Structural model of glassy CdGeAs2

Attempts to construct structural models of glassy CdGeAs₂ are described. Microcrystal models with normal and appropriately distorted crystallites gave poor fits with the experiment. Relaxed random network models, in which force constants and equilibrium distances and angles obtained by an energy relaxation of the crystal were used to minimize their free energy, provided excellent agreement with experiment if only even-numbered rings and, therefore, no wrong bonds were allowed to form.     

Structural analysis according to reduced data: VIII. Refinement of the extended model of aspherical atoms

A program for the refinement of the model of aspherical atoms within the Stewart-Hansen-Coppens formalism has been developed. Deformation scattering up to the 8th expansion order in multipoles has been taken into account for the first time. The program was tested for 11 crystals. The effect of the result of interpolation of radial scattering curves on the model parameters is considered. The importance of introduction of multipoles of high (5th–8th) orders into the model for a number of crystals is shown. The use of the extended multipole model for a silicon crystal revealed some new specific features of the electronic structure: consideration of multipoles up to the 7th order makes it possible to explain the intensity of the forbidden 222 reflection.     

Structural model of gelation processes of a sodium silicate sol destabilized by calcium ions: combination of SAXS and rheological measurements

This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels.     

Structural Investigations of Novel Indoloacridines

Abstract  5,6-Dihydro-9-chloro-7-phenylindolo[3,2-c]acridinium sulfates (3a-c) were formed from 2,3,4,9-tetrahydro-1H-carbazol-1-ones (1a-c) on reaction with 2-amino-5-chlorobenzophenone. 3b and 3c are isostructural and all three sulfates crystallize in the triclinic system with the space group P each with two crystallographically independent organic cations and one sulfate anion in the asymmetric unit. 3a exhibits the parameters a = 9.8896(17), b = 14.165(2) and c = 14.583(2) ?, α = 83.184(3)°, β = 78.985(3)° and γ = 75.903(3)°; 3b a = 10.5713(9), b = 14.5667(12) and c = 14.9947(13) ?, α = 90.916(1)°, β = 97.153(1)° and γ = 97.208(1)°; and 3c a = 10.5505(10), b = 14.6090(13) and c = 15.2960(14) ?, and α = 91.4310(10)°, β = 98.8630(10)° and γ = 98.9100(10)°. The structure of 3c exhibits voids of 286 ?³ of ill defined and disordered solvate molecules. The structure of 3a is both non-merohedrally twinned and also exhibits an interesting type of pseudosymmetry. All but the sulfate ion and the ethylene moieties also fit with a unit cell half the size with only one independent cation, and disordered ethylene moieties and sulfate anions. The choice of unit cell was made based on the diffraction pattern and the absence of disorder in the larger cell, pointing towards alternating ordered ethylene and sulfate units rather than randomly disordered ones. Index Abstract  [The structures of three indoloacridines 3a, 3b, and 3c with similar packing motifs are described. The structure of 3a is both non-merohedrally twinned and also exhibits an interesting type of pseudosymmetry by being a double volume ordered supercell of an alternative cell with disordered ethylene and sulfate anion moieties.]      

Structural Features of PLZT Films

Lead zirconate titanate (PZT) films doped with lanthanum, Pb(_1–х)La_х(Zr_0.48Ti_0.52) (х = 0, 0.02, 0.05, 0.08, or 0.01), have been investigated by electron microscopy and X-ray diffraction. Films were formed on Si–SiO₂–TiO₂–Pt substrates by chemical vapor deposition from a solution and annealed at temperatures T = 650 and 750°C. The main structural features of the films, differing them from undoped PZT films fabricated by the same method, have been established. It is found that doping with lanthanum delays the pyrochlore–perovskite transformation in the film bulk, i.e., in the regions distant from the film–substrate interface. The fraction of metastable pyrochlore phase increases with an increase in the La molar content in the films. The main reason for the delay is the deficit of lead in the intergranular perovskite space, especially in the upper part of the film. Annealing at T = 750°C reduces the content of pyrochlore phase but does not completely remove it, which was never observed for undoped PZT films. Doping with lanthanum leads to a change in the lattice period c and a tetragonal distortion of the perovskite lattice (c/a ratio). Hence, the [100] texture of the films obtained, in contrast to the typical [111] texture of PZT films, is due to the increase in the lattice mismatch between the film and platinum layer when lead atoms are replaced with lanthanum. Lattice distortions of “transrotational” character, whose value exceeds 160 deg/μm, are found to arise in growing crystals.     

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qⁿ. ²⁹Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q² state. The chemical shift of the ³¹P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The ²⁹Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q² and Q³ were calculated, showing that all glasses studied are predominantly Q² in structure, i.e. silica chains which readily dissolve. The changes in the chemical shifts of the Q² and Q³ species with composition have been interpreted as resulting from the preferential association of Na⁺ with Q³ and Ca²⁺ with Q².     

Structural investigation of five N-arylsulfonyloxazolidines

The crystal structures of (1R,5R,8S)-8-[(2R,4R)-4-ethyl-3-(2,4,6- trimethyl-benzenesulfonyl)-oxazolidin-2-yl]-1-hydroxy-bicyclo[3.3.1]nonan-3-one (1), formic acid (1S,2R)-2-{2-[(2R,4R)-4-ethyl-3-(toluene-4-sulfonyl)-oxazolidin-2-yl]-cyclohexylidene}-1-trimethylsilanyl-ethyl ester (2), (2R,4R)-2-{(4S,5R,6S)-6-tertbutyl-2,2,4,5-tetramethyl-[1,3]dioxan-4-yl}-4-ethyl-3-(toluene-4-sulfonyl)-oxazolidine (3), (2S)-2-[(2R,4R,5R)-4-methyl-5-phenyl-3-(toluene-4-sulfonyl)-oxazolidin-2-yl]-3,4-dihydro-2H-naphthalen-1-one (4) and (1R)-1-[(2R,4R,5R)-4-methyl-5-phenyl-3-(toluene-4-sulfonyl)-oxazolidin-2-yl]-3,4-dihydro-1H-naphthalen-2-one (5) have been determined: (1) crystallizes in the orthorhombic space group P2₁2₁2₁ with cell dimensions a = 11.120(1), b = 13.809(1), c = 14.676(1) Å, (2) crystallizes in the monoclinic space group P2₁ with cell dimensions a = 12.577(3), b = 9.589(1), c = 13.971(3) Å, = 95.77(1) °, (3) crystallizes in the monoclinic space group P2₁ with cell dimensions a = 7.990(4), b = 11.282(9), c = 14.473(4) Å, = 96.31(3)°, (4) crystallizes in the orthorhombic space group P2₁2₁2₁ with cell dimensions a = 7.674(1), b = 14.647(1), c = 20.620(1) Å, (5) crystallizes in the monoclinic space group P2₁ with cell dimensions a = 7.890(2), b = 11.319(1), c = 13.493(3) Å, = 104.12(1)°.     

Structural studies on fullerenes

Amongst the other physical properties, the structure of fullerene solids, and the structural transformations with temperature, pressure and doping have all evoked considerable interest and during the last two years tremendous progress has been made in this area. This paper provides a brief review of the studies on the structural properties of pristine and doped fullerenes. The results of our own investigations are also presented.     

Structural properties of liquid Fe-Si alloys

The atomic structure of liquid Fe-Si alloys within the whole concentration range (including pure components) was investigated at the temperature of 1550 °C by means of X-ray diffraction. Analysis of the reported data on physical properties of Fe-Si melts has been carried out. Interrelation between liquid-state structure and solid-state properties was considered. Fe-Si alloys in the liquid state are shown to be micro-inhomogeneous and they contain atomic micro-formations (clusters) that differ by atomic composition and packing. There are five types of clusters: two of them consist of atoms of one kind (Fe or Al); the composition of the other clusters depends on the stoichiometry of solid Fe₃Si, FeSi and Fe₂Si₅ phases. The entire concentration range of the Fe-Si system consists of four concentration intervals. Within each interval melts are constituted of the clusters of two kinds. The variation of the component concentrations in Fe-Si alloys results in changing of volume fraction of each type clusters, whereas the atomic composition and arrangement inside the clusters remains constant.     

Structural Studies of Electron Deficient Titanacyclobutanes

Abstract  

Two 16 electron titanacyclobutanes of the formula Ti(C₅H₄R)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇) (R=H, CH₃) have been prepared from the reaction of Ti(C₅H₅)₂(μ₂-CH₂)(μ₂-Cl)Al(CH₃)₂ or Ti(C₅H₄CH₃)₂(μ₂-CH₂)(μ₂-Cl)Al(CH₃)₂ with H₂C=C(CH₃)(i-C₃H₇). Structural parameters, most notably lengthened C–C bonds in the titanacyclobutane ring, for both complexes reveal the expected presence of (C–C)→Ti agostic interactions. The complexes are isomorphous, crystallizing in the monoclinic space group Cc. For Ti(C₅H₅)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇), a = 11.3459(3) ?, b = 16.2108(4) ?, c = 8.1646(2) ?, β = 105.5276(16)°, V = 1446.87(6)°, D_calc = 1.268 at 150(1) K. For Ti(C₅H₄CH₃)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇), a = 12.6591(2) ?, b = 16.2795(4) ?, c = 8.2462(2) ?, β = 107.2421(14)°, V = 1623.04(6) ?³, D_calc = 1.245 at 150(1) K.     

Structural studies of lead aluminate glasses

Lead aluminate melts were quenched rapidly with a roller quencher and bulk glasses were formed over a composition range from 72.5 to 80.0 mol% PbO. Pulsed neutron diffraction, ²⁷Al MAS NMR and Raman spectroscopy were used to study the structure of a series of these glasses. The results show that in the glasses the aluminium is four coordinated by oxygen across the compositional range, with a bond length of about 1.76 Å. The Pb–O peak in the neutron correlation function is asymmetric, and it can be modelled in terms of two bond lengths of ∼2.25 Å and ∼2.47 Å, with the majority of the coordination at the shorter distance. There is evidence that most or all of the lead ions are on asymmetric sites, coordinated by three oxygens in a trigonal pyramid arrangement. Both the neutron diffraction and Raman results indicate that the Pb–O bond lengths become shorter with increasing lead content.     

Structural study of 2,4,6-triisopropylbenzenesulfonyl chloride

The structure of 2,4,6-triisopropylbenzenesulfonyl chloride (TPSCl) (I) was studied by X-ray diffraction. Compound I is a highly selective condensing agent first suggested by Khorana for the formation of the C3′–C5′ interribonucleotide linkage in the oligo- and polyribonucleotide synthesis. I crystals are orthorhombic. At ?120°C, the unit-cell parameters are a = 14.184(4) Å, b = 11.344(5) Å, c = 19.883(6) Å, V = 3199(2) ų, d _calc = 1.257 g/cm³, Z = 8, sp. gr. Pbca. Molecule I adopts a strongly flattened boat conformation with the carbon atoms C-SO₂Cl (C ₁) and C-i-Pr (C4) of the benzene ring deviating from the bottom of the boat by 0.065(1) and 0.032(1) Å, respectively. Molecule I is sterically overcrowded, resulting in an increase in the bond lengths and bond-angle distortions in the fragment containing the SO₂Cl group and two ortho-i-Pr groups. Nonbonded contacts that are present in the molecule can be considered weak intramolecular hydrogen bonds (for example, the (CH₃)₂C-H?O=S bond). There is no rotation of the SO₂Cl, ortho-i-Pr, and CH₃ groups. The above-mentioned facts are, apparently, responsible for the specific selectivity of TPSCl. New readily available sterically hindered arenesulfonyl chlorides were designed.     

Structural studies of sputtered Ni---As---O films

The combined results of chemical analysis, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) studies demonstrate that the majority of Ni atoms in sputtered Ni---As---O films are best described as covalently bonded to six As atoms in a noncrystalline phase. The arrangement of nearest neighbors about the Ni atoms is very similar to that found in crystalline NiAs₂.     

Structural study of magnesium polyphosphate glasses

Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P₂O₅), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO₄ chains with rising MgO content is quantitatively described by the increasing concentration of Q¹ and Q⁰ groups determined by NMR. In the pyrophosphate region the Q¹ sites disproportionate to Q⁰ and Q² groups, whereas there is no disproportionation of the Q² sites at metaphosphate composition. The shortening of the real-space distances, r_m, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgO_n polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.     

Structural analysis of anodic alumina films

Structures of amorphous anodic alumina films have been investigated by the X-ray radial distrubution function and the correlation method. The amorphous anodic alumina films are considered to contain AlO₅ coordination polyhedra as well as AlO₄ tetrahedra and AlO₆ octahedra. Two types of conformation are taken into account by the correlation method; one (model A) is the conformation in which the above coordination polyhedra are arranged in a similar way to that found in the crystalline modifications of γ-Al₂O₃ within a short range of r < 4.5 Å and the other (model B) is the conformation in which coordination polyhedra are randomly arranged. The final model formed by mixing these two conformations (model A and B) in 1 : 1 ratio describes quantitatively the observed scattering intensity curves of the amorphous anodic alumina films.     

Structural studies of low-valent metal-fluorocarbon complexes

The crystal structure of bis(triphenylphosphine)hexafluoroacetone nickel(0), (Ph₃P)₂ , has been determined. The crystals are monoclinic: space group P2₁/n, a=18·123(5), b=19·879(5), c=9·745(2) Å, =93·46(2)° and Z=4. X-ray data were collected by counter methods using MoK radiation, and least-squares refinement led toR = 0·087 for the 1406 independent reflexions for which ¦F₀¦² > 2(¦F₀¦²). In the complex, the hexafluoroacetone molecule is associated with the nickel atom by sideways coordination of the C=O group to form a three-membered ring in which the bond lengths are Ni-C, 1·89(2), Ni-O, 1·87(1) and C-O, 1·32(2) Å. The whole Ni, P, P, C, O coordination group forms an approximate plane with the two CF₃ groups projecting on either side.