bluuebunny/crossref_metadata_2025 huggingface dataset:
Concentration and temperature dependence of decomposition in supercooled liquid alloys
Authors: Löffler J.F. , Thiyagarajan P. , Johnson W.L.
Published: July 2002

Small-angle neutron scattering experiments were performed on the bulk amorphous alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1®, subscripts indicate at.%) and on other alloys, where the (Zr,Ti) and (Cu,Be) contents were varied by following the line in composition space connecting Vit1 and Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4®). The small-angle neutron scattering data of the samples, annealed at temperatures between 603 K and 663 K, show interference peaks, giving evidence for spatially correlated arrangements of inhomogeneities. The Q values of the interference peaks, Q
max, decrease with increasing annealing temperature Ta and, at a given annealing temperature, with composition following the connecting line from Vit1 to Vit4. Down to the glass transition temperature T
g, the data follow a relation 1/L
2 ∝ (T
s − T
a) as predicted by the linearized Cahn theory, with L = 2π/Q
max the characteristic wavelength of the decomposition and T
s the apparent spinodal temperature. Below T
g, a different behavior is observed, which may be either due to a change in atomic diffusion or due to an insufficient relaxation of the samples.
A peak interpolation formula for Fourier map interpretation
Authors: Pavelčík F.
Published: July 2002

A weighted 19-point parabolic interpolation formula applicable to computer interpretation of Fourier maps is derived.
Determination of the photoelectron emission probability with inclined X-ray Laue diffraction
Authors: Afanas'ev A. M. , Imamov R. M. , Mukhamedzhanov E. Kh. , Chuzo A. N.
Published: July 2002

A simple relation has been established between the Fourier component of the probability density P(z) of photoelectron emission from different depths of a crystal and the angular dependence of the emission of photoelectrons formed in inclined X-ray Laue diffraction, which for the first time permitted the use of a direct method for the reconstruction of the P(z) function. Accurate measurements of the angular dependence of photoelectron emission were carried out on a silicon single crystal with diffraction of Cu Kα radiation for different energy ranges. Photoelectrons were recorded by a proportional gas counter specially designed for the energy analysis of photoelectrons under inclined Laue diffraction conditions. The laws predicted by the theory have been fully confirmed, and the corresponding P(z) functions have been obtained.
New surface patches for minimal balance surfaces. I. Branched catenoids
Authors: Fischer W. , Koch E.
Published: July 2002

Three new families of minimal balance surfaces have been derived. For this a new kind of surface patch,i.e.branched catenoid, has been used. A concave polygon with one point of self-contact and a convex polygon are the two generating circuits of such a minimal balance surface.
Search for satellite reflections and low-temperature study of the 1223 and 2212 Tl–Ba–Ca–Cu–O superconductors
Authors: Gao Y. , Li R. , Coppens P. , Sleight A. W. , Subramanian M. A.
Published: July 2002

The structures of the 1223 and 2212 phases of the T1-Ba-Ca-Cu-O system have been determined at 125 K. No satellite reflections were observed in extensive searches at both room and low temperature. The temperature dependence of the thermal parameters of the thallium atoms confirms the presence of a static disorder beyond that already incorporated in the refinements.
Extinction in finite perfect crystals: case of a sphere
Authors: Al Haddad M. , Becker P.
Published: July 2002

The extinction factor in finite perfect crystals is calculated from pure dynamical theory. In particular, a detailed solution is proposed for a sphere, in which case the extinction factor depends on the Bragg angle θ and the parameter (R/Λ), where R is the radius of the crystal and Λ the extinction length. An approximate solution based on the Laue geometry is proposed and corrections to take care of the complex boundary conditions are presented. An expression easily usable in refinement programs is proposed that fits the exact value to better than 1%.
X-ray interference on finite polyhedral crystals
Authors: Snigirev A. A. , Suvorov A. Yu.
Published: July 2002

An X-ray interference phenomenon associated with the superposition of reflected waves corresponding to the different diffraction types may be observed for finite polyhedral crystals. Using plane-wave solutions, the interference patterns are analyzed for the two specific geometries of the crystal profile. The computer simulations supplement the theoretical inferences and fit the experimental results.
Holographic methods in X-ray crystallography. II. Detailed theory and connection to other methods of crystallography
Authors: Szöke A.
Published: July 2002

Equations are derived for finding an unknown part of the electron density in the unit cell of a crystal when part of the structure of its contents is already known. These equations are based on the similarity of the X-ray diffraction pattern to a hologram. The X-ray field scattered by the known part of the structure is identified as the holographic reference beam. It interferes with the waves scattered from the unknown part of the structure. The interference pattern contains phase information that can be exploited to recover fully the unknown part of the structure. This paper discusses mathematical properties of the resulting equations and some methods for their solution. A strong similarity to inverse problems of image processing is pointed out and connections to other known methods of X-ray crystallography are established. In paper III [Maalouf, Hoch, Stem, Szöke & Szöke (1993). Acta Cryst. A49, 866–871], some modest numerical simulations are presented.
The periodic average structure of particular quasicrystals
Authors: Steurer Walter , Haibach Torsten
Published: July 2002

The non-crystallographic symmetry of d-dimensional (dD) quasiperiodic structures is incompatible with lattice periodicity in dD physical space. However, dD quasiperiodic structures can be described as irrational sections of nD (n > d) periodic hypercrystal structures. By appropriate oblique projection of particular hypercrystal structures onto physical space, discrete periodic average structures can be obtained. The boundaries of the projected atomic surfaces give the maximum distance of each atom in a quasiperiodic structure from the vertices of the reference lattice of its average structure. These maximum distances turn out to be smaller than even the shortest atomic bond lengths. The metrics of the average structure of a 3D Ammann tiling, for instance, with edge lengths of the unit tiles equal to the bond lengths in elemental aluminium, correspond almost exactly to the metrics of face-centred-cubic aluminium. This is remarkable since most stable quasicrystals contain aluminium as the main constitutent. The study of the average structure of quasicrystals can be a valuable aid to the elucidation of the geometry of quasicrystal-to-crystal transformations. It can also contribute to the derivation of the physically most relevant Brillouin (Jones) zone.
Minimal surfaces with self-intersections along straight lines. I. Derivation and properties
Authors: Koch E. , Fischer W.
Published: July 2002

A special kind of three-periodic minimal surface has been studied, namely surfaces that are generated from disc-like-spanned skew polygons and that intersect themselves exclusively along straight lines. A new procedure for their derivation is introduced in this paper. Several properties of each such surface may be deduced from its generating polygon: the full symmetry group of the surface, its orientability, the symmetry group of the oriented surface, the pattern of self-intersections, the branch points of the surface, the symmetry and periodicity of the spatial subunits demarcated by the surface, and the Euler characteristics both of the surface and of the spatial subunits. The corresponding procedures are described and illustrated by examples.
X-ray diffraction moiré topography as a means to reconstruct relative displacement fields in weakly deformed bicrystals
Authors: Ohler M. , Köhler S. , Härtwig J.
Published: July 2002

X-ray diffraction topographs of wafers produced by separation by implanted oxygen (SIMOX) show moiré fringes in both reflection and transmission geometry. These fringes reveal deformations of the order of 10−6 to 10−8 between the layer and the substrate of the SIMOX material. A new method for a quantitative analysis of moiré fringes is developed and allows reconstruction with a high sensitivity of the three components of the relative displacement field between layer and substrate directly from a set of topographs. This method is used for the interpretation of moiré topographs of entire 4 in SIMOX wafers and of regions around crystal defects. Finally, the capabilities of an analysis of moiré fringes are compared with those of the usual diffraction topography.
Towards an exit wave in closed analytical form
Authors: Van Dyck D. , Chen J. H.
Published: July 2002

A simple but sufficiently accurate expression is obtained for the exit wave of a crystal in zone-axis orientation. The exit wave at each atom column can be parametrized with only one parameter, which is a function of the projected `weight' of the column.
Determination of mean inner potential of germanium using off-axis electron holography
Authors: Li Jing , McCartney M. R. , Dunin-Borkowski R. E. , Smith David J.
Published: July 2002

Off-axis electron holography has been used to determine the mean inner potential of germanium using cleaved 90° wedge samples, where the wedge thickness profiles were checked by weak-beam dark-field extinction fringes. Dynamical contributions to the phase of the image were minimized by tilting to weakly diffracting conditions, as confirmed by reference to convergent-beam electron diffraction patterns. Small residual corrections were determined using multislice calculations. From a total of 18 separate measurements, it is concluded that the value of the mean inner potential is 14.3 (2) V, which agrees with recent theoretical calculations to within experimental error.
The joint probability distribution function of structure factors with rational indices. VI. Cases with reduced dimensionality
Authors: Giacovazzo Carmelo , Siliqi Dritan , Fernández-Castaño Cristina , Cascarano Giovanni Luca , Carrozzini Benedetta
Published: July 2002

The probabilistic formulas relating standard and mixed type reflections (these last show integral and half-integral indices) are derived. It is shown that probabilistic estimates can be obtained by using particular sections of the three-dimensional reciprocal space. The concept of structure invariant is extended to define the wider class of structure quasi-invariant. Their statistical behaviour is briefly discussed with the help of some practical tests.
Bonding effects and the crystal structures of (NH4)2[Cu(H2O)6](SO4)2 and its H2
18O substituted form at 9.5 K
Authors: Figgis Brian N. , Sobolev Alexandre N. , Simmons Charles J. , Hitchman Michael A. , Stratemeier Horst , Riley Mark J.
Published: July 2002

The crystal structures of the Tutton salts (NH4)2-[Cu(H2O)6](SO4)2, diammonium hexaaquacopper disulfate, formed with normal water and isotopically substituted H2
18O, have been determined by X-ray diffraction at 9.5 K and are very similar, with Cu—O(7) the longest of the Cu—O bonds of the Jahn–Teller distorted octahedral [Cu(H2O)6]2+ complex. It is known that structural differences accompany deuteration of (NH4)2[Cu(H2O)6](SO4)2, the most dramatic of which is a switch to Cu—O(8) as the longest such bond. The present result suggests that the structural differences are associated with hydrogen-bonding effects rather than with increased mass of the water ligands affecting the Jahn–Teller coupling. The Jahn–Teller distortions and hydrogen-bonding contacts in the compounds are compared with those reported for other Tutton salts at ambient and high pressure.
Aplanarity of CO3 groups: a theoretical investigation
Authors: Winkler Björn , Zemann Josef , Milman Victor
Published: July 2002

Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO_{3} groups in some carbonates such as dolomite, CaMg(CO_3)_2, aragonite, CaCO_3, and norsethite, BaMg(CO_3)_2, is a ground-state property. This distortion stabilizes dolomite by ∼500 J mol^{-1}. Up to at least 6 GPa, the aplanarity of CO_3 groups in dolomite is independent of pressure. In aragonite the aplanarity increases slightly on increasing pressure, while a significant tilting of the CO_3 groups occurs. The calculations do not support previous findings of anomalously low values for the pressure derivative of the bulk moduli, B^\prime, of aragonite and dolomite. Instead, the computed pressure dependences of the unit-cell volumes correspond to B^\prime = 5.0 (5) for aragonite and B^\prime = 4 (1) for dolomite, when fitted with a third-order Birch–Murnaghan equation-of-state.
Localized defects in radiation-damaged zircon
Authors: Ríos Susana , Malcherek Thomas , Salje Ekhard K. H. , Domeneghetti Chiara
Published: July 2002

The crystal structure of a radiation-damaged natural zircon, ZrSiO_{4} (\alpha-decay radiation dose is ca 1.8 × 10^{18} α-decay events g−1), has been determined. The anisotropic unit-cell swelling observed in the early stages of the amorphization process (0.17% along the a axis and 0.62% along the c axis compared with the undamaged material) is a consequence of the anisotropy of the expansion of ZrO_{8} polyhedra. Larger anisotropic displacement parameters were found for Zr and O atoms, indicating that the distortion produced by \alpha particle-induced localized defects mainly affects the ZrO_{8} unit. The overall shape of SiO_{4} tetrahedra remains essentially undistorted, while Si—O bonds are found to lengthen by 0.43%.
Structure of manganese diacetate tetrahydrate and low-temperature methyl-group dynamics
Authors: Nicolaï Béatrice , Kearley Gordon J. , Cousson Alain , Paulus Werner , Fillaux François , Gentner Fabien , Schröder Ludger , Watkin David
Published: July 2002

We have determined the crystal structure of manganese(II) diacetate tetrahydrate at 300 and 14 K by single-crystal neutron diffraction. Proton density distributions for each of the three crystallographically distinct methyl groups have been calculated by Fourier difference. At room temperature the observed densities are those of quasi-free rotors. At low temperature rather well localized protons are observed. Inelastic neutron scattering measurements performed with single crystals allow us to assign each of the three tunnelling lines to a particular crystal site. Classical molecular dynamics simulations give density distributions in qualitative agreement with the observations. With quantum mechanics proton distributions can be represented with rotational wavefunctions convoluted with static distributions of librational coordinates. The effective rotational potentials are temperature dependent.
Average structure of the composite crystal urea/octanedioic acid at room temperature within the superspace formalism
Authors: Peral I. , Madariaga G. , Petříček V. , Breczewski T.
Published: July 2002

The average structure of the composite urea/octanedioic acid has been refined using the superspace formalism [superspace group H′3121(00γ)00\bar{1}]. Modulation effects seem to be almost negligible. The guest substructure appears to be largely disordered and has been modelled using rigid units occupying 12 equiprobable different orientations inside the urea tunnels. Guest molecules are slightly tilted with respect to the tunnel axis favouring a stronger guest–guest intratunnel interaction.
Polymorphism of maleic hydrazide. I
Authors: Katrusiak Andrzej
Published: July 2002

The third polymorph (denoted MH3) of maleic hydrazide (3,6-dihydroxypyridazine in the monolactim form, 6-hydroxy-3-pyridazinone, C4H4N2O2) has been studied by X-ray diffraction and shown to be monoclinic, space group P21/n. Polymorph MH3 was found as the prevailing form along with the rare triclinic polymorph MH1, space group P\bar 1, but they were obtained separately from monoclinic MH2, space group P21/c. The structure of MH1, previously studied by photographic methods, has been redetermined. Polymorph MH3 exhibits the same scheme of molecular association into hydrogen-bonded ribbons as in MH1 and MH2, but the arrangements of the aggregates and details of their supramolecular conformations are different. The accommodation of the supramolecular conformations to the requirements of close packing of the aggregates in crystal lattices, as well as the symmetries of the polymorphs, are analyzed.
Studies of molecular structure parameters of 20-piperidin-2-yl-5α-pregnan-3β,20-diol and its N-methyl derivative: two inhibitors of Δ24(25) sterol methyl transferase and Δ24(24′) sterol methyl reductase of Trypanosoma cruzi
Authors: Atencio Reinaldo , Visbal Gonzalo , Pekerar Sara , Papale Jham , Urbina Julio A.
Published: July 2002

Molecular structural parameters of two potential drugs against Trypanosoma cruzi epimastigotes, 20-piperidin-2-yl-5α-pregnan-3β,20-diol (1) and 20-N-methylpiperidin-2-yl-5α-pregnan-3β, 20-diol (2) were studied using a combination of a stereoselective synthetic route, spectroscopic characterization and single-crystal X-ray analysis. Both compounds were synthesized with an R configuration at C20. This chirality is a consequence of the stereoselectivity observed during the formation of the intermediate 20-pyridin-2-yl-5α-pregnan-3β,20R-diol (4). NMR data indicated that the six-membered aza ring of (2) is conformationally more restrained, in CDCl3 solution, than (1). X-ray studies showed that maximum deviations among structural molecular parameters of (1) and (2) correspond to torsion angles along the C20—C22 bonds, leading to a different relative orientation of the N atom; a critical structural parameter for the binding properties of aza-sterols to Δ24(25) sterol methyl transferase. Cremer–Pople parameters of the five-membered rings of (1) and (2) lie in the observed range for a family of tetracyclic fused ring systems retrieved from the CSD. The φ2 parameter of (1) lies just on the mean of the family, while φ2 of (2) deviates significantly towards the lower limit.
Structure and pseudosymmetry of cholesterol at 310 K
Authors: Hsu Leh-Yeh , Kampf Jeff W. , Nordman Christer E.
Published: July 2002

The structure of cholesterol above the (304.8 K) phase transition, previously published in preliminary form [Hsu & Nordman (1983). Science, 220, 604–606], has been fully refined using augmented X-ray data. The crystals are triclinic, space group P1, with (reassigned) cell parameters a = 27.565 (10), b = 38.624 (16), c = 10.748 (4) Å, α = 93.49 (3), β = 90.90 (3), γ = 117.15 (3)°, and V = 10151 (7) Å3. The unit cell contains Z = 16 molecules, of which eight are related to the other eight by unusual twofold rotational pseudosymmetry. The structure is related to the room-temperature phase, with Z = 8, by a rearrangement of some of the molecules, and by a doubling of the a axis.
CRYSTMET: a database of the structures and powder patterns of metals and intermetallics
Authors: White Peter S. , Rodgers John R. , Le Page Yvon
Published: July 2002

CRYSTMET is a database of critically evaluated crystallographic data for metals (including alloys, intermetallics and minerals) and associated bibliographic, chemical and physical information. Also included are simulated powder diffraction patterns for all of the entries. The database currently contains almost 70000 entries and covers the literature exhaustively from 1922 to the present. The database is available on CD-ROM with search/analysis software for use on personal computers. This software can be used with any database in the appropriate format; currently CRYSTMET and the ICSD databases are available. This paper describes the database content, the procedures used in its construction, the software made available to the user and a number of potential uses for the data.
CIF: the computer language of crystallography
Authors: Brown I. David , McMahon Brian
Published: July 2002

The Crystallographic Information File (CIF) was adopted in 1990 by the International Union of Crystallography as a file structure for the archiving and distribution of crystallographic information. The CIF standard is now well established and is in regular use for reporting crystal structure determinations to Acta Crystallographica and other journals. The structure of CIF is flexible and extensible and is compatible with other evolving standards. It is well suited to relational and object-oriented models, and is being adopted by the crystallographic databases. This paper reviews the development of CIF and describes its salient features. Future extension of the standard to include implementation of methods will allow CIF to exploit the potential of advanced information-handling software.
Chemistry of the S=O bond. 13. Structures of isomers 1 (I) and 2 (II) of 4-methyl-1,3,2-dioxathiane 2-oxide and 1,3,2-dioxathiepane 2-oxide (III) at 256, 240 and 228 K
Authors: Hellier D.G. , Luger P. , Buschmann J.
Published: July 2002

The structures of the two isomers of 4-methyl-1,3,2-dioxathiane 2-oxide, C4H8SO3 (I, II), and 1,3,2-dioxathiepane 2-oxide, C4H8SO3 (III), have been determined by X-ray diffraction methods for single crystals that were grown in this case at low temperatures from the normally liquid compounds. Compound (I): orthorhombic, P212121, Mr
= 136.17, a = 7.204 (6), b = 13.670 (11), c = 6.041 (6) Å, V = 594.91 Å3, Z= 4, Dx
= 1.520 Mg m−3, Mo Kα (λ = 0.71068 Å), μ = 4.49 cm−1, F(000) = 288, T = 256 K, R = 0.042 for 778 unique reflections. Compound (II): orthorhombic, P212121, Mr
= 136.17, a = 18.238 (6), b = 10.265 (3), c = 6.616 (2) Å, V = 1238.60 Å3, Z= 8, Dx
= 1.460 Mg m−3, Mo Kα (λ = 0.71068 Å), μ = 4.31 cm−1, F(000) = 576, T = 240 K, R = 0.032 for 1597 unique reflections. Compound (III): monoclinic, P21/n, Mr
= 136.17, a = 6.060 (3), b = 11.185 (8), c = 9.186 (4) Å, β = 101.32 (4)°, V = 610.52 Å3, Z = 4, Dx
= 1.481 Mg m−3, Mo Kα (λ = 0.71068 Å), μ = 4.38 cm−1, F(000) = 288, T = 228 K, R = 0.037 for 1161 unique reflections. Compound (I) adopts a chair structure, which has a methyl group in an equatorial position and an axially oriented S=O group. Compound (II) is similar, except that the S=O group is equatorially oriented. Compound (III) adopts a twist-chair conformation with a local approximate endocyclic twofold axis passing through S(1) and the middle of the opposite bond, C(4)—C(5).
A synchrotron X-ray study of the electron density in Y2BaCuO5
Authors: Hsu R. , Maslen E. N. , Ishizawa N.
Published: July 2002

The deformation density (Δρ) for Y2BaCuO5, barium diyttrium cuprate, determined by single-crystal X-ray diffraction with synchrotron radiation, is affected to only a limited degree by the bonding interactions involving the O anions. Electron density is strongly depleted along the cation–cation contacts within the mirror plane in the structure and is transferred to regions between mirror planes that do not lie along short cation–cation vectors. The structural geometry for the CuO5 moiety, with the Cu atom in the +2 state, closely resembles that of the Cu2O5 group in YBa2Cu3O7−δ
, for which the +3 state involvement for Cu has been suggested. Space group Pnma, orthorhombic, Mr
= 458.68, a = 12.1793 (7), b = 5.6591 (5), c = 7.1323 (4) Å, V = 491.6 Å3, Z = 4, Dx
= 6.197 Mg m−3, λ = 0.9 Å, μ
0.9 = 28.893 mm−1, F(000) = 812, T = 293 K, R = 0.020, wR = 0.020, S = 3.09 (5) for 2225 unique reflections.
A critical review of the experimental valence charge density of GaAs
Authors: Pietsch U. , Hansen N. K.
Published: July 2002

The valence charge and difference densities of GaAs have been calculated without previous refinements of a charge density model using six different data sets of X-ray structure amplitudes published until now. Since the data sets have been measured by means of different experimental methods and due to the different data treatment, the individual structure factors differ on the absolute scale. Furthermore, different temperature factors have been published. In order to bring the data to a common level, we used the same two harmonic temperature factors and the same algorithm for correcting the different sets of experimental data for anomalous dispersion. Because of the non-centrosymmetry of the zinc blende structure, these procedures are not strictly model-independent. A simple bond charge model was used to obtain phases of the structure amplitudes and to perform the above-mentioned corrections. In general, the details of `experimental' charge densities depend sensitively on the balanced ratio among the structure factor moduli used. A smooth density map is only obtained if all F have the same high level of accuracy [δ(F)/F < 1%] and if `outliers' are omitted. Only four of the six data sets describe the covalent bond and the partial charge transfer between neighboring atoms, in qualitative agreement with our expectation based on the results of pseudo-potential calculations. However, some quantitative discrepancies remain, particularly in the height of the charge density maximum between nearest neighbours and in some details outside the bonding region.
Antiferroelectric modulations in Sb2WO6 and Sb2MoO6
Authors: Ling C. D. , Withers R. L. , Rae A. D. , Schmid S. , Thompson J. G.
Published: July 2002

The structure of Sb2WO6 [Mr
= 523.4, triclinic, F{\bar 1}, a = 11.132 (1), b = 9.896 (4), c = 18.482 (7) Å, α = 90.20 (4), β 96.87 (8), γ = 90.21 (5)°, Dx
= 6.88  g cm−3, Z = 16, Mo Kα, λ = 0.7107 Å, μ = 338.5 cm−1, F(000) = 3536] has been refined as an enlarged 2a × 2b × 2c
F-centred superstructure of the previously reported structure [Castro, Millan, Enjalbert, Snoeck & Galy (1994). Mat. Res. Bull.
29, 871–879] refined in the space group P1. The re-refinement follows the observation, initially by TEM, of satellite reflections at G± ½(111)*, where G represents a reflection of the P1 reciprocal lattice. A final value of 0.040 for R
1 = Σ
h
||F
obs(h)| − |F
calc(h)||/Σ
h
||F
obs(h)| was obtained for 3316 merged reflections with I(h) > 3σ[I(h)], compared with R
1 = 0.12 for the previous refinement. The refined structure is described in terms of an antiferroelectric modulation of a P121/a1 underlying parent structure in the original setting. Twinning of the crystal was successfully modelled in the refinement. Synthesis of the previously unknown phase Sb2MoO6 [Mr
= 435.5, triclinic, F{\bar 1}, a = 10.758 (1), b = 9.673 (2), c = 17.57 (1) Å, α = 90.00 (5), β = 96.98 (3), γ = 90.05 (2)°, Z = 16, Dx
= 4.97 g cm−3] is also reported, along with evidence for its isostructuralism with Sb2WO6.
Trimorphism of 2,2'-dipyridylamine: structures, phase transitions and thermodynamic stabilities
Authors: Schödel H. , Näther C. , Bock H. , Butenschön F.
Published: July 2002

Crystals of an unknown third crystalline modification of 2,2′-dipyridylamine were obtained from acetone solution and their crystal structure was determined at both 150 K and room temperature. In contrast to the known orthorhombic and triclinic forms, which contain hydrogen-bridged dimers, the molecules of the monoclinic polymorph are arranged in tetramers. The crystallographic results for the monoclinic form are presented here, as well as a detailed comparison of crystal and molecular structures of the three polymorphs. Studies by differential thermal analysis (DTA) and optical microscopy, performed with single crystals, show a transformation of the orthorhombic phase at ~323 K and of the monoclinic form at ~368 K. According to powder diffraction studies, transformation of the low melting orthorhombic polymorph results in a mixture of monoclinic and triclinic phases, whereas the monoclinic modification transforms into the triclinic phase just below its melting point at 368 K. The single crystals of both forms are destroyed during the conversion and, therefore, in both cases a reconstructive transition via nucleation and growth should occur. The conditions for the crystallization of the distinct modifications and their relative thermodynamic stabilities are investigated in different solvents and at different temperatures. Independent of the solvent chosen, the orthorhombic form is the most stable below 263 K. In the range between 263 and 313 K the monoclinic form appears to be thermodynamically advantageous and above that temperature, the stability order is changed in favour of the triclinic polymorph. Based on the experimental results, a qualitative free energy-temperature diagram is provided.
Neutron diffraction study of [Pt(en)2][Pt(en)2I2](ClO4)4 at 20 K: structure and evidence of a new phase transition
Authors: Bardeau J.-F. , Bulou A. , Klooster W. T. , Koetzle T. F. , Johnson S. , Scott B. , Swanson B. I. , Eckert J.
Published: July 2002

The structure at 20 K of fully deuterated [Pt(en)2][Pt(en)2I2](ClO4)4, where en = ethylenediamine, has been determined by neutron diffraction on a single-crystal. It is shown that the symmetry is monoclinic with cell parameters a = 16.650 (7), b = 5.760 (2), c = 14.751 (2) Å and β = 99.19 (1)°, that is, with respect to the previously reported room-temperature structures, a doubling of the c parameter is observed. The space group is C2/c and the refinements lead to an R value of 0.0374. All the bond lengths (C—C, C—N, C—D, N—D and Cl—O) and angles are consistent with the expected values and the main difference with respect to the previously reported room-temperature structures results in an ordering of the stacking of the ethylenediamines. The iodines are distributed over two sites, which gives two Pt—I distances, 2.712 (3) Å, attributed to the PtIV—I bond, and 3.048 (3) Å, attributed to PtII—I. Actually, very weak diffraction peaks that provide evidence for an additional doubling of the a and b parameters are also observed and a space group C2 is therefore proposed. This could result from a very slight ordering of the chains with respect to one another, but, due to the weakness of such signals and the large number of atoms to be considered, no reliable refinement has been obtained in C2. On the other hand, on heating the crystal the diffraction peaks with I odd greatly decrease in intensity between 150 and 160 K, which unambiguously argues for the existence of a phase transition presumably connected to an order—disorder of the ethylenediamines.
Bond length–bond order relations and calculated geometries for some benzenoid aromatics, including phenanthridine. Structures of 5,6-dimethylphenanthridinium triflate, [N-(6-phenanthridinylmethyl)-aza-18-crown-6-κ5
O,O',O'',O''',O''''](picrate-κ2-O,O')potassium, and [N,N'-bis(6-phenanthridinyl-κN-methyl)-7,16-diaza-18-crown-6-κ4
O,O',O'',O''']sodium iodide dichloromethane solvate
Authors: Kiralj R. , Kojić-Prodić B. , Žinić M. , Alihodžić S. , Trinajstić N.
Published: July 2002

The crystal structures of the title compounds are studied in order to investigate the role of novel fluoroionophores in complexation of sodium and potassium. In the potassium complex seven coordination, including the picrate ligand, is encountered. An additional coordination site is via the phenanthridine nitrogen at 3.252 (2) Å (second coordination). The complex is of C
1 symmetry and the aza-18-crown-6 macrocylic ring exhibits a crown-type conformation. The 7,16-diaza-18-crown-6 macrocycle accommodates a six-coordinate sodium with two additional ligands, via nitrogen from phenanthridine units. The complex cation shows a crystallographic twofold symmetry. The macrocycle is not of the crown-type conformation. In both complexes the alkali metals are shifted out of the cavity centres towards a picrate ligand in [N-(6-phenanthridinylmethyl)-aza-18-crown-6-κ5
O,O′,O′′,O′′′,O′′′′](picrate-κ2
O,O′)potassium and the phenanthridine units in [N,N′-bis-(6-phenanthridinyl-κN-methyl)-7,16-diaza-18-crown-6-κ4
O,O′,O′′,O′′′]sodium iodide dichloromethane solvate. Semi-empirical and molecular mechanics calculations based on various force fields were used for the optimization of phenanthridine geometry. The values obtained are compared with experimental data. Valence bond calculations of bond lengths in some benzenoid aromatic systems (C—C bonds in benzenoid hydrocarbons, azabenzenoid hydrocarbons and picrate-like systems; C—N bonds in the azabenzenoids; C—O bonds in the picrate-like systems), as well as some analogous Hückel molecular orbital calculations (C—C bonds in the benzenoid hydrocarbons and the azabenzenoids), were found to agree with the observed values (average differences up to 0.015 Å). These approaches can be used by means of bond length-bond order relations for prediction of bond lengths in the phenanthridine units as well as in the picrate.
Molecular electric moments and electric field gradients from X-ray diffraction data: model studies
Authors: Spackman M. A. , Byrom P. G.
Published: July 2002

Model X-ray data sets, with and without the inclusion of experimental thermal motion parameters, have been computed via Fourier transformation of ab initio molecular electron densities for 12 different molecular crystals. These datasets were then analysed with three different multipole models of varying sophistication and, from the multipole functions, molecular dipole and second moments, as well as electric field gradients (EFG's), at each nuclear site were computed and compared with results obtained from the original ab initio wavefunctions. The results provide valuable insight into the reliability of these properties, extracted in the same way from experimental X-ray data. Not all molecular systems display identical trends, but a general pattern is discernible. Specifically, dipole moments are typically underestimated by a small but significant amount (~ 10–15%), the trace of the second moment tensor is well determined but overestimated by a few per cent and electric field gradients at protons are confirmed to be well within reach of a careful charge density analysis of X-ray diffraction data.
Distortion Characteristics Across the Structural Phase Transition in (Cu1−x
Zn
x
)WO4
Authors: Schofield P. F. , Knight K. S. , Redfern S. A. T. , Cressey G.
Published: July 2002

Rietveld analysis of neutron powder diffraction data on the sanmartinite (ZnWO4)-cuproscheelite (CuWO4) solid solution has enabled the comparison of microscopic and macroscopic order parameters associated with the P2/c–P\overline 1 structural phase transition. The macroscopic spontaneous strain, calculated from the lattice parameters, conforms well with a second-order Landau model. Furthermore, this is also true of the symmetry-related atomistic M—O order parameter and the quadratic elongation of the MO6 octahedra. It is clear that the Jahn–Teller effect, associated with the divalent Cu cation, is the driving force for the phase transition and the excess elongation evident in the M—O(2) bond giving rise to the non-symmetry predicted strain element e
22. The existence of a large region of order parameter saturation at the copper-rich end of the solid solution is also associated with the MO6 elongation and the breakdown of the homogeneous strain field of the zinc solute atoms.
Ab Initio Structure Determination of Two Polymorphs of Cyclopentadienylrubidium in a Single Powder Pattern
Authors: Dinnebier R. E. , Olbrich F. , van Smaalen S. , Stephens P. W.
Published: July 2002

The structures of two polymorphic phases of solid RbC5H5 have been solved ab initio by high-resolution powder X-ray diffraction measured in a single powder pattern. The compound crystallizes in space group Pbcm (phase I), Z = 8, with unit-cell parameters a = 9.3396 (1), b = 10.9666 (1), c = 10.5490 (1) Å, and in space group Pnma, Z = 4 (phase II), with unit- cell parameters a = 10.7990 (2), b = 8.6923 (2), c = 5.7061 (2) Å. Both phases show polymeric zigzag chains, so-called bent-polydecker sandwich structures, with an. average Rb—Cpz (Cp-ring-centroid) distance of 2.97 Å. Phase II contains a single chain along the a axis, whereas phase I contains two similar but crystallographically independent chains perpendicular to each other along the b axis and along the c axis, leading to a more complex coordination sphere around the Rb atoms. The chains are bent with Cpz—Rb—Cpz angles ranging from 123.5 to 136.5°.
`Inverted Twinning' in Intergrowth Tungsten Bronzes
Authors: Kihlborg L. , Sundberg M.
Published: July 2002

A new type of twinning occurring in intergrowth tungsten bronzes (ITB) is described, revealed by high-resolution electron microscopy. Across the twin boundary the two structure elements of hexagonal tungsten bronze- and tetragonally distorted ReO3-types are interchanged and grow in strict geometrical relationship to each other. The structure is thus `inverted' and in the general case the two `twin' parts represent different members of the structure family. Some members remain invariant upon inversion, however. This defect is most often seen as ribbons in an ITB matrix in Mo-doped samples Cs
x
Mo
y
Wl−y
O3, which require a lower synthesis temperature than pure tungsten bronzes. They may be frozen-in stages of a slow ordering process. A similar type of twinning might be found in other intergrowth structures.
Energetic Materials: The Preparation and Structural Characterization of Three Biguanidinium Dinitramides
Authors: Martin A. , Pinkerton A. A. , Gilardi R. D. , Bottaro J. C.
Published: July 2002

Three biguanidinium salts of the energetic dinitramide anion have been prepared and structurally characterized from room-temperature X-ray diffraction data. Biguanidinium mono-dinitramide, (BIGH)(DN), triclinic, P\overline 1, a = 4.3686 (4), b = 9.404 (2), c = 10.742 (1) Å, \alpha = 83.54 (1), \beta = 80.386 (9), \gamma = 79.93 (1)°, V = 426.8 (1) Å3, Z = 2, D

x
= 1.62 g cm−3. Biguanidinium bis-dinitramide, (BIGH2)(DN)2, monoclinic, C2/c, a = 11.892 (2), b = 8.131 (1), c = 13.038 (2) Å, \beta = 115.79 (1)°, V = 1135.1 (3) Å3, Z = 4, D

x
= 1.84 g cm−3. Biguanidinium bis-dinitramide monohydrate, (BIGH2)(DN)2.H2O, orthorhombic, P212121, a = 6.4201 (6), b = 13.408 (1), c = 14.584 (2) Å, V = 1255.4 (4) Å3, Z = 4, D

x
= 1.76 g cm−3. All three structures are characterized by extensive hydrogen bonding. Both the mono- and diprotontated cations consist of two planar halves twisted with respect to each other. The dinitramide anion has a surprisingly variable and asymmetric structure. The two halves of the anion are twisted with respect to each other; however, the twist varies from 5.1 to 28.9°. In addition, the two ends of the anion have significantly different geometries, e.g. the `equivalent' N—N bond lengths differ by up to 0.045 Å.
Cu12Sb4S13: A Temperature-Dependent Structure Investigation
Authors: Pfitzner A. , Evain M. , Petricek V.
Published: July 2002

Synthetic tetrahedrite, Cu12Sb4S13, obtained by reaction of the elements, has been investigated at various temperatures in the 295–573 K range. It crystallizes in the cubic system with a = 10.3293 (6) Å, V = 1102.1 (2) Å3, space group I\overline 43m and Z = 2 at room temperature. The structure refinement converged to a residue of R = 0.0165 (at room temperature, wR = 0.0200) for 389 independent reflections and 34 refined parameters. A Gram–Charlier non-harmonic development of the atomic displacement factor for both independent Cu atoms was used. The results show a disorder for the three-coordinated Cu atom, within and perpendicular to the plane of the three surrounding S atoms. However, although the non-harmonic probability density deformation increases with raising the temperature within this plane, it barely changes in the perpendicular direction. This suggests two different types of disorder: static in-plane and dynamic out-of-plane, therefore underlining a possible diffusion pathway for copper ions. To check the significance of the observed effects, the errors for the one-particle potentials and the probability density function maps were calculated by means of a Monte-Carlo method.
Resonance-Induced Hydrogen Bonding at Sulfur Acceptors in R
1
R
2C=S and R
1CS2
− Systems
Authors: Allen F. H. , Bird C. M. , Rowland R. S. , Raithby P. R.
Published: July 2002

The hydrogen-bond acceptor ability of sulfur in C=S systems has been investigated using crystallographic data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. The R1R2C=S bond lengths span a wide range, from 1.58 Å in pure thiones (R
1 = R
2 = Csp
3) to 1.75 Å in thioureido species (R
1 = R
2 = N) and in dithioates —CS^{-}_2. The frequency of hydrogen-bond formation at =S increases from 4.8% for C=S > 1.63 Å to more than 70% for C=S > 1.70 Å in uncharged species. The effective electronegativity of S is increased by conjugative interactions between C=S and the lone pairs of one or more N substituents (R
1
R
2): a clear example of resonance-induced hydrogen bonding. More than 80% of S in —CS^{-}_2 accept hydrogen bonds. C=S...H—N,O bonds are shown to be significantly weaker than their C=O...H—N,O analogues by (a) comparing mean S...H and O...H distances (taking account of the differing non-bonded sizes of S and O and using neutron-normalized H positions) and (b) comparing frequencies of hydrogen-bond formation in `competitive' environments, i.e. in structures containing both C=S and C=O acceptors. The directional properties and hydrogen-bond coordination numbers of C=S and C=O acceptors have also been compared. There is evidence for lone-pair directionality in both systems, but =S is more likely (17% of cases) than =O (4%) to accept more than two hydrogen bonds. Ab
initio calculations of residual atomic charges and electrostatic potentials reinforce the crystallographic observations.
Chiral Molecular Alloys: Patterson-Search Structure Determination of L-Carvone and DL-Carvone from X-ray Powder Diffraction Data at 218 K
Authors: Sañé J. , Ruis J. , Calvet T. , Cuevas-Diarte M. A.
Published: July 2002

The crystal structures of pure L-carvone [(R)-(−)-2-methyl-5-(1-methylethenyl)-2-cyclohexen-1-one, C10H14O] and the equimolar mixture DL-carvone (RS) have been determined by Patterson-search methods at low resolution from laboratory X-ray powder diffraction data (218 K). Crystal data: (L) a = 6.8576 (3), b = 6.8831 (5), c = 19.988 (2) Å, P212121 space group, Z = 4; (DL) a = 6.9744 (3), b = 6.8094 (6), c = 20.038 (7) Å, Pcmn space group, Z = 4. The L-carvone structure has been refined by the Rietveld method as a rigid body, allowing the rotation of the isopropenyl group (R\rho, = 0.030 and R

wp
= 0.043). Although the structure of DL-carvone could be unambiguously established, the Rietveld refinement was not possible due to the existence of preferred orientation in the sample and the difficulty in modelling the disorder. The molecular packing is essentialy the same for both compounds and can be explained as a stacking of two different molecular layers in the [001] direction. In each layer the molecules are placed with their long axis perpendicular to the layer plane, in a head-to-tail manner. The great similarity between the molecular shapes of L and D enantiomers favours the positional disorder in DL-carvone. This result confirms the mixed crystal formation for the chiral carvone system as proposed in recent thermodynamic studies. The DL-carvone crystal must be considered as a pseudo-racemate, since both enantiomers are randomly distributed over all the lattice sites.
Retention of -4 Symmetry in Compounds Containing MAr4 Molecules and Ions
Authors: Lloyd M. A. , Brock C. P.
Published: July 2002

A search of the Cambridge Structural Database shows that MAr4 molecules usually retain the low-energy \overline 4 conformation in the crystal. MAr^{q}_{4} ions also usually occupy sites if the counterion can conform to \overline 4 (or higher) symmetry. The ability of the aryl groups to fit well around an empty \overline 4 site is crucial to the symmetry retention. Molecules (and ions) in the tetragonal structures pack in columns that parallel the c axis; the columnar cross sections are approximately square. Aryl groups in adjacent columns can interleave to form a sort of herringbone arrangement. The tetragonal packing of columns is sufficiently favorable that it sometimes occurs even if the molecules must be disordered. The columnar motif is also found in some lower-symmetry structures of MAr4 molecules and MAr^{q}_{4} ions; these structures may be pseudosymmetric.
Thermal Expansion of Chemical Bonds
Authors: Brown I. D. , Dabkowski A. , McCleary A.
Published: July 2002

Using the bond-valence model, a relationship is developed between the thermal expansion of a chemical bond, its amplitude of thermal vibration and its force constant. An empirical expression found between bond valence and the force constants derived from vibrational spectroscopy allows all of these quantities to be predicted from either the expected or the observed bond valence. The thermal expansion predicted by these relations is in excellent agreement with the average expansion observed around cations in inorganic solids, but individual bonds are found to expand more or less than this depending on strains and constraints within the structure. Comparison between the theoretical and observed amplitudes of thermal vibration gives a quantitative measure of correlation between the thermal motions of atoms that form the bond. The theory also shows how the parameters used in calculating bond valences from bond lengths should be corrected for temperature.
Li+/Na+ β-Alumina: A Combined Single-Crystal Neutron and X-ray Diffraction Study
Authors: Edström K. , Gustafsson T. , Thomas J. O. , Farrington G. C.
Published: July 2002

The crystal structure and ionic distribution in the conduction plane of the partially exchanged Na+ \beta-alumina system Li0.75Na0.47Al11O17.11 has been determined from single-crystal X-ray diffraction at 30 and 298 K, in combination with a single-crystal neutron diffraction study at 295 K. At 30 K the Li+ ions occupy two sites: one in (2/3, 1/3, 1/4) and one ∼1.0 Å out of the z = 1/4 conduction plane. The Na+ ions are also found in two sites, both in the conduction plane. At 298 K the Li+ ions are found in three sites; the same two sites as at 30 K, as well as at a third site between these two. The Na+ ions occupy the same sites as at 30 K. The combination of X-ray and neutron diffraction is used to resolve the distribution of Na+ and Li+ ions. The column O5 atom in the Al3—O5—Al3 bridge between the spinel blocks is found to be disordered. Charge compensation is achieved through a Frenkel defect of two interstitial Al atoms, one on either side of the conduction plane, together with an extra oxygen in the conduction plane. One such defect occurs, on average, in every 4.5 unit cells.
Phase Transitions in Tris(3,5-dimethylpyrazol-1-yl)methane. The Structure of the High-Temperature Phase from X-ray Powder Diffraction
Authors: Ochando L. E. , Rius J. , Louër D. , Claramunt R. M. , Lopez C. , Elguero J. , Amigó J. M.
Published: July 2002

The crystal structure of the sublimated form (m.p. = 424 K) of tris(3,5-dimethylpyrazol-l-yl)methane has been solved by a Patterson search method from laboratory X-ray powder diffraction data. Crystal data: trigonal symmetry with the unit-cell parameters a = 16.152 (1) and c = 5.353 (1) Å, space group P3, C16H22N6, Z = 3, 293 K. After indexing the powder pattern by two methods, the unit-cell parameters found were refined by a least-squares technique. A whole pattern-fitting program was used to extract the integrated intensities. The structure was solved taking a related compound as a search model and the final Rietveld refinement converged to R

wp
= 0.077 and R

p
= 0.059. This study is one of the first examples of Patterson search structure determination from an hemihedral space group using powder data. The complexity of the structural determination is increased by the presence of three molecules in the asymmetric unit.