bluuebunny/crossref_metadata_2025_split huggingface dataset:
Prediction of hydrogen positions in complex structures
Authors: Milman Victor , Winkler Björn 
Published: December 2003

Abstract
The structures of a number of complex inorganic systems, including the positions of the hydrogen atoms, have been refined using ab initio density functional modeling of interatomic interactions. The calculated O–H, N–H, P–H bond lengths in various structures, as well as the geometry of various hydrogen bonds, satisfy basic crystal chemistry requirements and represent a substantial improvement over the experimentally measured ones.
AlB2-related intermetallic compounds – a comprehensive view based on group-subgroup relations
Authors: Hoffmann R.-D. , Pöttgen Rainer 
Published: December 2003

Abstract
46 structure types of binary and ternary intermetallic compounds have been derived from the well known, simple aristotype AlB2. Many compounds with the general compositions RX
2, RT
2, RTX, and R
2
TX
3 (R = alkaline earth, rare earth or actinoid metal; T = transition metal; X = main group element) belong to this family. The structural similarities between these compounds are manifested by an ordering of the transition metal atoms and main group element atoms on the boron positions of the AlB2 type. Also an ordering of vacancies on the boron positions occurs in some compounds. Striking structural motifs of the RX
2, RT
2, RTX, and R
2
TX
3 compounds are planar or puckered six-membered rings consisting of X
6, T
6, T
3
X
3, and T
2
X
4 separated by the respective R atoms. The crystal chemical relationships between these compounds are discussed on the basis of a group-subgroup scheme in the concise and compact Bärnighausen formalism with respect to the aristotype AlB2. Two different branches of AlB2 related structures are discussed. In the first, the hexagonal/trigonal branch, six-membered rings carry at least a three-fold axis, while in the orthorhombic/monoclinic branch the distortions are more pronounced. The pseudo-hexagons show significant tilting and a loss of hexagonal/trigonal symmetry. Significant variations of the chemical bonding occur for some structure types, and therefore they may only be considered to be isopointal rather than isotypic. These "isopointal conversions" are based on a gradual changeover from two- to three-dimensionally infinite polyanions. Besides the group theoretical group-subgroup scheme we also discuss chemical bonding in these AlB2 related intermetallic compounds.
Microstructural and magnetic properties of epitaxial Co/Cu(111) multilayers
Authors: El-Sayed K. , Soliman A. , Pelzl J. 
Published: December 2003

Abstract
A series of epitaxial Co/Cu(111) superlattices has been grown on Al2O3 (112̅0) substrates with a Nb(110) buffer layer by molecular-beam epitaxy. By applying profile analysis technique it was possible to determine: (1) the crystallite size, (2) the microstrains for Co and Cu of the multilayers and (3) the percentage of the hexagonal and face centred cubic structures in the Co layers. Ferromagnetic resonance (FMR) measurements at x-band frequencies are used to study in-plane magnetic properties of the investigated superlattices. A good relationship between the microstructural and the in-planemagnetic properties were obtained.
The crystal chemistry of the humite minerals: spectroscopic studies and structure refinement of an unusual iron-rich clinohumite
Authors: Platonov A. N. , Langer K. , Wildner M. , Polshin E. V. , Matsyuk S. S. 
Published: December 2003

Abstract
An orange-red mineral with strong pleochroism from magnesian skarns of the Salminski rapakivi massif which was originally described as an olivine,is identified as an unusually iron-rich clinohumite.The composition of the mineral is,as obtained by combining data of electron microprobe analyses,Mössbauer analyses and unconstrained structure refinements: [Mg5.2(Fe3.42
2+Mn0.07Ni0.01Zn0.04Ca0.01Fe0.11
3+ Ti0.04
4+)Σ=3.70] × [Si4.03O16[F1.21(OH,O)0.79]Σ=2.00]. Thus,the Fe-end member fraction is X
Fe = 0.42, i.e. unusually high when compared with those of the usual magnesian clinohumites, e.g. = 0.06 in the crystal studied structurally by Robinson et al.(1973). Fe is the sumof octahedral,Mg-substituting cations which could - due to the similarity of their scattering curves and the very low fractions except for that of iron - not be discriminated in the structure refinement. Such refinements yield the following Fe-fractionations, f
Fe, between the different octahedral sites of the structure: f
Fe(M1
c
) =0.17, f
Fe(M1
n
) = 0.33, f
Fe(M26) = 0.32, f
Fe(M25) = 0.13, f
Fe(M3) = 0.05 (ΣFe = 1.00). The analysis of the Mössbauer spectra yields a fractionation pattern very close to that obtained by XRD. The Fe-fractioning in the ferrous clinohumite with X
Fe = 0.42 is nearly the same as that in magnesian clinohumite with X
Fe = 0.06, such similarities being obviously related to the fact that a slightly distorted hexagonally close anion package forms the basis of all the humite mineral structures. The structural changes on iron incorporation pertain predominantely changes in the individual and mean M–O distances and O–M–O angles. Polyhedral distortions Δ
oct and σ
oct
2 increase most drastically, compared to magnesian clinohumite with X
Fe = 0.06, in the M26 octahedra. The polarized electronic absorption spectra are dominated by two features: an intense, broad band centered at 22800 cm-1 strongly polarized E ∥ X (nα ∢ b ca. 10°), which was identified as Fe2+(M25)-Ti4+(M3) charge-transfer transition in the M25M3M3M25 octahedral chain fragments of the structure. The strong polarization of this band together with the position of the UV absorption edge,causes the pleochroismof the mineral.The second prominent feature in the polarized single crystal spectra is a complex band system in the NIR which is assigned to dd-transitions of Fe2+(M1
c
+ M1
n
), bands at 10800 and 7600 cm-1, and of Fe2+(M26), bands at 9400 and 8800 cm-1. Such transitions are in analogy with the behaviour of Fe2+ in the M1 and M2 sites of the olivine structure.
Superprotonic high temperature phase and refinement of the low temperature structure of CsHSeO4
Authors: Zakharov M. A. , Troyanov Sergej I. , Kemnitz Erhard 
Published: December 2003

Abstract
The crystal structure of the high temperature superprotonic phase of CsHSeO4-I (tI28) has been investigated by single crystal X-ray diffraction at 408 K. It crystallizes tetragonal, a = 5.880(1), c = 14.415(3) Å, V = 498.4(2) Å3, Z = 4, space group I41/amd. In the SeO4 tetrahedron, the Se-O distance is 1.66 Å after correction for libration. A layer-like hydrogen bonding system is suggested. The low temperature structure, CsHSeO4-II(mP28), was refined from X-ray diffraction data with an accuracy which made it possible to localize the H atom. Crystallographic relations between the two phases during the phase transition are discussed.
Crystal structure determination of a series of small organic compounds from powder data
Authors: Goubitz K. , Sonneveld E. J. , Schenk H. 
Published: December 2003

Abstract
The crystal structures of 2,4-di-bromo-aniline (A; C6H5NBr2 ), 4-iodo-anisole (B; C7H7OI), 2-iodo-benzenemethanol (C; C7H7OI), 2-amino-benzothiazole (D; C7H6N2S) and 2-amino, 5-bromo-pyridine (E; C5H5N2Br) have been determined from X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine the structures. The crystals of (A) and (B) are orthorhombic, the crystals of (C), (D) and (E) are monoclinic. (A): Space group P212121, Z=4 with cell parameters a = 11.18(1), b = 16.17(1), c = 4.110(3)Å; (B): Space group Pca21, Z = 4, a = 6.288(4), b = 7.361(4), c = 16.93(1)Å; (C): Space group P21/n, Z = 4, a = 13.23(1), b = 4.652(3), c = 12.82(1)Å, β = 109.69(4)°; (D): Space group P21/c, Z = 4, a = 14.58(2), b = 4.094(4), c = 11.62(1)Å, β = 94.12(6)°; (E): Space group P21/c, Z = 4, a = 13.80(1), b = 5.839(5), c = 7.687(7)Å, β = 106.04(5)°. Chemical diagrams of all five compounds are depicted in Fig. 1. Soft constraints have been applied to the molecules during Rietveld refinement. The final R

p
values obtained were 7.3 (A), 2.9 (B), 11.1 (C), 16.4 (D) and 7.9% (E) respectively. All compounds were measured on a Guinier camera. In addition, the structure of compound (A) was confirmed by single-crystal structure determination.
A CH/π interaction at a highly solvated macrocycle in the solid state — the p-tert-butylcalix[8]arene–chloroform 1:8 clathrate
Authors: Schatz J. , Schildbach F. , Lentz A. , Rastätter S. , Debaerdemaeker T. 
Published: December 2003

Abstract
The crystal structure of the p-tert-butylcalix[8]arene–chloroform 1:8 clathrate (C88H112O8 · 8 CHCl3:P1̅, a = 13.886(2) Å, b = 21.891(4) Å, c = 21.930(4) Å, α = 64.00(2)°, β = 78.97(2)°, γ = 88.75(2)°, Z = 2) features a strong CH/π interaction of one chloroform molecule with the π system of one phenol ring of the calix[8]arene moiety; the seven other chloroform molecules occupy interstitial places in the crystal lattice of a highly solvated macrocyclic system (c = 0.566 mol dm–3). A thermogravimetric analysis also shows that one solvent molecule is bound more tightly than the others. The observed binding motif is presumably based on a subtle interplay between favourable interactions (CH/π and to some extent Cl/π) and induced fit between host and guest.
Mineral phases of the Earth´s mantle
Authors: Fiquet G. 
Published: December 2003

AbstractOur knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.
The crystal structure of synthetic Rb2Sb8S12(S2) · 2 H2O, a new member of the hutchinsonite family of merotypes
Authors: Berlepsch P. , Miletich R. , Makovicky E. , Balić-Žunić T. , Topa D. 
Published: December 2003

Abstract
The crystal structure of synthetic Rb2Sb8S12(S2) · 2 H2O was determined from single crystal X-ray diffraction data: space group P21/n, Z = 2, a = 7.0750(17) Å, b = 25.403(8) Å, c = 8.0527(19) Å, β = 97.842(10)° (R
1 = 4.88% for 1050 reflections with Fo > 4σ[Fo]). The structure is composed of slabs with formal composition [Sb8S12] that are based on (110)PbS cut-outs of the PbS archetype, are interspaced by channels with Rb+ and H2O, and separated by S2
2-disulfide groups which also interconnect the slabs. Rb2Sb8S12(S2) · 2 H2O is a N
1,2 = 1,2 member of the hutchinsonite family of merotypes.
Sequential re-arrangement of V2O5-layers during the paraelastic-to-ferroelastic phase transition in ∊ʹ-Li
x
V2O5
Authors: Katzke H. 
Published: December 2003

Abstract
The paraelastic-to-ferroelastic phase transition in the lithium vanadium oxide bronze ∊ʹ-Li0.527V2O5 was studied by low-temperature, high-resolution Guinier diffractometry. At the transition the symmetry changes from orthorhombic Pmmn to monoclinic P21
m11 at Tc
= 285(5) K. At all temperatures anisotropic broadening of the reflections occurs, indicating the presence of stacking faults in this layer structure. X-ray intensity calculations in terms of a one-dimensional model of stacking disorder were carried out in order to elucidate the structural changes induced by the phase transition, and to determine the critical behaviour of the order parameter Q. The symmetry relations between the Pmmn and P21
m11 phases allow the development of the spontaneous strain which is, in this system, proportional to the change in cos α. A bilinear coupling between es
and Q was found.
High temperature crystal structure of a 3:2 mullite from neutron diffraction data
Authors: Brunauer G. , Boysen H. , Frey F. , Hansen T. , Kriven W. 
Published: December 2003

Abstract
High temperature neutron investigations up to 1950 K of 3:2-sintered mullite show a smooth reversible behaviour which can be described by a rotation of more or less rigid TO4 (T = Al(2),Si)-tetrahedra and accompanying distortions of the Al(1)O6-octahedra. Large atomic dispacement parameters along the c-direction, including static parts, match to respective structural modulations in the mullite structure. A comparison of a pristine and a preheated sample reveals an irreversible depopulation of the Al(2) atomic site in favour of the Al(*) site.
Crystal structure of 5′-phenyl-1,1′:3′,1″-terphenyl-4-carboxylic acid, a 27 atoms organic compound by powder method
Authors: Łasocha W. , Czapkiewicz J. , Milart P. , Schenk H. 
Published: December 2003

Abstract
The crystal structure of a complex organic compound containing 27 independent non-hydrogen atoms in asymmetric unit has been solved from powder diffraction data collected at ESRF Grenoble. Structure model was found using PATSEE program. By Rietveld method the structure was completed and refined to final values of discrepancy factors RF
and Rwp
equal to 14.6 and 13.2%, respectively. The space group is P21/c, unit cell parameters a = 11.0769(5), b = 22.387(1), c = 7.3885(3) Å, β = 91.923(4)°.
The crystal structure of sodium oxamate NaC2O3NH2 from powder diffraction data
Authors: Helmholdt R. B. , Sonneveld E. J. , Chernyshev V. V. , Schenk H. 
Published: December 2003

Abstract
The crystal structure of sodium oxamate, NaC2O3NH2, has been determined from synchrotron powder diffraction data. First Guinier-Johannson photographs have been taken to derive the unit cell. Grid search and Rietveld refinement have been used to determine the structure. The crystal structure is monoclinic, P21/c, Z = 4, a = 3.5513(1) Å, b = 5.2561(2) Å, c = 20.4451(8) Å, β = 99.329(2)° and V = 376.58 Å3. The final RP
value obtained with Rietveld refinement was 6.5 %
Crystal and molecular structures of R3PAu(3-Hmba): R=Cy and Ph; 3-H2mba=3-mercaptobenzoic acid at 173 K: Steric control of aurophilic association
Authors: Smyth D. R. , Vincent B. R. , Tiekink Edward R. T. 
Published: December 2003

Abstract
The characterisation of R3PAu(3-Hmba) [R = Cy (1) and Ph (2); 3-H2mba = 3-mercaptobenzoic acid] is reported. The gold atoms exist in linear arrays defined by P- and S-donor atoms. The subtle combination of steric bulk of the phosphine ligands and steric profile of the thiolate ligand is demonstrated to exert control over the appearance of hydrogen-bonded or Au...Au-mediated dimer motifs.
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
(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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.