bluuebunny/crossref_metadata_2025 huggingface dataset:
Single-crystal neutron diffraction of urea-phosphoric acid: evidence for H-atom migration in a short hydrogen bond between 150 K and 350 K
Authors: Wilson C. C. , Shankland K. , Shankland N.
Published: December 2003

The structure of urea-phosphoric acid has been refined using single-crystal neutron diffraction data collected at seven temperatures in the range 150 K to 350 K. The structure is orthorhombic, space group Pbca, Z=8, a=17.453(5)-17.679(8), b=7.454(3)-7.483(4), c=8.963(3)-8.992(4) Å, V=1166.0–1189.6 Å3. The molecules are linked by a short intermolecular O···H···O hydrogen bond, with O···O ≈ 2.4 Å and <O···H···O ≈ 170°. At 150 K, this bond is markedly asymmetric, with O···H = 1.168(4) Å and H···O = 1.252(4) Å. However, at 300 K and above, the H-atom is essentially centred between the two O-atoms. This systematic shift in position of the H-atom (up to ≈ 0.04 Å) is significant in relation to the esd´s of the measurements. H-atom migration along a hydrogen bond with temperature has been observed previously with other species, including quinolinic acid, and the hydrogen phthalate anions of lithium hydrogen phthalate monohydrate. There is no evidence to support the notion that the H-atom within the O···H···O hydrogen bond in urea-phosphoric acid is disordered, and the diffraction data are consistent with the previously reported hypothesis of a single-minimum potential energy surface for the short hydrogen bond.
Determination, prediction, and understanding of structures, using the energy landscapes of chemical systems - Part I
Authors: Schön J.C. , Jansen Martin
Published: December 2003

In the past decade, new theoretical approaches have been developed to determine, predict and understand the struc-ture of chemical compounds. The central element of these methods has been the investigation of the energy landscape of chemical systems. Applications range from extended crystalline and amorphous compounds over clusters and molecular crystals to proteins. In this review, we are going to give an introduction to energy landscapes and methods for their investigation, together with a number of examples. These include structure prediction of extended and mo-lecular crystals, structure prediction and folding of proteins, structure analysis of zeolites, and structure determination of crystals from powder diffraction data.
Hydrogen bond network of the layered phosphates γ-Zr(H2PO4)(PO4)⋅2H2O and γ-Hf(H2PO4)(PO4)⋅2H2O determined by neutron powder diffraction
Authors: Salvadó M.A. , Pertierra P. , García-Granda S. , Barcina L. M. , Llavona R. , Rodríguez J.
Published: December 2003

The hydrogen atom positions of the layered phosphates Zr(PO4)(H2PO4)⋅2H2O (γ-ZrP) and Hf(PO4)(H2PO4)⋅2H2O (γ-HfP) have been determined from neutron powder diffraction data and the complete crystal structures have been refined by the Rietveld method. The compounds crystallize in the monoclinic space group P21. The final agreement factors are: R
wp = 1.47, R
exp = 0.91 for γ-ZrP and R
wp = 1.73, R
exp = 1.43 for γ-HfP. The two compounds are isostructural and posses a dense hydrogen-bonding network. They are formed by layers consisting of two planes of metallic atoms joined by phosphate and dihydrogenphosphate groups. The interlayer space form channels where each water molecule accepts and donates an hydrogen bond to another water molecule forming a zig-zag chain. Additionally each water molecule acts as acceptor and donor in two hydrogen bonds with oxygen atoms of the same layer. Consecutive layers are, in this way, indirectly held together by hydrogen bonds through the water molecules.
Absorption spectra of calcium tartaric and cadmium oxalate crystals, grown in gel and doped by chromium, cobalt and nickel
Authors: Popov K.
Published: December 2003

This article is devoted to the insertion of admixture atoms to crystals during growth in gel and their influence on the optical properties of the obtained crystals. The crystals of tetra-hydrate calcium tartaric (CaC4H4O6·4H2O) and trihydrate cadmium oxalate (CdC2O4·3H2O) [1,2] were grown in silica-gel and doped during this process by chromium, cobalt and nickel atoms. This procedure causes changes of the optical properties of the crystals, in particular changes of the color. The extinction of the obtained crystals measured by us provides a quantitative description of this evolution.
Structure and ferroelasticity of tetramethylammonium-hexafluoroosmate(IV), (TMA)2[OsF6]
Authors: Küppers H. , Göbel O. , Bruhn C. , Schmahl W. W.
Published: December 2003

(TMA)2[OsF6] has at room temperature a rhombohedral structure (space group R3̅) which is pseudo-cubic (K2[PtCl6]-type). a = 8.0565(6) Å, c = 20.020(3) Å (hexagonal setting). Double refraction decreases linearly with increasing temperature and disappears above 434 K which indicates cubic symmetry of the high temperature phase. The change from the cubic high temperature structure to the rhombohedral structure is characterized by a ferrorotative tilt of the [OsF6] octahedra and the TMA tetrahedra. Evidence of ferroelasticity follows from the switching of the trigonal c-axis by the application of directional pressure. The phase transitions of (TMA)2[OsF6] (Fm3̅ m → R3̅) and of the recently reported (TMA)2[PtF6] and (TMA)2[IrF6] (Fm3̅m → I4/m) are described by Landau’s theory.
Determination, prediction, and understanding of structures, using the energy landscapes of chemical systems – Part II
Authors: Schön J. C. , Jansen Martin
Published: December 2003

In the past decade, new theoretical approaches have been developed to determine, predict and understand the struc-ture of chemical compounds. The central element of these methods has been the investigation of the energy landscape of chemical systems. Applications range from extended crystalline and amorphous compounds over clusters and molecular crystals to proteins. In this review, we are going to give an introduction to energy landscapes and methods for their investigation, together with a number of examples. These include structure prediction of extended and mo-lecular crystals, structure prediction and folding of proteins, structure analysis of zeolites, and structure determination of crystals from powder diffraction data.
Comparing two molecular structures by using half-normal probability plots
Authors: Okada K.
Published: December 2003

To compare the molecular structures between colorless and colored forms of fluoran basic dyes, we developed a computer program COMPARE to compare two sets of isooriented orthogonal coordinates of atoms, intramolecular distances less than 4.65 Å and anisotropic displacement parameters by using half-normal probability plots. As a result the differences in the molecular structure between colorless and colored forms were observed with high sensitivity on all moieties of molecules, especially in hydrogens.
A new Rietveld refinement of κ-Al2O3
Authors: Smrčok L'. , Langer V. , Halvarsson M. , Ruppi S.
Published: December 2003

Rietveld refinement and the final model for pure κ-alumina are presented. The starting model was obtained through total energy calculations based on first principles. The resulting geometry parameters are within intervals of expected values for octahedral and tetrahedral coordina-tions. This result thus serves as a validation of the compu-tational approach as a useful alternative to classical methods in the first step of structure determination.
In situ structure solution of helical sulphur at 3GPa and 400ºC
Authors: Crichton W. A. , Vaughan G. B. M. , Mezouar M.
Published: December 2003

The structure of a 2-chain helical form of sulphur with 9 atoms per unit-cell has been determined from powder synchrotron x-ray diffraction data obtained at 3 GPa and 400ºC, using a combination of global optimization, simulated annealing and Rietveld refinement techniques. Final refinement of the structure in trigonal space group P3221 (no. 154) results in a = 7.0897(2)Å, c = 4.30238(9)Å and V = 187.282(10)Å3. There are two unique S sites per unit cell, the general S1 6c position with x = 0.7727(9), y = 0.3067(11) and z =.6105(12) and the S2 3b with x = 0.8755(8) Å. Bond length and angle analysis shows S1-S1 = 2.070(4)Å and S2-S2 = 2.096(7)Å with an helical S1-S1-S1 divalent angle of 102.7(2)° and 101.7(3)° for S2-S2-S2. The 2 helices formed, contrary to other polymeric sulphur phases (principally, Ψ-S [1]), are non-chiral and evidently have repeats along the (001) translation; that is one turn includes each of the 3 equivalent atoms about the 32-screw axis (S1 helix) and vertex (S2) of the cell. This phase, although temperature quenchable, is observed to back transform to a 4.04Å phase at pressures less than 0.5 GPa. It is likely that this phase represents the true form of the phase XII of Vezzoli and Walsh.
Phonon dispersion curves by inelastic neutron scattering to 12 Gpa
Authors: Klotz S.
Published: December 2003

Recent progress in high pressure techniques allows the measurements of phonon dispersion curves to ~12 GPa by inelastic neutron scattering on triple axis spec-trometers. Provided the structure is not too complex, a vari-ety of low-compressibility solids may be studied over the entire stability range of their ambient pressure forms. This article reviews results obtained during the last five years on the lattice dynamics of a number of “simple” systems (Ge, GaSb, PbTe, FeO, Zn, Fe) where the pressure-induced frequency shifts of the acoustic branches have been studied in considerable detail. In several of these solids pronounced “mode softening” is found under pressure. Grüneisen parameters and elastic constants have been determined and the results were compared to predictions of first-principle calculations.
Crystal structure of kristiansenite: a case of class IIB twinning by metric merohedry
Authors: Ferraris G. , Gula A. , Ivaldi G. , Nespolo Massimo , Raade G.
Published: December 2003

The structure of the new disilicate kristiansenite, Ca2ScSn(Si2O7)(Si2O6OH), has been solved and refined from a crystal polysynthetically twinned by metric merohedry. The Bravais lattice is mC, with parameters a = 10.028(1), b = 8.408(1), c = 13.339(2) Å, α = 90.01(1), β = 109.10(1), γ = 90.00(1)°, but the space-group type is C1 (Z = 4). The twin law is m´, and the two components of the twin have nearly identical volumes: as a consequence, the Laue group of the twin is practically 2/m. By taking into account the twinning, an anisotropic refinement of the structure in C1 converged to R1 = 0.0242 for 259 refined parameters and 4862 observed reflections. The effects of the twinning by metric merohedry and of the volume ratio of the components on the symmetry of the diffraction pattern are discussed. The triclinic structure approximates within about 0.1 Å the monoclinic symmetry, the lower symmetry resulting mainly from cation ordering. Kristiansenite represents a new type of silicate structure and the first known case with the presence of protonated and normal disilicate groups at the same time. The disilicate groups and the other polyhedra centred on cations lie on different alternating (101) planes.
The crystal structures of phenacite-type Li2(MoO4), and scheelite-type LiY(MoO4)2 and LiNd(MoO4)2
Authors: Kolitsch U.
Published: December 2003

The crystal structures of flux-grown Li2(MoO4), LiY(MoO4)2 and LiNd(MoO4)2 were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoK
αX-radiation, CCD area detector, room temperature). Li2(MoO4) is isostructural with phenacite, Be2(SiO4) and has space group R3̅, a = 14.330(2), c = 9.584(2) Å, Z = 18 (R1 = 1.6%). The structure consists of a three-dimensional network of corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra. Average Li–O and Mo–O distances are 1.965 Å (Li1) and 1.967 Å (Li2), and 1.764 Å (Mo). The atomic arrangement is characterised by a narrow open channel along the three-fold axis. Comparisons are drawn with other phenacite-type compounds. Low-temperature single-crystal X-ray studies of Li2(MoO4) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO4)2 and LiNd(MoO4)2 crystallise in the scheelite-type structure, with space group I41/a and Z = 2 (R1 = 1.7 and 2.7 %, re-spectively). The first has a = 5.148(1), c = 11.173(2) Å, V = 296.11(10) Å3, the latter a = 5.243(1), c = 11.440(2) Å, V = 314.47(10) Å3. Both Li and Y/Nd are completely disordered on a jointly occupied site. Mean (Y,Li)– and (Nd,Li)–O distances are 2.40 and 2.47 Å, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. The stoichiometry of LiRE(MoO4)2 (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO4)2 (M = alkali metal, X = Mo, W) compounds is briefly addressed.
A synchrotron X-ray diffraction analysis of near-stoichiometric LiNbO3
Authors: Etschmann B. , Ishizawa N. , Streltsov V. , Oishi S.
Published: December 2003

Single-crystal diffraction data was collected at 120 and 294 K for an approximately spherical LiNbO3 crystal, 16 μm in diameter, using focused λ=0.7516 (1) Å wavelength synchrotron X-radiation and a fast avalanche photodiode (APD) detector. Extinction was significantly less than that recorded for previous single-crystal X-ray diffraction experiments. Anharmonic atomic displacements of Nb and Li were refined using a Gram-Charlier expansion to the fourth order. There was no significant change in the topography of the difference electron density (Δρ) map after anharmonic refinement. An accumulation of electrons present at the vacant site in the 294 K Δρ map was not evident in the 120 K Δρ map, indicating a migration of Li atoms to the vacant site as the temperature increased. The orientation of the electron density around Nb flipped by 90° going from 120 to 294 K.
Polymorphism and multiple twinning of (Et4N)2ZnCl4
Authors: Sondergeld P. , Fuess H. , Ishihara H. , Schmahl W. W.
Published: December 2003

The phase sequence of The phase sequence of (Et4N)2ZnCl4 has been determined based on thermal analysis and 35Cl NQR spectroscopy. On cooling two phase transitions are observed at 223.0(5) and 217.7(9)K, respectively. On heating only one phase transition is found at 228.4(3)K. Therefore three phases are distinguished below the decomposition point (556(5)K) with a metastable phase in the temperature region 217-229K, a twinned low-temperature phase below and a disordered high-temperature phase above this region, respectively. The structure of the low-temperature phase has been determined from X-ray 4-circle diffraction data. The space group of this phase is C2ca(Aba2) with a = 11.366(6)Å, b = 13.347(4)Å and c = 15.246(6)Å at T = 190K. From the group-subgroup relations between the high- and the low-temperature phase the structure of the intermediate phase has been derived and confirmed by the analysis of the transition entropies.
Equations of state for solids under strong compression
Authors: Holzapfel Wolfgang
Published: December 2003

Various approaches for the representation of equation of state data for solids under strong compression are discussed. The theoretical background for reasonable extrapolations to higher pressures and higher as well as lower temperatures is described. The distinction between ideal, regular, and anomalous behaviour allows to gain deeper insight into the electronic changes occurring in various solids under strong compression. The discussion of experimental data for various regular solids leads finally to an estimate of the accuracy obtained in the present realisation of a practical pressure scale based on equation of state measurements.
Symmetry and structural relationships among ABW-type materials
Authors: Kahlenberg V. , Fischer R. X. , Baur W. H.
Published: December 2003

The symmetry relationships among zeolite-type ABW compounds are described in terms of the reductions of symmetry from the highest possible topological symmetry in space group Imam to eight possible subgroups. This family of compounds, also known as Icmm-type compounds, covers a wide range of chemical compositions. The distortion modes inducing the lowering of symmetry are described based on rotation and ordering processes. Atomic site relationships with their evolution from the aristotype are given for the recently described monoclinic P1121/n structures with an ABW-type framework.
Compression mechanism of cubic silica sodalite [Si12O24]: a first principles study of theIm3̅mtoI4̅3mphase transition
Authors: Knorr K. , Winkler B. , Milman V.
Published: December 2003

The high pressure behaviour of cubic silica sodalite has been investigated byab initiocalculations based on density functional theory. The experimentally observed phase transition from the fully-expanded ideal sodalite framework (space groupIm3̅m) to a collapsed framework with space groupI4̅3mcould be successfully reproduced to appear at 0.12 GPa. This transition is discontinuous. The mechanism of the transition can be understood in terms of a tilt of the framework and of deformations of the SiO4-tetrahedra. Discontinuities of the enthalpy, cell volume, Si-O bond lengths and angles at 1.15 and 3.01 Gpa point at further possible pressure-induced iso-symmetric phase transitions in the space groupI4̅3m.
Polymorph stability and phase transitions of trigonal Al1-

x
Ga
x
PO4 mixed crystals
Authors: Barz R.-U. , David F. , Schneider J. , Gille P.
Published: December 2003

Single crystals of Al1-

x
Ga
x
PO4 of various mole fractions x were grown from phosphoric acid solutions under hydrothermal conditions using the slow-heating method. In order to relate the parameters of this low-quartz structure analogue and the observed phase transitions to the respective mole fraction, the crystals were investigated by means of electron probe microanalysis, X-ray powder diffraction and differential thermal analysis. The results are discussed in terms of the segregation effects occuring during growth. The limiting geometrical parameters for the existence of the high-quartz phase are compared to the values given in the literature.
X-ray studies of phase transitions of (Ca,Sr)-åkermanite solid solutions
Authors: Kusz J. , Böhm H.
Published: December 2003

The phase transitions of the solid solutions (Ca1-

x
Sr
x
)2MgSi2O7 (0 ≤ x ≤ 0.32) of modulated melilites are studied by X-ray diffraction. The variation of the intensity and the FWHM of satellite reflections reveal different transition temperatures. Two ranges of different ordering are found in the range of incommensurate modulation. The mechanism of the phase transition into the incommensurately modulated structure is discussed by including the diffuse scattering above the transition.
Crystal structures of sulfamates MeLi(NH2SO3)2 (Me: K, Rb and Cs) and physical properties of KLi(NH2SO3)2 (refractive indices, thermal expansion, elastic properties)
Authors: Meinhart A. , Haussühl E. , Bohatý L. , Tillmanns E.
Published: December 2003

New sulfamates KLi(NH2SO3)2, RbLi ⋅ (NH2SO3)2 and CsLi(NH2SO3)2 were prepared from non-stoichiometric (ratio Li: Me = 3:1) aqueous solutions of LiNH2SO3 and MeNH2SO3 (Me = K, Rb, Cs) at room temperature. Large single crystals of KLi(NH2SO3)2 were grown from these solutions by controlled lowering of the temperature between 293 K and 288 K. The crystal structures were investigated using single-crystal X-ray diffraction methods. KLi(NH2SO3)2: space group P21, Z = 2, a = 5.196(1) Å, b = 8.475(1) Å, c = 8.868(1) Å, β = 105.05(1)°, T
meas. = 293 K, R1 = 0.026; RbLi(NH2SO3)2: space group P1̅, Z = 4, a = 7.844(1) Å, b = 9.804(1) Å, c = 10.825(1) Å, α = 108.77(1)°, β = 93.29(1)°, γ = 98.64(1)°, T
meas. = 293 K, R1 = 0.033; CsLi(NH2SO3)2 : space group P1̅, Z = 2, a = 5.535(1) Å, b = 7.877(1) Å, c = 9.857(1) Å, α = 74.07(1)°, β = 76.33(1)°, γ = 87.07(1)°, T
meas. = 200 K; R1 = 0.023. The common feature of the three crystal structures are alternating layers of Me+ and lithium cations together with anionic [NH2SO3]– tetrahedra. Li+ coordination polyhedra and [NH2SO3]– anions, linked via common corners, form one type of these sheets. Additionally, these layers are stabilized by hydrogen bonds. Me+ coordination polyhedra are linked to each other and to the above mentioned layers by common corners and edges. RbLi(NH2SO3)2 and CsLi(NH2SO3)2 possess similar structures. Generally, the observed interatomic distances and angles agree with crystal chemical expectations. Differential scanning calorimetry measurements showed that KLi(NH2SO3)2 undergoes a first-order structural phase transition at 401 K and melts at 423 K. Between 153 K and 423 K no indications of further phase transitions were observed. In addition, the refractive indices and the tensor of thermal expansion were measured. Further, the elastic and thermoelastic constants of KLi(NH2SO3)2 were determined from ultrasonic resonant frequencies of thick plane-parallel plates and their shift upon variation of temperature, respectively. The elastic behaviour of KLi(NH2SO3)2 is similar to the average elastic properties of LiNH2SO3 and KNH2SO3.
Crystal structure of 12H+-2,4-benzo-1,5-dioxa-8,12,16-triaza-cyclooctadec-2-ene-7,17-dione picrate
Authors: Hundal G. , Kumar S. , Hundal M. S. , Singh H.
Published: December 2003

The title compound [C16H24N3O4]+[C6H2N3O7]- resembles 18 membered macrocycle, which has two ether O atoms, two pairs of amide N and O atoms and an amine N as potential donor atoms. The macrocycle has undergone protonation at the amine N atom. The picrate ion interacts through its phenolic O atom with NH2
+ group via N-H⋅⋅⋅O hydrogen bond. NH2
+ also shows a strong intramolecular H bonding with one of the amide oxygens of the macrocyclic ring. This results in significant conformational changes in one half of the macrocyclic ring. Apart from it, an increased flexibility of the system and a -CH2⋅⋅⋅π interaction renders the cavity non-suitable for accepting small molecules like water or alcohol in it.
ACORN — theory and practice
Authors: Jia-xing Y. , Woolfson M. M. , Wilson K. S. , Dodson E. J.
Published: December 2003

ACORN is a flexible and general procedure for theab initiosolution of protein structures when atomic resolution data are available. It has two components — ACORN-MR that locates a single atom or a fragment of the structure and ACORN-PHASE that refines the structure starting with the phase estimates from the fragment. ACORN-MR depends heavily on a correlation coefficient (CC) between the observed normalized structure factors and those from the fragment to find the fragment‘s correct orientation and/or position. ACORN-PHASE has three components, a Patterson-superposition sum function, real-space Sayre-equation refinement and, most importantly, Dynamic Density Modification (DDM). The concepts that underline DDM are explained by a theoretical approach. ACORN has been shown in trials to be able to solve structures with up to 5000 independent non-hydrogen atoms in the asymmetric unit. Several previously unknown structures have also been solved. At lower resolution, if anomalous scattering or isomorphous replacement data are available, then substructures can be determined using the ACORN approach. The quality of the final maps from ACORN, without further processing, enables automatic interpretation in terms of structure to be readily made.
Qantitative determination of phase for macromolecular crystals using multiple diffraction methods and dynamical theory
Authors: Chang S.-L. , Stetsko Y. P. , Lee Y.- R.
Published: December 2003

Multiple diffraction techniques for quantitative phase determination of macromolecular crystals are briefly described and overviewed. The modified second-order iterative approximation [Chang, S. L.; Tang, M. T.: Acta Cryst. A44 (1988) 1065] is employed to derive analytic expressions for the phase sensitivity of multiple diffraction and the visibility of multiple diffraction line profiles. Calculated visibility and sensitivity of three-beam cases of the protein, aldose reductase, are illustrated. The functions used for intensity profile fitting in the quantitative phase determination procedure are also modified accordingly. This calculation provides an effective way of choosing appropriate reflections for experimentally generating useful three-beam diffractions for phase analysis. Other practical issues are also addressed.
Structural variations in the aegirine solid-solution series (Na,Li)FeSi2O6 at 298 K and 80 K
Authors: Redhammer G. , Roth G.
Published: December 2003

The structural variations within the aegirine (NaFeSi2O6) – Li–aegirine (LiFeSi2O6) solid-solution series have been determined from structure refinements of single–crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C2/c. Replacing Na+ by Li+ in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi2O6, the bond length decreases by 7.1% with increasing replacement of Na+ by Li+. The decrease of the M2–oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li–aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe3+O6 – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na1-

x
Li
x
FeSi2O6 with Li+ concentrations x > 0.75 crystallize in the space group P21/c. For the compounds with x < 0.75, no phase transition from C2/c → P21/c was observed down to low temperatures (30 K). A x – T phase diagram for the stability of the low temperature P21/c structure is provided. The characteristica of the P21/c low temperature form of the (Na,Li)FeSi2O6 compounds agree with those of pure LiFeSi2O6, recently described by Redhammer et al. (2001).
Crystal structures of R-thiocamphor
Authors: Brunelli M. , Fitch A. N. , Mora A. J.
Published: December 2003

The crystal structure of the globular organic molecule R-thiocamphor (C10H16S) has been studied at ambient and low temperatures by high resolution powder X-ray diffraction using synchrotron radiation. At around 260 K thiocamphor transforms on cooling to an ordered orthorhombic structure from the orientationally-disordered body-centered cubic phase with a = 8.1477(1)Å at room temperature. The structure of the low-temperature phase was solved by auto-indexing, then by intensity extraction and direct methods using the EXPO package and refined by the Rietveld Method. The low temperature structure has a = 9.0490(1)Å, b = 14.5887(3)Å, c = 7.2931(1)Å, space group P212121, Z = 4, ρ
calc = 1.161 g cm-3.
Synthesis and X-ray structure of 1,3-dimethylurea-picric acid (1/1) complex
Authors: Artali R. , Bombieri G. , Carvalho C. C.
Published: December 2003

The synthesis and the X-ray crystal and molecular structure of the N,N'-Dimethylurea picrate [C3H9N2
+ C6H2N3O7
-] are reported. It crystallizes in the triclinic system, space group P1̅ with a = 6.882(7), b = 9.050(2), c = 11.149(12) , α = 97.54(13), β = 104.02(9), γ = 96.78(14), V = 659.7(2)Å3, Z = 2, D
calc = 1.60 g/cm3.The structure consists of infinite layers of anions alternating with cations linked together through an extensive hydrogen bonding involving most of the potential donors and acceptors. Semiempirical calculations for the picrate complex reproduce well the observed geometry.
The crystal structure of Na2Ca2Si2O7 - a mixed anion silicate with defect perovskite characteristics
Authors: Kahlenberg V. , Hösch A.
Published: December 2003

Single crystals of a new sodium calcium silicate with composition Na2Ca2Si2O7 have been synthesized. The title compound is monoclinic, space group I12/a1, a = 10.4906(5) Å, b = 10.3762(6) Å, c = 24.5349(12) Å, β = 91.217(6)°, V = 2670.1(2) Å3, Z = 16. The crystal used for the data collection showed twinning by pseudo-merohedry, a feature we took account of by separating the reflections from the different twin individuals. Structure determination was based on direct methods followed by difference Fourier calculations and least squares refinements (R(|F|) = 0.056 for 2414 independent observed reflections (I>2σ(I)) and 245 parameters). Na2Ca2Si2O7 belongs to the group of mixed anion silicates comprising insular [SiO4]-groups and [Si3O10]-trimers. Both anion groups are located in layers perpendicular to [001]. Ten symmetrically independent M-sites occupied by Na and/or Ca crosslink between the tetrahedral anion groups. Another very interesting understanding of the structure can be obtained from the comparison with the ABO3 perovskite structure type. The compound can be regarded as a superstructure of perovskite (a ≈ 2√2a
p, b ≈ 2√2a
p, c ≈ 6a
p) with 22.2% oxygen vacancies. The vacancies are ordered in such a way that the silicon atoms residing on the B-sites of the perovskite structure are coordinated to four oxygens. The relationship with perovskite can be expressed in the following crystal chemical formula: [(Na,Ca)][(Na,Ca)1/4Si3/4][O7/9∀2/9]3.
Calculation of the BO3 triangle to BO4 tetrahedron ratio in borates
Authors: Parthé E.
Published: December 2003

A bonding model, valid for alimited group of borates, is formulated which allows the ratio of triangularly to tetrahedrally coordinated B atoms to be calculated from the chemical formula. The anion complex, which is formed by covalently bound atoms, has non-directed ionic bonds with the surrounding cations. To complete its electron octet, each O atoms of the complex forms exactly two covalent bonds either with 2 B and/or with 1 B and 1 H atom, depending on the number of OH groups. The B atoms have no preference for either triangular or tetrahedral O coordination. However, a change of a B-O base triangle to an isoelectronic base tetrahedron decreases the number of O atoms which can be used for B-O-H bonds and increases the number of shared O atoms engaged in B-O-B bonds - and vice versa. The ratio of B-O triangles to tetrahedra is adjusted such that O atoms can form the proper number of B-O-B and/or B-O-H bonds. Using the model one can also prove that a borate with OH groups and all its dehydration products (independent whether they are crystallized or a glass) should have the same ratio of three- to four-coordinated B atoms. Under the presupposition that the bonding model is applicable, one can specify a simple mixture of base triangles and/or base tetrahedra with which it should be possible to construct an anion complex. The observed complex is either built up with this simple mixture or with a mixture of base polyhedra derived from the simple one by cross-substitution which does not change the triangle to tetrahedron ratio. The realization of the bonding model is in principle possible only within a restricted composition range in which lie one quarter of the known borates. The equation to calculate the triangle to tetrahedron ratio has been tested on all borates within this range. An agreement was found for 85 percent of the 222 investigated crystal structures.
The crystal structure of InYGe2O7 germanate
Authors: Juárez-Arellano E. A. , Bucio L. , Ruvalcaba J. L. , Moreno-Tovar R. , García-Robledo J. , Orozco E.
Published: December 2003

A new indium yttrium germanate presenting the thortveitite structure with symmetry described by the space group C2/m (No. 12) has been prepared by high temperature solid state reaction as polycrystalline powder material. This crystallizes in the monoclinic system, with cell parameters a = 6.8286(1) Å, b = 8.8836(2) Å, c = 4.9045(1) Å, β= 101.8340(7)º, V = 291.195(9) Å3 and Z = 2. The structure was characterized by X-ray powder diffraction and Rietveld refinement of the diffraction pattern. The In3+ and Y+3 cations occupy the same octahedral site forming a hexagonal arrangement on the ab planes. In their turn, the hexagonal arrangements of InO6/YO6 octahedral layers are held together by sheets of isolated diorthogroups constituted by a double tetrahedra sharing a common vertex. In this compound, the Ge2O7 diorthogroup shows the C2h symmetry implying a Ge-O-Ge angle of 180º, being an important feature of the thortveitite structure, which has been controversial in some reported papers. A remarkable photo-luminescence effect (in comparison with glass) was observed when the sample was irradiated with α-particles beam during the RBS experiments employed to analyze the chemical stoichiometry.
Colored patterns and the subgroups of their symmetries effecting color permutations
Authors: De Las Peñas Ma. Louise Antonette , Paras Agnes T.
Published: December 2003

In this paper, we study colorings corresponding to the partition of the form

of the symmetry group G of an uncolored pattern where Ji
, H, K are subgroups of G such that K ≤ Ji
≤ H ≤ NG
(K) and

is a complete set of right coset representatives of H in G. In particular, we consider those colorings obtained when Y is partitioned into one set, two sets or singletons and determines the subgroup H* consisting of elements of G effecting color permutations.
Structural topology of potassium uranyl chromates: crystal structures of K8[(UO2)(CrO4)4](NO3)2, K5[(UO2)(CrO4)3](NO3)(H2O)3, K4[(UO2)3(CrO4)5](H2O)8 and K2[(UO2)2(CrO4)3(H2O)2](H2O)4
Authors: Krivovichev Sergey V. , Burns Peter C.
Published: December 2003

Crystals of four potassium uranyl chromates, K8[(UO2)(CrO4)4](NO3)2 (1), K5[(UO2)(CrO4)3](NO3)·(H2O)3 (2), K4[(UO2)3(CrO4)5](H2O)8 (3) and K2[(UO2)2·(CrO4)3(H2O)2](H2O)4 (4), have been synthesized by evaporation of aqueous solutions of (UO2)(NO3)2(H2O)6 and K2CrO4. The structure of (1) (triclinic, P1̅, a = 7.0397(6), b = 9.7341(9), c = 9.7568(9) Å, α = 105.846(2), β = 97.992(2), γ = 93.271(2)°, V = 633.8(1) Å3, Z = 1) was solved by direct methods and refined to R1 = 0.044 (wR2 = 0.098). It is based upon clusters of composition [(UO2)(CrO4)4] that consist of UrO4 square bipyramids (Ur: UO2
2+ uranyl ion) that share corners with four different CrO4 tetrahedra. The clusters are arranged in layers parallel to the (100) plane. K+ cations and NO3
- groups are located between the clusters. The crystal structure of (2) (orthorhombic, P212121, a = 6.1112(11), b = 12.136(2), c = 27.464(4) Å, V = 2036.9(6) Å3, Z = 4) was solved by direct methods and refined to R1 = 0.047 (wR2 = 0.071). it contains chains of composition [(UO2)(CrO4)3] that are parallel to the b axis. The chains consist of UrO5 pentagonal bipyramids that share all equatorial corners with different CrO4 tetrahedra. Planes containing the equatorial ligands of the UrO5 pentagonal bipyramids are approximately parallel to the (100) plane. K+ cations, NO3 groups and H2O groups are between the chains. The crystal structure of (3) (monoclinic, P21/c, a = 8.2336(7), b = 18.8042(17), c = 21.2413(18) Å, β = 89.979(2)°, V = 3288.7(5) Å3, Z = 4) has been solved by direct methods from a crystal twinned on the (001) plane and refined to R1 = 0.060 (wR2 = 0.125). The structure is based upon topologically complex sheets of composition [(UO2)3(CrO4)5] with UrO5 pentagonal bipyramids that share their corners with CrO4 tetrahedra. The sheets are parallel to the (010) plane. K+ cations and H2O groups are located in the interlayer between the sheets. The crystal structure of (4) (monoclinic, P21/c, a = 10.7417(5), b = 14.5290(7), c = 14.1387(6) Å, β = 108.135(1)°, V = 2097.0(2) Å3, Z = 4) was solved by direct methods and refined to R1 = 0.036 (wR2 = 0.067). It is based upon sheets of composition [(UO2)2(CrO4)3(H2O)2] that are parallel to the (010) plane. K+ cations and additional H2O groups are in the interlayer. Nodal representations of uranyl chromate structural units are developed, and a comparison with related structures are presented.
Oriented graphite single-crystal inclusions in diamond
Authors: Glinnemann Jürgen , Kusaka Katsuhiro , Harris Jeffrey W.
Published: December 2003

A crystallographic study of 13 specimens of diamonds with euhedral single-crystalline graphite inclusions in their centres is presented. All inclusions belong to the hexagonal graphite modification (space group P63/mmc; a
0 ≈ 2.46 Å, c
0 ≈ 6.70 Å) and are up to 300 μm in diameter. Comparison of the measured c-lattice parameters of the graphite crystals with lattice parameters of graphite at elevated isostatic pressures indicates remnant pressures of up to 2.6 GPa acting on the inclusions. All samples exhibit distinct orientation relations between graphite and diamond. In 12 samples the direction [001] of graphite (G[001]) approximately parallels one of the directions 〈111〉 of cubic diamond (D〈111〉). The largest deviation is about 4°. A further division of these 12 samples is: (a) The three G〈100〉 directions have angles of about 4° with three D〈110〉 directions. This orientation relation is observed in five samples. (b) The angle between the same directions is about 34° in six samples. (c) In one sample this angle is about 16°. In a 13th specimen G[001] approximately parallels one of the three D〈100〉. The deviation is of about 6°, and one of the G〈100〉 directions nearly parallels one of the D〈110〉. These orientation relations are analysed with a simplified application of the “coincidence site lattice (CSL)” concept. A 14th specimen is different to all others, as it exhibits a large (about 300 μm edge length) region with a sharp hexagonal borderline filled with a “patchwork” of tiny graphite “islands”. Several parallel lamellae of up to 5 μm thickness result in a hexagonal pyramidal form. This graphite inclusion is suggested to be protogenetic with respect to the diamond.
Compositional and structural variations in the ternary system Li – Al – Si
Authors: Spina Laurent , Jia Yong Zhong , Ducourant Bernard , Tillard Monique , Belin Claude
Published: December 2003

The Li–Al–Si ternary system has been investigated and four ternary phases characterized using both powder and single crystal X-ray diffraction data. LiAlSi and Li7Al3Si4 crystallize in the F4̅3m, cubic system, Li18Al2Si6 in tetragonal I41/amd and Li15Al3Si6 in hexagonal P63/m space group. The structure of LiAlSi (a = 5.94 Å), previously reported from powder data, has been confirmed. The new compound Li7Al3Si4 has been identified and, due to a higher lithium content, its cell parameter is slightly larger (a = 6.115 Å). The tetragonal cell (a = 6.179(1) Å, c = 12.199(4) Å) of Li18Al2Si6 may be considered as a (1 × 1 × 2) supercell of the cubic one. Structural variations are strongly correlated to compositional fluctuations and atomic substitutions, in every case the SI–Al network displays some covalent character. In the Li15Al3Si6 hexagonal cell, bonding is more covalent leading to a heterographite-like SI–Al network.
Crystal structures of 2,2′-bipyridine adducts of two cadmium O-alkyl dithiocarbonates: rationalisation of disparate coordination geometries based on different crystal packing environments
Authors: Chen Daulin , Lai Chian Sing , Tiekink Edward R. T.
Published: December 2003

Two distinct coordination geometries are found in the structures of Cd(S2COR)2(2,2′-bipyridine). For R = CH2CH2OCH3 (1) a N2S2 donor set defines a distorted tetrahedral geometry for cadmium as the xanthate ligands adopt a monodentate coordination mode. By contrast, a N2S3 donor set is found in the structure where R is butyl (2) as one of the xanthate ligands is chelating. An analysis of the crystal structures shows that both lattices feature extensive C–H...π interactions and that in (1) there are C–H...O interactions that are not present in (2). In (2) there are π...π interactions that are absent in (1). A qualitative argument based on crystal packing considerations is proposed to explain the differences in molecular structures.
The first crystal structure of a free neutral form of a nicotine derivative
Authors: Evain Michel , Felpin François-Xavier , Laurence Christian , Lebreton Jacques , Le Questel Jean-Yves
Published: December 2003

The single crystal X-ray diffraction analysis of a racemic form of a nicotine derivative, 5-(2-hydroxy-2-methylbut-3-ynyl)-3-(1-methyl-2-pyrrolidinyl)pyridine (2), provides structural information on molecular geometry and intermolecular interactions that could be relevant for the transport and docking of nicotine ligands to nicotinic acetylcholine receptors (nAChRs), since it is the first crystal structure of a free neutral form of a nicotine derivative. Each enantiomer of the two independent molecules of the asymmetric unit is characterized by (i) anantiorientation of the N2 methyl substituent with respect to the pyridine ring (ii) an envelope form of the pyrrolidine ring (iii) an orientation roughly perpendicular of the pyridine and pyrrolidine rings. The crystal packing is characterized by the formation of OH...N1 hydrogen-bond cyclic dimers that are packed through van der Waals interactions. This selectivity of hydrogen-bonding site in favor of the N1 pyridine nitrogen found in the solid state for (2) agrees with the greater hydrogen-bond acceptor strength of the N1 pyridine nitrogen compared to the N2 pyrrolidine one previously found in solution for nicotine (1).
Reversible crystal-to-crystal phase transition in dibromobis(4-vinylpyridine)zinc
Authors: Hu Chunhua , Huster Jürgen , Englert Ullrich
Published: December 2003

In the solid state the mononuclear tetrahedral complex dibromobis(4-vinylpyridine)zinc shows a reversible crystal-to-crystal phase transition which was studied by temperature dependent single crystal X-ray diffraction experiments. At low temperatures the compound crystallizes in space group P1̅ with one ordered complex molecule in general position. Results derived from an intensity data set collected at 120 K are reported. Above the phase transition at 207 K both the cell metric and the symmetry of the diffraction pattern allow the transformation to a centered monoclinic unit cell in space group C2/c. In this high temperature phase the molecule is located on a twofold crystallographic axis and shows disorder with respect to the orientation of the vinyl substituent. The crystal and molecular structure based on a data collection at 293 K is described.
Intermetallic compounds with ordered U3Si2 or Zr3Al2 type structure – crystal chemistry, chemical bonding and physical properties
Authors: Lukachuk Mar'yana , Pöttgen Rainer
Published: December 2003

The crystal chemistry of binary and ternary intermetallic compounds with tetragonal U3Si2 or Zr3Al2 type structure is reviewed. Besides the preparation techniques and crystal chemistry, we especially focus on the structure–property relations and chemical bonding peculiarities.
Structural studies on the Fresnoite type compound Ba2VSi2O8
Authors: Höche Thomas , Esmaeilzadeh Saeid , Withers Ray L. , Schirmer Henning
Published: December 2003

Single crystals of Ba2VSi2O8 have successfully been grown from a high-temperature solution and single crystal X-ray diffraction used to refine its fresnoite type average crystal structure. In contrast to other recently investigated representatives of the fresnoite family of compounds, Ba2VSi2O8 did not at first appear to be incommensurately modulated. Neither single crystal X-ray diffraction nor powder neutron diffraction gave any indication for the existence of satellite reflections in addition to the strong average structure Bragg reflections. A careful electron diffraction study, however, has revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave-vectors q
1 = 0.297 [110]* ½c* and q
2 = 0.297 [11̅0]* + ½c*. The reciprocal space positioning of these incommensurate primary modulation wave-vectors, the overall (3 + 2)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from the single crystal XRD refinement are all consistent with their arising from condensed Rigid Unit Mode (RUM) modes of distortion of the Ba2VSi2O8 parent structure. The weakness of the satellite reflections in Ba2VSi2O8 relative to, for example, Ba2TiGe2O8 or Sr2TiSi2O8 arises from the significantly smaller amplitude of the condensed RUM mode as is clear from the magnitudes of the refined average structure anistropic displacement ellipsoids.
Rare earth disilicates R
2Si2O7 (R = Gd, Tb, Dy, Ho): type B
Authors: Fleet Michael E. , Liu Xiaoyang
Published: December 2003

The type B structure of the single rare earth element disilicates has been revised using room temperature and pressure single-crystal X-ray diffraction measurements on Gd2Si2O7, Tb2Si2O7 and Ho2Si2O7 synthesized at 2–2.5 GPa, 1400–1500 °C in a piston-cylinder apparatus and Dy2Si2O7 synthesized hydrothermally at 0.1 GPa. Crystal data are: triclinic, space group P1̅, Z = 4; Gd2Si2O7 – a = 6.6609(2), b = 6.7081(2), c = 12.1390(4) Å, α = 94.277(1), β = 90.577(1), γ = 91.441(1)°, R = 0.033, and Dx
= 5.929 g/cm3; Tb2Si2O7 – a = 6.6331(3), b = 6.6799(2), c = 12.0967(2) Å, α = 94.128(1), β = 90.609(2), γ = 91.541(1)°, R = 0.038, and Dx
= 6.041 g/cm3; Dy2Si2O7 – a = 6.6158(2), b = 6.6604(2), c = 12.0551(4) Å, α = 94.373(2), β = 90.836(2), γ = 91.512(2)°, R = 0.031, and Dx
= 6.156 g/cm3; and Ho2Si2O7 – a = 6.5960(2), b = 6.6328(3), c = 12.0214(6) Å, α = 94.479(1), β = 90.856(1), γ = 91.749(2)°, R = 0.034, and Dx
= 6.313 g/cm3. Substitution of Gd3+, Tb3+, Dy3+, and Ho3+ cations into the type B structure results in proportional linear decrease in size of the REEOn polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.
Synthesis of 4-(3-coumarinyl)-3-benzyl-4-thiazolin-2-one 4-methylbenzylidenehydrazone and determination of the isomeric structure by X-ray analysis
Authors: Özbey Süheyla , Karalı Nilgün , Gürsoy Aysel
Published: December 2003

In this study 4-(3-coumarinyl)-3-benzyl-4-thi azolin-2-one 4-methylbenzylidenehydrazone 3 was synthesised. An independent proof of the thiazolylhydrazone structure of 3 was achieved by single crystal X-ray diffraction analysis. Elemental analyses and spectral data (IR, 1H-NMR, 13C-NMR and EIMS) of 3 was also determined. The coumarin ring system and the thiazole ring are almost planar, the dihedral angle between them is 62.93(5)°. The phenyl groups make dihedral angles of 86.09(7) and 31.67(7)° with the thiazole ring plane. The crystal structure is stabilised by intra-, intermolecular hydrogen contacts and C–H...π type interactions.
Small-angle neutron scattering and phase separation in alloys
Authors: Kostorz G.
Published: December 2003

The basic principles of small-angle neutron scattering and the two-phase model, frequently used for the interpretation of scattering data, are presented. Some studies on phase separation in alloy single crystals serve to illustrate the possibilities of the method. Small-angle neutron scattering of Ni-rich Ni—Ti studied during ageing at elevated temperatures indicates a complex sequence of phases during decomposition, involving two stages of metastable coherent precipitation preceding the formation of the stable hexagonal phase.
Disorder in decagonal quasicrystals
Authors: Frey F. , Weidner E.
Published: December 2003

Complementary neutron and x-ray diffuse scattering may provide insight into structural super-ordering and disordering of decagonal quasicrystals (d-phases), and, in consequence, into the formation and stability of aperiodically ordered alloys. Neutron diffraction makes a contrasting almost isoelectronic atomic species possible, as well as a separation of elastic and inelastic diffuse intensity contributions. Experimental work at high temperatures is comparatively unproblematic. The method suffers, however, from the difficulty in obtaining sufficiently sized mono-grain samples and a lack of dedicated neutron diffraction instruments. Recent results, with a main focus on high-temperature (<1000°C) investigations of disordered decagonal Al—Ni—Co phases are reported and some tentative models are discussed.