set of general guidelines for structure refinement using the Rietveld (whole-profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld refinement are discussed with a view to guiding newcomers in the field. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features specific to data from neutron (both constant-wavelength and time-of-flight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) refinement of profile parameters, (v) Fourier analysis with powder diffraction data, (vi) refinement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation ofRvalues and (x) some common problems and possible solutions.

When multiple elastic diffraction peaks are obtained from an x-ray or neutron source, data analysis is commonly performed using a Rietveld refinement applied to the entire pattern, rather than simply performing single peak fits. In the simplest case the crystal structure is assumed to be ideal despite the presence of stresses which, coupled with the elastic and plastic anisotropy of individual grains, can result in a nonisotropic response of the polycrystal. A first step to account for this anisotropy in the refinement is to include an anisotropic strain parameter. In an earlier work [J. Appl. Phys. 82, 1554 (1997)] we included elastic anisotropy into a Rietveld refinement and discussed its validity in the elastic and plastic regimes for a cubic crystal structure. Here we extend the discussion to include anisotropy in hexagonal crystal structures. The agreement between single peak fits and the Rietveld refinement modeled single peak positions is considered for hexagonal close packed beryllium in the prese...

A method of rapid capillary sample loading of either dry powder or polycrystalline slurry has been developed. By embedding a Millipore filter in a Kapton capillary tube, one can then draw either loose dry powder or polycrystalline slurry into the tube and pack the solid against the filter, creating a dense column of powder suitable for a Debye–Scherrer powder diffraction experiment.

Macrostrain variations in engineering components are frequently examined using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement time. At reactors, macrostrain behavior is inferred from a single lattice reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordingly it is natural to fit the entire diffraction spectra using a Rietveld [J. Appl. Cryst. 2, 65 (1969)] refinement. This means that an idealized crystal structure is fit to the measured distorted crystal structure, which includes deviation of the measured lattice reflections from the ideal due to elastoplastic strain anisotropies, which are dependent on the particular lattice reflection (hkl) considered. We show that elastic macrostrains calculated from lattice parameter changes in Rietveld refinements (without accounting for hkl dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source. Moreover good refinements on the complete pattern are obtained with short measurement times compared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expected in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these arguments is demonstrated by analysis of a uniaxial tensile load test and a reanalysis of previously reported data taken on a deformed stainless steel ring. The plastic strain predictions compare favorably with a finite element model.

The crystal structure of the two phases of chlorodifluoromethane (CHClF2) were determined at 70 K and 10 K using Rietveld refinement of neutron powder profiles. The upper phase is tetragonal P42/n (C4 4 h) with eight molecules in the unit cell occupying general positions. The lattice constants are a = 10•3711(1) A and c = 5•5915(2) A. The lower phase is monoclinic P112/n (C2 4 h) with eight molecules in the unit cell with dimensions a = 10•1106(2), b = 10•4830(2), c = 5•5868(2) A and γ = 90•319(2)°. The asymmetric unit has two molecules in general positions. The phase transition is of a displacive type but there are no large changes in the molecular positions or orientations at the transition.

The crystal structure of Pr{sub 12}O{sub 22} was determined utilizing Rietveld analysis of powder neutron diffraction data (P2{sub 1}/c, {alpha} = 6.6850(2) {Angstrom},b = 11.6004(4) {Angstrom}, c = 12.8271(4) {Angstrom}, {Beta} = 99.974(1){degrees}, V = 979.67(8) {Angstrom}{sup 3}; total variables 130, R{sub p} = 3.25%, wR{sub p} = 5.02%). This structure is different from the models previously proposed, and the basic structural element is the isolated oxygen vacancy, as observed in another more-oxidized member of the R{sub n}O{sub 2n-2} series, Tb{sub 11}O{sub 20}.

The crystal structure of the lower-temperature polymorph of the n = 9 member of the homologous series Pr{sub n}O{sub 2n-2}, Pr{sub 9}O{sub 16} ({zeta})m was determined utilizing Rietveld analysis of neutron time-of-flight powder diffraction data.The refined structure (P{bar 1}, a = 6.7396(8) {angstrom}, b = 8.711(1) {angstrom}, c = 6.6726(8) {angstrom}, a = 97.424(1) {angstrom}, {beta} = 99.973(1){degrees}, {gamma} = 75.301(1){degrees}, V = 371.7(1) {angstrom}{sup 3}; Z = 1; R{sub p} = 3.29%, and wR{sub p} = 4.64%) is an agreement with the model proposed on the basis of high-resolution electron microscopy. This structure is a superstructure of fluorite (F). The structural element is a pair of vacant oxygen sites separated by 1/2[111]{sub F} across a metal atom. These units are further apart from each other than those in the more reduced member of the series, Pr{sub 7}O{sub 12}. The existence of a higher-temperature form of Pr{sub 9}O{sub 16} was not confirmed.

The General Structure Analysis System, GSAS, has recently been modified to include magnetic neutron-scattering cross-sections. Low-temperature diffraction data have been taken on the hexagonal noncollinear antiferromagnet UPdSn on both the HIPD and the NPD powder diffractometers at LANSCE. The low-resolution data reveal that the magnetic structure has orthorhombic symmetry between 23 K and 37 K, and monoclinic symmetry below 23 K. The high-resolution data reveal that there are structural distortions with corresponding symmetry changes in each of these phases, while the paramagnetic phase above 37 K remains hexagonal. Using GSAS, we have refined data sets from both diffractometers simultaneously, including both magnetic and structural cross-sections. The magnetic results from Rietveld refinement are in good agreement with model fitting to the integrated intensities of seven independent magnetic reflections and these, in turn, agree with measurements made on the same sample using the constant-wavelength reactor technique.

In powder-diffraction measurements made by elastic scattering of thermal neutrons, diffuse scattering shows up as a weak, oscillating component in the scattering data. For years, we observed such diffuse scattering from plutonium (Pu) and its alloys, but we did not know how to analyze it. This analysis is important for any material to account for all scattering, and it is especially important for Pu because of its role in the nuclear weapons program. We used neutron pow-der diffraction at Los Alamos Neutron Science Center (LAN-SCE) to help us discover the cause of diffuse scattering in our experiments. We analyzed the data with tools recently incorporated into the General Structure Analysis System (GSAS)—a powder-diffraction data-analysis package used to refine and determine crystal structure. We found that the observed diffuse scattering comes from lattice vibrations (not from some other exotic mechanism) and is quite an ordinary, non-Pu-specific phenomenon.

Neutron-diffraction investigations of powder and consolidated ultrafine-grain-sized chromium samples indicate that antiferromagnetic order in the body-centered-cubic phase of this material can be suppressed to well below the Neel temperature of coarse-grained and single-crystal chromium. The suppression is correlated strongly with decreasing grain size. Antiferromagnetic order was not observed in powder or in consolidated samples with grain sizes less than 16 nm, indicating that free surfaces and grain boundaries play the same role in preventing antiferromagnetic order with the structure of bulk chromium. Antiferromagnetic order was observed in nanocrystalline samples with grain sizes greater than 19 nm at 20 K. No correlation is seen between the Neel temperature and the degree of long-range microstrain, or with contents of light-element impurities in the samples. Even in cases where antiferromagnetic order is detected, the transversely polarized [ital AF][sub 1] spin-density-wave magnetic phase is never seen. While this may suggest that spin-density-wave phases do not occur in nanocystalline chromium, the presence of the longitudinally polarized [ital AF][sub 2] spin-density-wave phase cannot be ruled out unambiguously.

The crystal structure of Mg(OD)2 has been refined from time-of-flight (TOF) neutron diffraction data. Mr = 60.304, trigonal, P{\bar 3}m1, a = 3.1455 (1), c = 4.7646 (3) A, V = 40.831 (4) A3, Z = 4, Dx = 2.453 g cm−3, neutron time of flight, Rietveld refinement, Rwp = 3.84%, Rp = 2.83%, reduced χ2 = 2.98 for 70 variables, data collected at 305 K. The O–D bond length is 0.937 (1) A (0.956 A corrected for `riding' motion). The use of a pseudo-Voigt profile function to allow for strain and particle-size broadening in TOF neutron diffraction of fine anisotropic powders is described.

We present a feasibility study to extract quantitative texture and precise crystal structure information of polycrystalline monoclinic NiTi shape-memory alloys from a simultaneous refinement of 52 time-of-flight neutron-diffraction patterns taken in 13 orientations in the diffractometer. The multiple-data-set capabilities and the generalized spherical harmonic texture model of the GSAS program system were employed.

The crystal structure of phase III of chloroiodomethane-d 2 was determined at 13 K using Rietveld refinement of neutron powder profiles. The space group is Pnma (D16 2h) and there are four molecules in its orthorhombic unit cell with dimensions a = 6·383(4) Å, b = 6·706(1) Å and c = 8·867(4) Å. The molecules form a 'herringbone' pattern in layers perpendicular to the b axis, similar to that in the halogen and the hydrogen halides crystals.

The structures of Tb[sub 7]O[sub 12] and Tb[sub 11]O[sub 20] have been determined by Rietveld analysis of high-resolution neutron diffraction data. Tb[sub 7]O[sub 12] is rhombohedral of space group R[bar 3] with a = 6.5082(3), [alpha] = 99.3420(1), V = 263.32(3), and Z = 1. This structure is isomorphous with Pr[sub 7]O[sub 12]. Tb[sub 11]O[sub 20] is triclinic of space group P[bar 1] with a = 6.50992(4), b = 9.8298(6), c = 6.4878(4), [alpha] = 90.019(2), [beta] = 99.966(1), [gamma] = 95.881(1), V = 406.68(7), and Z = 1. Its structure is different than previously proposed. The vacant oxygen coordination sites are not paired across metal atoms. Establishment of these structures will contribute to unraveling the structural principle underlying oxygen-deficient fluorite-related structures. 25 refs., 3 figs., 6 tabs.

The structure of cyanamide ND2CN, was determined at 13K using Rietveld refinement of neutron powder profiles. The space group was confirmed to be Pbca with 8 molecules in a unit cell with dimensions a = 6·559(2), b = 9·169(2) and c = 6·775(2) A. Molecules are linked in a three dimensional array by hydrogen bonds. A satisfactory model for the observed structure was developed using standard exp-6 potentials for all interactions except D-N which was represented by a modified exp-6 potential, a collection of point charges or a simple attractive exponential potential.

The intermetallic compound UPdSn has been studied by means of dc magnetic susceptibility and neutron diffraction. While it has been assumed in previous work that the space group is P6 3 /mmc and there is disorder between the Pd and Sn atoms, we find a significantly better fit to our data with the related space group P6 3 mc which allows Pd/Sn ordering. There are two magnetic phase transitions at approximately 25 and 40 K. Above 40 K, there is no long-range magnetic order, but between 25 and 40 K, a canted antiferromagnetic structure forms, in which the moments lie in, or close to, the y-z -plane of a double-sized orthorhombic cell, with an angle of approximately 54° away from the z -axis. Below 25 K, the moments rotate out of the y-z -plane by approximately 45° to form a second related non-collinear structure. At the lowest temperature measured, the uranium moment is (2.05±0.13)μ B .

Powdered samples of {beta}-MoO{sub 3} have been produced by the gentle heat treatment of freeze-dried molybdic acid at 350{degree}C for 1 hr. The samples, yellow-green in appearance, contained varying amounts of the thermodynamically stable {alpha}-MoO{sub 3}, depending upon the time and temperature of heat treatment. Neutron diffraction data were collected at 300 K. all peaks, not attributable to {alpha}-MoO{sub 3}, were indexed on the basis of a monoclinic cell, P2{sub 1}/c, {alpha} = 7.1228(7), b = 5.3660(6), c = 5.5665(6), {beta} = 92.01(1){degree}, V = 212.62(6){angstrom}{sup 3}. The structure, which is related to ReO{sub 3}, contains two crystallographically independent octahedra. Both show evidence of disorder at the Mo and O sites. Two distinct orientations of a short mo-O distance, suggestive of the type of molybdenyl bond observed in both the {alpha} and {beta}{prime}-forms, are primarily responsible for the observed disordering.

The structure of the newly discovered “1201” lead cuprate superconductor (Pb, Cu) (Sr, La) 2 CuO 5−δ with T c =27.5 K at onset and a shielding fraction 38% at 5 K and 10 Oe, has been determined using neutron powder diffraction. The structure is similar to the other 1201 materials TlBa 1.2 La 0.8 CuO 5 and Tl 0.5 Pb 0.5 Sr 2 CuO 5 (where the former superconductors with a T c of K and the latter is not superconducting), belonging to the space group P4/mmm. The doping scheme in (Pb, Cu)(Sr, La) 2 CuO 5−δ combines the doping scheme used in these two compounds, in that both the TI and Sr sites are doped. The starting stoichiometry, the refined scale factors for the impurity phases and the refined site occupancies for oxygen suggests that the stoichiometry (relative to Cu) of the superconducting phase is Pb 0.60 Cu 0.40 Sr 1.08 La 0.92 Cu0 4.96 . Calculation of the average hole concentration in these compounds from charge summation is difficult with these compounds because the TI/PbO layers provide polarizable charge reservoirs that can participate in substantial covalent bonding and because of the probable mixed-valent nature of Tl and Pb. Nevertheless, bond valence sums calculated for the Cu ions in the CuO 2 layers for the three 1201 cuprates do provide a correlation with the values of T c or the absence of superconductivity.

A polycrystalline powder can be represented in reciprocal space as a set of nested spherical shells positioned with their centers at the origin1(Figure 9.1). These shells arise from the reciprocal lattice points from the myriad (e.g. ≈109 mm−3 for 1 μm crystallites) of small crystals, ideally with r...

The CuO structure has been refined from time-of-flight neutron diffraction data collected at 11 K. Crystal data: C2 c , a = 4.6833(2) A, b = 3.4208(1) A, c = 5.1294(2) A, β = 99.567(1)°, V = 81.031(7)A3. The CuO distances are 1.951(1) and 1.961(1) A (each 2 ×) with corresponding OCuO angles of 84.315(7)° and 95.685(7)°. Data for a mixture of AgO and Ag2SO4 were collected at 9 K. Crystal data for AgO: P2 1 c , a = 5.8517(3) A, b = 3.4674(2) A, c = 5.4838(3) A, β = 107.663(3)°, V = 106.02(1)A3. The formula for AgO is better written as Ag(I)Ag(III)O2. The Ag(III) atoms are 4-coordinated: AgO = 2.008(3) (2x) and 2.036(3) (2x), and the Ag(I) atoms are 2-coordinated by O: AgO = 2.147(3) (2x). A comparison of the CuO, AgO, PdO, and MgO structures is presented.

The results of an investigation of ${\mathrm{Bi}}_{2}$${\mathrm{CuO}}_{4}$ using x-ray and neutron powder diffraction, dc magnetometry, and electron-spin resonance are presented. Simultaneous refinement of the room-temperature x-ray and neutron-diffraction data was used to obtain accurate cell parameters and atomic positions. Neutron-diffraction data at 13 and 300 K show that the appropriate space group is P4/ncc at both temperatures and reveal the appearance at the lower temperature of two magnetic peaks, which can be indexed as (100) and (210) reflections. While they are clearly indicative of long-range antiferromagnetic order, on the basis of these powder data alone one cannot determine the moment direction. However, on the assumption that the moments lie along the c axis, the copper magnetic moment is (0.56\ifmmode\pm\else\textpm\fi{}0.04)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. dc magnetometry was performed at temperatures from 1.66 to 400 K and fields ranging from 0.5 to 50 kOe. The magnetization showed no field saturation even at 1.66 K and 45 kOe. The susceptibility showed a maximum near 50.4 K with a Curie tail observed at low temperatures. Antiferromagnetic interactions dominated at all temperatures. The magnetic behavior is like that of a three-dimensional antiferromagnetic system. ESR experiments were done over the temperature range 4.3--300 K. For temperatures above 50 K, one broad line with g=2.09 was observed. The resonance field shifted to higher values for T37 K, with an eventual splitting of the line below 15 K. These ESR signals can be associated with antiferromagnetic resonance modes, consistent with an ordered antiferromagnetic phase.

Cristallisation dans Aa avec a=11,648, b=45,99 et c=12,428 A, β=99,82 o , Z=4; affinement jusqu'a R=0,038. La maille asymetrique consiste en une molecule anisole et deux molecules hexaphenylbenzene. L'hexaphenylbenzene adopte une conformation helice avec les cycles phenyl tordus en moyenne de 75 o . Les molecules hexaphenylbenzene forment un arrangement en couches inhabituel alternant entre l'empilement centre et l'empilement hexagonal. L'anisole occupe un ensemble de trous pseudo-octaedriques dans cet arrangement

Doping of La3+ for Ba2+ in TlBa2CuO5, to introduce holes into the CuO2 planes, leads to superconductivity in the material TlBa1.2La0.8CuO5 (with Tc ≈ 52 K). The structure (P4/mmm) has been refined using neutron powder diffraction data collected at 305 K ( a = 3.8479 (3) A and c = 9.0909 (6) A ) and 15 K ( a = 3.8444 (1) A and c = 9.0744 (3) A ). An increase in the Cu-O in-plane bond distance from that in TlBa2CuO5 is consistent with a lowering of the formal oxidation state of copper from its value of 3+. Substantial static site disorder is observed in the Tl-O (3) planes, presumably to allow for better Tl3+-O coordination. Examination of the thermal parameters for the CuO6 octahedra shows that this disorder in turn produces a local tilting of the octahedra and a resultant puckering of the CuO2 planes.

The structure of Ag 0.167 TiS 2 has been determined at several temperatures from time-of-flight neutron powder diffraction data. At 305 K, Ag 0.167 TiS 2 is a stage-II complex which belongs to the space group P3¯m1 , with a = 3.4161(4) Aand c = 12.100(2) A. When the temperature is lowered to 13 K, the lattice parameters decrease, but no phase change or superlattice formation is observed. Upon heating to 1123 K, this compound transforms to a stage-I structure belonging to the space group P3¯m1 , with a = 3.4676(1) Aand c = 6.2247(4) A. No further phase changes are observed when the temperature is raised to 1323 K. When this compound is slowly cooled to ambient temperature from 1323 K, it transforms quantitatively to the original stage-II structure. The Rietveld refinements of Ag 0.167 TiS 2 at six temperatures are presented and discussed.