bluuebunny/crossref_metadata_2025 huggingface dataset:
A double cell for controlling nucleation and growth of protein crystals
Authors: Przybylska M.
Published: July 2002

A simple device for protein crystallization is described that consists of two connected cells, one for the hanging- or sitting-drop vapour diffusion method and the other for changing the concentration of the reservoir solution. It has been found useful for decoupling crystal nucleation from crystal growth, for improving the size and the stability of crystals, and in the preparation of heavy-atom derivatives.
Synchrotron X-ray and neutron powder diffraction studies of the structure of α-CrPO4. Erratum
Authors: Attfield J. P. , Cheetham A. K. , Cox D. E. , Sleight A. W.
Published: July 2002

Owing to a printer's error, the equation given in the Discussion of the paper by Attfield, Cheetham, Cox & Sleight [J. Appl. Cryst. (1988). 21, 452–457] is incorrect. The correct equation is (Δρ/σ(Δρ)) = (1/P) \sum_{P}|p
2 − p
1|/(σ
2
1 + σ
2
2)1/2.
Molecular conformation determination in unknown crystal structures by Patterson methods
Authors: Wilson C. C.
Published: July 2002

A technique is described by which the conformation of a flexible molecular fragment can be obtained when some information is available on the orientation of part of the structure. Using calculations based on the reciprocal-space rotation function, it is found to be possible to determine the preferred conformation of the relevant group in an unknown crystal structure when merely orientational and not translational (positional) information is available. In structures where this conformation is the main interest, important chemical information is thus extracted.
Compton scattering experiments with monochromatic W Kα1 radiation
Authors: Manninen S. , Honkimäki V. , Suortti P.
Published: July 2002

Construction and performance of a spectrometer based on a high-voltage generator and W-anode X-ray tube are described. W Kα
1 radiation is monochromated using a bent Ge (400) crystal. The obtained monochromatic flux is 107 photons s−1 and the beam size at the focal point 15 × 0.5 mm. Comparison is made with a 241Am γ-ray source, which has an almost equal photon energy. The W Kα
1 beam does not exhibit the low-energy tail typical for the γ-ray source but, on the other hand, a small contribution of doublet energy was observed. Directional Compton profiles of pyrolytic graphite were measured with both sources and essentially the same features were found.
Electron diffraction from phospholipids – an approximate correction for dynamical scattering and tests for a correct phase determination
Authors: Dorset D. L. , McCourt M. P. , Tivol W. F. , Turner J. N.
Published: July 2002

An approximate experimental correction of electron diffraction intensities from an epitaxically crystallized phospholipid bilayer for dynamical scattering is described. This correction, which is useful for certain low-angle centrosymmetric data sets, compares intensities recorded at high and low electron-accelerating voltages to ascertain which reflections are most affected by n-beam interactions. When applied to experimental intensity data from 1,2-dihexadecyl-sn-glycerophosphoethanolamine (DHPE), the correction facilitates a direct phase determination based on the probabilistic estimate of three-phase invariants because a more accurate estimate of the hierarchy of |El
| values is obtained. When a multisolution technique is used, incorporating algebraic unknowns for certain phase values, the best phase assignment can be assessed by comparison of the single convolution of phased structure factors to the observed structure-factor magnitudes for the low-voltage data. This approach exploits an approximate analogy made earlier by Moodie between the Sayre equation and the phase grating series and is valid as long as the single convolution adequately models experimental low-voltage data (a condition favored by light-atom structures in a low-angle region of reciprocal space). In real space, the correct structure can also be readily identified as the one having the smoothest density profile.
A fast and portable microspectrophotometer for protein crystallography
Authors: Hadfield A. , Hajdu J.
Published: July 2002

Spectroscopic measurements on crystals during X-ray data collection provide additional information on the composition of the crystal and can be used in the interpretation of structural data. This paper describes a portable microspectrophotometer to obtain UV–visible–near-IR spectra from single crystals during X-ray measurements. The instrument consists of a deuterium lamp, optical fibres, a pair of mirror lenses and a monochromator equipped with a photodiode array detector. Spectra can be recorded in short periods of time (a few milliseconds) from a measurement area of 0.10 mm diameter or smaller. The device can be used to monitor spectral changes in crystals during time-resolved X-ray experiments so that the X-ray camera can be triggered at the right moment as determined by the spectrum, thereby eliminating much of the present guesswork from such studies.
Remeasurement of the profile of the characteristic Cu Kα emission line with high precision and accuracy. Erratum
Authors: Härtwig J. , Hölzer G. , Wolf J. , Förster E.
Published: July 2002

Two errors in the paper by Härtwig, Hölzer, Wolf & Förster [J. Appl. Cryst. (1993), 26, 539–548] are corrected. On p. 540, the fifth line after equation (1) should read `δ
1, δ
2 and κ are small tilts of the crystals and the collimator, respectively'. In the reference list, the reference to Wilson (1958) should read Wilson, A. J. C. (1958). Proc. Phys. Soc. London, 72, 924–925.
Estimation of single-crystal elastic constants from textured polycrystal measurements
Authors: Wright S. I.
Published: July 2002

A method for estimating single-crystal elastic constants given the elastic constants and texture of a polycrystal is outlined for hexagonal crystal symmetry. The method is based on the Voigt and Reuss polycrystal-average formulations. Sample calculations are detailed for titanium diboride and wire-drawn and rolled copper. The influences of possible sources of error on the estimation scheme are examined.
Diffraction Peak Displacement in Residual Stress Samples Due to Partial Burial of the Sampling Volume
Authors: Spooner S. , Wang X.-L.
Published: July 2002

Near-surface measurement of residual strain and stress with neutron scattering complements and extends the surface residual stress measurements by X-ray diffraction. However, neutron diffraction measurements near surfaces are sensitive to scattering volume alignment, neutron beam wavelength spread and beam collimation and, unless properly understood, can give large fictitious strains. An analytic calculation and a numerical computation of neutron diffraction peak shifts due to partial burial of the sampling volume have been made and are compared with experimental measurement. Peak shifts in a strain-free nickel sample were determined for conditions where the sample surface is displaced so that the scattering gage volume is partially buried in the sample. The analytic and numerically computed peak shifts take into account the beam collimation, neutron source size, monochromator crystal mosaic spread and the collection of diffracted intensity with a linear position-sensitive counter.
Studies on Interaction Parameters of Poly(N-isopropylacrylamide) in 1-Propanol–Water Mixtures by Small-Angle X-ray Scattering
Authors: Shimizu S. , Kimura M. , Maruyama T. , Kurita K.
Published: July 2002

Small-angle X-ray scattering from semidilute solutions of poly(N-isopropylacrylamide) in 1-propanol–water mixtures has been measured in a poor-solvent regime. The binary and ternary cluster integrals of polymer segments were determined from the concentration dependence of the correlation length at several temperatures just below the lower critical-solution temperature. The contributions of the segment–segment interactions to the entropy S
int and enthalpy H
int have been calculated from the temperature dependence of interaction parameters (binary and ternary cluster integrals) and it has been found that both values are positive. The dependence of S
int on the concentration of propanol does not show a simple linear relationship.
Ordering Phenomena in ABA Triblock Copolymer Gels
Authors: Reynders K. , Mischenko N. , Kleppinger R. , Reynaers H. , Koch M. H. J. , Mortensen K.
Published: July 2002

Temperature and concentration dependencies of the degree of order in ABA triblock copolymer gels are discussed. Two factors can influence the ordering phenomena: the conformation of the midblocks (links of the network) and the polydispersity of the endblock domains (nodes of the network). The latter is defined by the scattering density profile (polymer chain packing) at the domain boundary: a sharp boundary corresponds to less polydisperse domains and to increased order in the arrangement of the network nodes. The structure of the network can be described in terms of a highly distorted crystalline lattice with close-packed spheres or with cubic (presumably BCC) equilibrium morphology. The appearance of the latter is never detected in the gels with a stretched conformation of the midblock.
Isothermal Structural Evolution of SnO2 Monolithic Porous Xerogels
Authors: Brito G. G. S. , Pulcinelli S. H. , Santilli C. V. , Craievich A. F.
Published: July 2002

Monolithic samples of SnO2 xerogel were produced by careful control of the gelation and drying steps of material preparation. In these samples, small and nanoporous aggregates stick together, yielding a monolithic (nonpowdered) material. The material was analyzed by in situ small-angle X-ray scattering (SAXS) during isothermal treatment at temperatures ranging from 473 to 773 K. At 473 K, the SAXS intensity does not change significantly with time. All experimental scattering intensity functions for T > 473 K are composed of two wide peaks, which evolve with increasing time. Each of them was associated with one of the modes of a bimodal distribution of pore sizes corresponding to a fine (intra-aggregate) and a coarse (inter-aggregate) porosity. The SAXS intensities of the maxima of both peaks increase with increasing treatment time, while the position of their maxima, associated with an average correlation distance, decreases. The time dependences of the SAXS intensity corresponding to both families of pores qualitatively agree with those expected for a two-phase separating system exhibiting dynamic scaling properties. The time evolutions of the several moments of the structure function of samples heat treated at 773 K exhibit a good quantitative agreement with the theory of dynamic scaling for systems evolving by a coagulation mechanism. The kinetic parameters are the same for both peaks, indicating that the same mechanism is responsible for the structural evolution of both families of pores.
Calibration of spiral-readout image-plate detectors
Authors: White Mark A. , Watowich Stanley J. , Fox Robert O.
Published: July 2002

An improved method for intensity-uniformity calibration of diffraction data collected on spiral-readout image-plate (IP) systems is described. This technique is applicable to all types of spiral-readout IP detectors. The procedure utilizes an attenuated direct-beam scan of the IP to generate a radial-sensitivity calibration table. Exposure and scanning of the calibration frame are done on the same time scale as typical data collections, and require no additional equipment. Specific examples are presented for use with Mac Science DIP2000 systems. The new radial calibration is shown to reduce significantly structure-basedRfactors. The improved radial calibration is also shown to lowerRmergewhen the IP is offset from the beam center. In addition to improving data quality and statistics, this method provides a quick and simple diagnostic tool to monitor changes in the sensitivity of the IP detector as a function of age.
Laser-induced transient structure and stress in a platinum(111) crystal studied by time-resolved X-ray diffraction
Authors: Chen P. , Tomov I. V. , Rentzepis P. M.
Published: July 2002

Nanosecond time-resolved X-ray diffraction has been employed to study the transient structure deformation of a Pt(111) crystal induced by laser pulse heating. A direct-imaging X-ray CCD system with high spatial resolution allowed the detection of structural changes of the order of 10−3 Å. The time evolution of lattice strain was measured with 12 ns resolution. The lattice spacings were found to suffer a transient change from 2.2653 to 2.2695 (10) Å, and the strain reached its maximum value 10 ns after laser irradiation.
CPRL: a program to plot the cylindrically projected reciprocal lattice for fiber diffraction patterns
Authors: Finkenstadt V. L. , van der Plas J. L. , Millane R. P.
Published: July 2002

The positions of reflections on the diffraction pattern from a polycrystalline fiber are described by a cylindrical projection of the reciprocal lattice. The characteristics of the projection depend on the crystal system and the orientation of the fiber axis relative to the unit-cell axes. The programCPRLplots the positions of the reflections on a diffraction pattern from an oriented polycrystalline fiber (the cylindrically projected reciprocal lattice), for a triclinic unit cell with any orientation of the fiber axis. The results provided by the program facilitate interpretation of fiber diffraction patterns for complex systems.
Determination of symmetries and idealized cell parameters for simulated structures
Authors: Hundt R. , Scho¨n J. Christian , Hannemann A. , Jansen M.
Published: July 2002

A robust algorithm is presented that determines the symmetries present in an atomic configuration and idealizes the cell parameters according to the crystal system suggested by the symmetries detected. No information besides the coordinates of the atoms within some arbitrary unit cell of the crystal is required.
An improved low-power-consumption heating stage for X-ray powder diffraction
Authors: Valdrè Giovanni
Published: July 2002

An improved low-power-consumption easy-to-operate simple heating stage for use in connection with an X-ray powder diffractometer is described and discussed. No modification is required to the conventional specimen chamber, which can be used safely up to a specimen temperature of at least 1200 K with a temperature accuracy of a few degrees. The stage is interchangeable with conventional specimen holders, without the need for realignment of the diffractometer. To prevent specimen oxidation, provisions are made for heating in an inert gaseous environment. Also, care has been taken to prevent scattered radiation escaping from the specimen chamber. The stage allows the study of structural transformations with temperature and so far it has been used successfully for the characterization of very different materials, such as hyaloclastite, smectites, montmorillonites and chlorocadmates with perovskite-like structures. Although the device has been designed for the Philips PW1820 X-ray diffractometer, it can be applied to other types of X-ray diffractometer.
Contrast in X-ray section topographs of perfect silicon crystals using the Laue–Laue three-beam case of diffraction
Authors: Heyroth F. , Höche H.-R. , Eisenschmidt C.
Published: July 2002

Section topographs taken in three-beam diffraction geometry and in the neighbourhood of this geometric position are presented. The experiments were performed using polychromatic and monochromatic X-ray radiation. The interference of the two diffracted waves produces new contrast phenomena compared to the two-beam case. The contrast changes are discussed on the basis of the Bethe approximation by means of the effective structure factor and with regard to the dispersion surface of the three-beam case.
Isomorphism and mixed crystals in 2-R-naphthalenes: evidence of structural subfamilies and prediction of metastable forms
Authors: Haget Y. , Chanh N. B. , Meresse A. , Bonpunt L. , Michaud F. , Negrier P. , Cuevas-Diarte M. A. , Oonk H. A. J.
Published: July 2002

Solid–liquid binary phase diagrams and isothermal unit-cell parameters as a function of composition are given for the high-temperature form (P21/a,Z= 2) mixed crystals generated by 2-fluoronaphthalene, naphthalene and 2-naphthol with four other β derivatives of naphthalene. The study leads to the distinction between two high-temperature forms (or types of packing). The first one (type 1) is taken by five 2-R-naphthalenes (R= F, Cl, Br, SH, CH3; the first subfamily), the second one (type 2) by naphthalene itself and 2-naphthol (R= H, OH; the second subfamily). The crystallographic data also allow an estimation of unit-cell parameters for the metastable forms of naphthalene (type 1,a= 8.0,b= 5.95,c= 8.6 Å, β = 116°) and 2-fluoronaphthalene (type 2,a= 8.336,b= 5.915,c= 9.000 Å, β = 122.23°), supporting an interpretation, in terms of crossed isodimorphism, of the observed two-phase regions in systems in which the components do not belong to the same subfamily.
Extension of Fe MAD phases in the structure determination of a multiple [FeS] cluster containing hydrogenase
Authors: Peters John W. , Bellamy Henry D.
Published: July 2002

The structure of the hydrogenase (CpI) fromClostridium pasteurianumhas been solved by MAD phasing taking advantage of the presence of 20 Fe atoms in the native ~60 kD protein. The Fe atoms in this protein are distributed into five [FeS] clusters that could easily be identified in Patterson maps. Although the individual sites for the Fe atoms within the [FeS] cluster could not be identified directly, phasing to the maximum resolution (2.5 Å) of the MAD data was achieved by an iterative assignment and refinement of the positions of individual Fe atoms based mainly on the known structure of several of the metal clusters. After the majority of the Fe atoms had been successfully assigned and refined in this manner, excellent quality electron-density maps were obtained which allowed the amino acid chain to be traced.
Porous structure of NiO-based xerogels
Authors: Fischer H. , Sharma P.K. , Craievich A.F.
Published: July 2002

NiO-based xerogels were prepared by sol-gel processing starting from the NiCl2-BuOH-H2O system. Structural features associated to the porosity of xerogels were studied by small-angle X-ray scattering. Xerogels heat-treated at low temperatures (423 and 573 K) are composed of hydrated NiCl2 small crystals and exhibit an essentially single mode nanoporous structure with an average pore radius of ∼75 Å. Xerogels heat-treated at 973 and 1173 K are composed of aggregates of NiO crystals which only contain intergrain mesopores with radii exceeding 200 Å. A mixture of phases is observed in xerogels heat-treated at 773 K. Xerogels with a water concentration of 2 mol/(mol NiCl2) exhibit a narrow nanopore size distribution. Water concentrations higher or lower than 2 mol promote the formation of nanoporous structures with wider size distributions.
Microbeam small-angle scattering experiments and their combination with microdiffraction
Authors: Riekel C. , Burghammer M. , Müller M.
Published: July 2002

The brilliance of undulator X-ray sources can be used to develop microfocusing optics for wide-angle (WAXS) and small-angle (SAXS) X-ray scattering. At the ESRF microfocus beamline, a beam size of 10 μm is obtained by a pinhole collimating system coupled to a double focusing mirror. This allows resolving the first order of collagen (67 nm). Glass capillary optics provides a beam size close to one micron, however, with a more limited resolution. A high-resolution CCD detector allows combined SAXS/WAXS experiments for one detector setting.
New types of space-filling polyhedra with fourteen faces
Authors: Rosa M. E. , Fortes M. A.
Published: July 2002

A study of the staggered packing of identical hexagonal prisms leading to four-connected periodic structures with polyhedral cells of fourteen faces has been undertaken. Special attention was given to those packings that lead to periodic structures with two polyhedra per lattice point, and such that the two polyhedra are related by a pure rotation and/or enantiomorphism. The general solution for packings of this type was obtained and the topology of the intervening polyhedra was determined. It is shown that polyhedra with eight hexagonal faces and six square faces, topologically isomorphic to the truncated octahedron, can be packed with or without a rotation. The polyhedra which can be packed with the respective enantiomorphs (with or without rotation) have four square faces, four pentagonal faces and six hexagonal faces. Each type of packing is compatible with Bravais lattices of any category and each topological solution is compatible with a range of convex shapes.
On integrating the techniques of direct methods with anomalous dispersion: the one-phase structure seminvariants in the monoclinic and orthorhombic systems. I. Theoretical background
Authors: Velmurugan D. , Hauptman H. A.
Published: July 2002

Conditional probability distributions are derived for the one-phase structure seminvariant in the presence of anomalous scattering for the monoclinic and orthorhombic systems. The results are presented in a compact tabular form. The applications of these results are discussed in the following paper [Velmurugan, Hauptman & Potter (1989).Acta Cryst.A45, 163-165].
Geometrical effects in diffraction analysis
Authors: Bernhoeft N.
Published: July 2002

The use of X-ray and neutron scattering as a tool to study phase transitions is well established. As techniques improve and experiments are made under successively higher resolution, the need to consider the role of both the distribution of diffracting length scales and the incident-beam coherence volume is emphasized. The interplay of diffracting length scales and the beam coherence volume no longer permits calculation of diffraction profiles in terms of the sample intensity response convolved with an instrumental resolution function. Rather, the probe and sample now enter the calculation on an equal footing at the level of the scattering amplitudes. Under these conditions, it is found that the summation of coherent scattering amplitudes leads to characteristic profiles in wave-vector and, in the case of resonant X-ray scattering, energy space. In this latter case, in the vicinity of strong absorption edges, as used for example in resonant magnetic X-ray diffraction, the energy dependence of diffraction profiles may uniquely allow spatial localization of the scattering volume below the sample surface. This observation may considerably augment the range and power of resonant X-ray scattering.
Chemical bonding effects in the determination of protein structures by electron crystallography
Authors: Chang Steven , Head-Gordon Teresa , Glaeser Robert M. , Downing Kenneth H.
Published: July 2002

Scattering of electrons is affected by the distribution of valence electrons that participate in chemical bonding and thus change the electrostatic shielding of the nucleus. This effect is particularly significant for low-angle scattering. Thus, while chemical bonding effects are difficult to measure with small-unit cell materials, they can be substantial in the study of proteins by electron crystallography. This work investigates the magnitude of chemical bonding effects for a representative collection of protein fragments and a model ligand for nucleotide-binding proteins within the resolution range generally used in determining protein structures by electron crystallography. Electrostatic potentials were calculated by ab initio methods for both the test molecules and for superpositions of their free atoms. Differences in scattering amplitudes can be well over 10% in the resolution range below 5 Å and are especially large in the case of ionized side chains and ligands. We conclude that the use of molecule-based scattering factors can provide a much more accurate representation of the low-resolution data obtained in electron crystallographic studies. The comparison of neutral and ionic structure factors at resolutions below 5 Å can also provide a sensitive determination of charge states, important for biological function, that is not accessible from X-ray crystallographic measurements.
A feasible set approach to the crystallographic phase problem
Authors: Marks L. D. , Sinkler W. , Landree E.
Published: July 2002

The connection between the crystallographic phase problem and the feasible set approach is explored. It is argued that solving the crystallographic phase problem is formally equivalent to a feasible set problem using a statistical operator interpretable via a log-likelihood functional, projection onto the non-convex set of experimental structure factors coupled with a phase-extension constraint and mapping onto atomic positions. In no way does this disagree with or dispute any of the existing statistical relationships available in the literature; instead it expands understanding of how the algorithms work. Making this connection opens the door to the application of a number of well developed mathematical tools in functional analysis. Furthermore, a number of known results in image recovery can be exploited both to optimize existing algorithms and to develop new and improved algorithms.
Structural characterization of protonated benzeneseleninic acid, the dihydroxyselenonium ion
Authors: Sørensen Henning Osholm , Stuhr-Hansen Nicolai , Henriksen Lars , Larsen Sine
Published: July 2002

The structure of the dihydroxyphenylselenonium ion (C_{6}H_{7}O_{2}Se^{+}) has been determined in its benzenesulfonate (C_{6}H_{5}O_{3}Se^{-}) and p-toluenesulfonate (C_{7}H_{7}O_{3}S ^{-}) salts. Whereas the former salt is disordered, the latter less dense salt is well defined. This difference in crystallization behaviour is attributed to a C—H...O hydrogen bond involving the methyl group of the p-toluenesulfonate ion. The two salts display very similar hydrogen-bond arrangements and differ only with respect to the stacking of the phenyl groups. The dihydroxyselenonium ion is a strong acid with a pK value of −0.9 determined from the variation of the 77Se chemical shift. A comparison with the two deprotonated species reveals a systematic increase in the Se—O bond lengths and the pyramidal configuration around Se with the number of protons attached.
X-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinites
Authors: Guelylah A. , Madariaga G. , Petricek V. , Breczewski T. , Aroyo M. I. , Bocanegra E. H.
Published: July 2002

The potassium manganese tetrafluoroberyllate langbeinite compound has been studied in the temperature range 100–300 K. Using DSC measurements, a phase transition has been detected at 213 K. The space group of the low-temperature phase was determined to be P1121 using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P213 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690 (8), b = 20.136 (2), c = 10.0329 (4) Å, γ = 90.01 (1)°]. A precise analysis of the BeF4 tetrahedra in the low-temperature phase shows that two independent tetrahedra rotate in opposite directions along the doubled crystallographic axis. A symmetry mode analysis of the monoclinic distortion is also reported. This is the first report of the existence of such a phase transition in K2Mn2(BeF4)3 and also of a new type of low-temperature structure for langbeinite compounds.
Chemical bonding in energetic materials: β-NTO
Authors: Zhurova Elizabeth A. , Pinkerton A. Alan
Published: July 2002

The electron density and related properties of the quasi-stable β form of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO; space group P21/c) have been determined from a low-temperature [100 (1) K] X-ray diffraction experiment. Intensities were measured with a 2K CCD Bruker diffractometer using Ag Kα radiation. Two detector settings, several φ settings, 0.3° ω scans and 160 s exposure time per frame gave R
int = 0.0215 for 68 989 (4080 unique) reflections and (sin θ/λ)max = 1.23 Å−1. The Hansen–Coppens [Acta Cryst. (1978), A34, 909–921] multipole model as implemented in the XD program gave R = 0.0333 (all reflections), which allowed calculation of the electron density, Laplacian and electrostatic potential distributions. The bonding (3,−1) critical points and the molecular dipole moment of 3.2 (1) D were also obtained. Chemical bonding in the molecule is discussed.
Structural complexity of a polar perhydrotriphenylene inclusion compound brought to light by synchrotron radiation
Authors: Weber Thomas , Estermann Michael A. , Bürgi Hans-Beat
Published: July 2002

The complex diffraction pattern of the heavily disordered co-crystals of perhydrotriphenylene and 1-(4-nitrophenyl)piperazine (5C18H30·C10H13N3O2) has been investigated with synchrotron radiation and an area detector. Five (almost) complete, three-dimensional data sets have been obtained from the tips and the centre of a needle-like crystal at room temperature and 120 K. They revealed a rich variety of features including one,- two- and three-dimensional diffuse scattering, as well as incommensurate satellites. At the centre and one tip of the crystal the symmetry appears to be orthorhombic, whereas at the other tip the symmetry of the satellites and of some of the diffuse scattering is clearly monoclinic, indicating that the crystal is not homogeneous. Most of the scattering could be assigned to R/S occupational disorder of the chiral host molecules, to positional disorder of the guest molecules or to local distortions of the average structure. Assignments are based on the disorder deduced from the average structure and the molecular form factors of host and guest molecules which show characteristic patterns in reciprocal space. Two smaller, orthorhombic twin fragments and an additional phase with hexagonal symmetry have also been found.
Transition from the incommensurately modulated structure to the lock-in phase in Co-åkermanite
Authors: Schaper Andreas K. , Schosnig Michael , Kutoglu Ali , Treutmann Werner , Rager Helmut
Published: July 2002

The adaptation of the incommensurate structure modulation in Ca2CoSi2O7 (dicalcium cobalt disilicate) single crystals to decreasing temperature has been examined using in situ high-resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the commensurate lock-in phase of Co-åkermanite exhibits a pronounced hysteresis of a highly strained metastable state with a characteristic microdomain morphology. A network of domain walls surrounding single orientation domains develops out of the room-temperature tartan pattern, the domains increase in size and their alignment changes from crystallographic to random. At 100 K the phase transition becomes almost complete. In parallel, the evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close-packed configuration of overlapping octagons in the commensurate low-temperature lock-in phase. Thereby, the octagon represents the ordered distribution of low-coordinated Ca clusters within a nanodomain extending over 4 × 4 subunits, on average [Riester et al. (2000). Z. Kristallogr.
215, 102–109]. The modulation wavevector was found to change from q
1,2 = 0.295 (a* ± b*) at 300 K to q
1,2 = 0.320 (a* ± b*) at 100 K.
Structure of C15-, C17- and C19-mono-acid β-triacylglycerols
Authors: Helmholdt Robert B. , Peschar René , Schenk Henk
Published: July 2002

The crystal structures of β-1,2,3-tris(pentadecanoyl)glycerol (β-C15C15C15), β-1,2,3-tris(heptadecanoyl)glycerol (β-C17C17C17) and β-1,2,3-tris(nonadecanoyl)glycerol (β-C19C19C19) have been determined from high-resolution X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine and refine the structures, respectively. As in β-1,2,3-tris(tridecanoyl)glycerol (β-C13C13C13) and the even-numbered mono-acid triacylglycerols, all three odd-numbered monoacid triacylglycerols crystallize in space group P\bar 1 with Z = 2 in an asymmetric tuning-fork conformation and have a lateral acyl chain packing resulting in a layered structure.
Structural investigation of homonuclear Pt2 and heteronuclear PdPt complexes containing a metal–metal bond bridged by hydrido and sulfido ligands
Authors: Clegg W. , Capdevila M. , González-Duarte P. , Sola J.
Published: July 2002

The complex [Pt2(μ-H)(μ-S)(dppe)2](PF6) undergoes a displacive order–disorder transformation at ca 230 K. The low-temperature structure is ordered with one cation–anion pair as the asymmetric unit in space group P21/n. At room temperature the b axis is halved and the space group is P2/n, imposing crystallographic twofold rotation symmetry on both ions; the anion shows major disorder and there is probably minor disorder in the cation, but its internal geometry remains essentially unchanged. The heteronuclear complex [PdPt(μ-H)(μ-S)(dppe)2](PF6) is isostructural with the Pt2 complex at room temperature. All three structures have been determined crystallographically and both complexes have been extensively characterized by NMR spectroscopy, unambiguously confirming the genuine heteronuclear nature of the mixed-metal complex and the presence of the bridging hydride ligand.
Atomic displacement parameters for garnets: a lattice-dynamical evaluation
Authors: Pilati T. , Demartin F. , Gramaccioli C. M.
Published: July 2002

Atomic displacement parameters (a.d.p.'s), together with vibrational spectra (Raman and IR) and thermodynamic functions, have been calculated for some minerals of the garnet group, such as pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12), andradite (Ca3Fe2Si3O12) and almandine (Fe3A12Si3O12). For this purpose, a rigid-ion Born–von Karman model has been applied, using empirical atomic charges and valence force fields derived from a best fit to the vibrational spectra of a group of orthosilicates and oxides. Agreement with the experimental data is good, with the only exception of pyrope and almandine: for these minerals the calculated a.d.p.'s of the Mg2+ atom in the former and of the corresponding Fe2+ atom in the latter are too low. This result confirms the unusual behaviour of these atoms, for which dynamic disorder has been claimed. However, if the values of the specific heat and entropy are considered and compared with our calculations, this situation can be best explained assuming the transition to static disorder of the Mg2+ and Fe2+ atoms to occur at low temperature.
Structure of the charge-transfer salt 2,2',5,5'-tetraselenafulvalene–7,7,8,8-tetracyano-p-quinodimethane (TSeF–TCNQ)
Authors: Corfield P. W. R. , La Placa S. J.
Published: July 2002

2,2′,5,5′-Tetraselenafulvalene-7,7,8,8-tetracyano-p-quinodimethane (1/1), TSeF–TCNQ, C6H4Se4.C12H4N4, Mr
= 596.13, monoclinic, P21/c, a = 12.5048 (4), b = 3.8724 (2), c = 18.5041 (7) Å, β = 104.131 (3)°, V = 868.9 Å3, T = 297 K, Z = 2, F(000) = 560, Dx
= 2.28 Mg m−3, λ(Cu Kα) = 1.5418 Å, μ = 115.7 cm−1, R(F) = 0.018, wR(F) = 0.030 for 1393 unique reflections with σ(F
2) > 3F
2. The crystals are isostructural with the corresponding tetrathiafulvalene salt, TTF–TCNQ, with both cations and anions arranged in segregated stacks parallel to b. There are Se...N distances of 3.17 and 3.18 Å, along a, between cation and anion stacks, and H...H distances of 2.58 Å, along c, between neighboring cation stacks, all shorter than the corresponding distances in TTF–TCNQ.
Structure of 1,5-bis(dimethylamino)-1,5-diphenylpentadienylium perchlorate and modelling of pentadienylium salts (cyanine dyes)
Authors: Declercq J.-P. , Dubourg A. , Payrastre C. , Mazières M.-R. , Madaule Y. , Wolf J.-G.
Published: July 2002

C21H25N2
+.ClO4
− (orange crystals), Mr
= 404.9, orthorhombic; P212121, a = 7.810 (2), b = 15.007 (3), c = 18.326 (3) Å, V = 2147.8 (7) Å3. Graphite-monochromated Mo Kα source at room temperature; λ = 0.71069 Å, μ = 0.22 mm−1, Z = 4; F(000) = 856, Dx
= 1.25 g cm−3, R = 0.057 for 1392 independent data. As for the unsubstituted pentamethinium dyes, the chain lies in an all-trans planar configuration. Molecular modelling calculations reproduce the X-ray structure with good accuracy, even in the case of more crowded pentadienylium cations, i.e. 1,5-bis(dimethylamino)-1,3,5-triphenylpentadienylium perchlorate.
Structures of room-temperature and ferroelectric (35 K) phases of deuterated betaine calcium chloride dihydrate
Authors: Ezpeleta J. M. , Zúñiga F. J. , Paulus W. , Cousson A. , Hlinka J. , Quilichini M.
Published: July 2002

The structures of the room-temperature and the lowest-temperature ferroelectric phases of fully deuterated betaine (trimethylammonioacetate) calcium chloride dihydrate have been determined by means of elastic neutron diffraction. In previous structural studies the structural distortions of the incommensurate phase at 130 K and the fourfold phase at 90 K were analyzed in terms of symmetry modes. The same approach has been applied in the present case and, as a result, a quantitative comparison of the distortions at different temperatures can be performed. This shows that the Λ
3 (for q ≠ 0) and B
2u
(for q = 0) modes, corresponding to the order parameter, are predominant for the three phases. In addition, it has been found that the structure of this primary mode remains essentially unaltered in the three phases. There is only an increase in its global amplitude as the temperature decreases. Accordingly, it can be expected that the structures of the remaining intermediate modulated phases in the transition sequence are similar to that of the first incommensurate phase, except for the change of wavevector values and greater amplitude of the distortion. Crystal data: (CD3)3NCD2COOCaCl2.2D2O, Mr
= 279.0, λ = 0.8308 Å, Z = 4, F(000) = 552. At room temperature: Pnma, a = 10.95 (1), b = 10.15 (1), c = 10.82 (1) Å, V = 1203 (2) Å3, Dx
= 1.54 g cm−3, μ = 0.10 cm−1, final R = 0.048 for 748 unique observed reflections. At 35 K: Pn21
a, a = 10.905 (9), b = 10.00 (1), c = 10.80 (1) Å, V = 1178 (2) Å3, Dx
= 1.57 g cm−3, μ = 0.11 cm−1, final R = 0.042 for 1726 unique observed reflections.
Ab initio powder structure determination and thermal behavior of a new lead(II) phenylphosphonate, Pb(O3PC6H5)
Authors: Cabeza A. , Aranda M. A. G. , Martinez-Lara M. , Bruque S. , Sanz J.
Published: July 2002

The crystal structure of Pb(O3PC6H5) has been solved from X-ray powder diffraction data. Crystal data: triclinic, a = 13.6907 (5), b = 9.3327 (4), c = 7.0432 Å, α = 106.188 (3), β = 94.927, γ = 96.977°, V = 851.04 (8) Å3, space group P{\bar 1} and Z = 4. Initial positional parameters for Pb atoms were obtained from direct methods using 556 structure factors in the 2θ range 9–63°. All remaining non-H atoms were located from successive difference-Fourier maps. The final agreement factors were R
wp = 0.055, Rp
= 0.042 and RF
= 0.017. There are 22 atoms in the asymmetric part of the unit cell and 66 positional parameters have been refined with the help of soft constraints. The lead coordination environment is irregular due to a marked inert pair effect. The three-dimensional structure of this organic inorganic compound is layered with inorganic layers built up from CPO3 tetrahedra and PbO6 units. 31P MAS and CP-MAS (cross-polarization magic angle spinning) NMR studies have been carried out to confirm the triclinic symmetry with two independent structural units. In optimum conditions two 31P resonance signals are observed, indicating that there are two crystallographically independent P atoms with closely similar environments. The thermal behavior of this material has been studied and the thermal decomposition products identified.
Synthesis and structure refinement of a new type of layered intergrowth phase: Sb3+x
Nb3−x
TiO14 and Sb3+x
Ta3−x
TiO14
Authors: Ling C. D. , Schmid S. , Thompson J. G. , Withers R. L. , Sterns M.
Published: July 2002

Attempts to synthesize Sb-based analogues of the known Bi-based Aurivillius phase, Bi3TiNbO9, have led to the discovery of an isomorphous pair of a new type of layered intergrowth phase of general stoichiometry Sb3+x
Nb3−x
TiO14 and Sb3+x
Ta3−x
TiO14. Electron diffraction proved essential for the initial recognition and structural characterization of these phases. The structures of Sb3+x
Nb3−x
TiO14, x = 0.89, and Sb3+x
Ta3−x
TiO14, x = 1.26, are reported. The structures are described as ordered intergrowths of β-Sb2O4-type and Sb2O5-type structures, with NbV/TaV substituting for SbV in the former and NbV/TaV/TiIV substituting for SbV in the latter.
A re-examination of the relationship between lattice strain, octahedral tilt angle and octahedral strain in rhombohedral perovskites
Authors: Thomas N. W.
Published: July 2002

A new expression is proposed for the relationship between lattice strain 90°− αpc ( αpc: pseudo-cubic angle) and mean BO6 octahedral tilt angle <ω> in rhombohedral perovskites ABO3. It is derived from volumetric arguments, leading to a cubic equation which incorporates lattice strain 90° −αpc and octahedral elongation explicitly. Numerical solutions of this equation are derived for equally spaced values of octahedral strain, giving rise to a set of parametric curves which relate ω to 90° − αpc for different values of η. These curves can be represented as polynomials of the fourth degree, thereby enabling their routine use in the analysis of rhombohedral perovskite structures. It is anticipated that these parametric curves will supersede earlier work [Megaw & Darlington (1975). Acta Cryst. A31, 161–173], in which an analytical expression was derived linking 90° − αpc, ω and octahedral strain for positive lattice strains only. By comparison, the relationship proposed here accommodates both negative and positive lattice strains. Correlations between values of <ω>, η, 90° − αpc and space-group symmetry are found, with an analysis of known rhombohedral and orthorhombic Pnma structures revealing the importance of cationic charges in determining symmetry. Since the polyhedral volume ratio VA/VB
may be quantitatively related to <ω>, allowed values of tilt angle, lattice strain and octahedral elongation may be inferred for a given composition, which has characteristic values of VA
and VB
.
A Re-investigation of the Crystal Structure of the Perovskite PbZrO3 by X-ray and Neutron Diffraction
Authors: Corker D. L. , Glazer A. M. , Dec J. , Roleder K. , Whatmore R. W.
Published: July 2002

The crystal structure of the perovskite lead zirconate PbZrO3 has been redetermined using single-crystal X-ray diffraction (Mo Kα radiation, λ = 0.71069 Å). Single-crystal data at 100 K: space group. Pbam, a = 5.884 (1), b = 11.787 (3), c = 8.231 (2) Å, V = 570.85 Å3 with Z = 8, μ = 612.6 cm−1, D

x
= 8.06 Mg m−3, F(000) = 1168, final R = 0.033, wR = 0.061 over 555 reflections with I > 2σ(I). An investigation is made into previous contradicting reports of a possible disorder in the O atoms and their origin by examining the crystal pseudo-symmetry. Information distinguishing an ordered and disordered oxygen substructure is shown to reside in weak l odd reflections. Because of their extremely low intensities these reflections have not contributed sufficiently in previous X-ray structure investigations and hence, to date, conclusive evidence differentiating between ordered and disordered models has not been possible. By collecting single-crystal X-ray data at low temperature and by using exceptionally long scans on selected hkl, l odd, reflections, a new accurate structure determination is presented and discussed, showing the true ordered oxygen positions. Because of the large difference in scattering factors between lead and oxygen when using X-rays, a neutron diffraction Rietveld refinement using polycrystalline samples (D1A instrument, ILL, λ = 1.90788 Å) is also reported as further evidence to support the true ordered oxygen structure revealed by the low-temperature X-ray analysis.
Experimental Electron Density Study of NaH2PO4 at 30 K
Authors: Ichikawa M. , Gustafsson T. , Olovsson I.
Published: July 2002

The deformation electron density of sodium dihydrogen phosphate, NaH2PO4, at 30 K has been studied as a reference for the electron-density characteristics of hydrogen-bonded ferroelectrics containing phosphate ions. Clear peaks of deformation electron density (0.3–0.6 e Å−3) are seen in the middle of each P—O bond and electron deficiency (−0.2 to −0.4 e Å−3) on the opposite side of each P—O bond peak. The peak height is higher and the peak shape is more distinct in P—O bonds than in P—O(H) bonds as a whole; the distribution of deformation density in the region of lone-pair electrons is more diffuse. The O—H...O bond deformation-density peaks of around 0.2 e Å−3 appear in the middle of the O—H bond, followed by a deeper electron depletion of around −0.4 e Å−3 on the H...O contract, just outside the H atom. The effective charges, defined as the integral of the deformation electron density, have the following values: around +0.2 for Na, +1.8 for P, −0.8 for O and +0.6 for H. The correlation of the deformation-density peaks and the depth of the negative peak of the H atom with the hydrogen-bond distance are discussed.
Towards extinction-free experimental diffraction data on Al2O3
Authors: Graafsma H. , Souhassou M. , Puig-Molina A. , Harkema S. , Lecomte C. , Kvick Å.
Published: July 2002

Using 58 keV (0.214 Å) synchrotron radiation we have obtained virtually extinction-free data on Al2O3. Refinement of a multipole model against these data produced deformation densities of high quality, which compare well with theoretical maps.
2-Phenyl-4-hydroxyphthalazin-1-one: A Benzoannelated Derivative of Maleic Hydrazide
Authors: Becker D. , Botoshansky M. , Gasper N. , Herbstein F. H. , Karni M.
Published: July 2002

The monoclinic crystals (space group P21/a, Z = 8) of 2-phenyl-4-hydroxyphthalazin-1-one, C14H10N2O2, have two independent molecules (A and B) in the asymmetric unit. Both occur as the lactim–lactam (hydroxy–one) structure, which is also found in the parent molecule maleic hydrazide (both triclinic and monoclinic polymorphs), dichloromaleic hydrazide and luminol (3-aminophthalhydrazide). The molecular arrangement is based on strings of alternating A and B molecules linked by hydroxyl...carbonyl hydrogen bonds, with only van der Waals interactions between adjacent strings. Comparison is made of the measured bond lengths for (monoclinic) maleic hydrazide and values from high-level ab initio calculations, and reasonably good agreement is obtained, with indications of improvements when allowance is made for electron correlation and hydrogen bonding.
Correlation of Internal Torsional Motion with Overall Molecular Motion in Crystals
Authors: Schomaker V. , Trueblood K. N.
Published: July 2002

The simple one-parameter Dunitz–White model for internal torsional motion accompanying overall molecular motion in crystals ignores the correlations between this torsion and the overall translation and libration. These correlations are explicitly considered here, in quadratic approximation (based on the linear approximation for displacements). For each attached rigid group (ARG) undergoing a torsional libration, there are, in addition to the mean-square libration amplitude, six correlations to be considered, three with the overall molecular libration and three with the overall translation. Because it is impossible from the observed quadratic mean displacements to distinguish the torsional motion from the overall molecular libration parallel to the torsional axis, the present analysis emphasizes the overall libration parallel to the torsional axis, which we term \Lambda. In the general case there are only six determinable parameters for each ARG. If the torsional axis of the ARG lies on a molecular symmetry element, the number of parameters is reduced to three or fewer. Examples of analyses with these correlations included, and without them, are compared.
Group-Theoretical Analysis of Octahedral Tilting in Perovskites
Authors: Howard C. J. , Stokes H. T.
Published: July 2002

A group-theoretical analysis is made of the structures derived from the aristotype cubic perovskite (Pm3¯m) by the simple tilting of rigid octahedral units. The tilting is mediated by the irreducible representations R^+_4 and M^+_3 or the two in combination. These result in 15 possible structures, compared with the 23 possibilities suggested previously by Glazer [Acta Cryst. (1972), B28, 3384–3392]. The analysis makes the group–subgroup relationships apparent.