USAXS studies of monosaccharide gels. I. Dependence of the glucofuranose-based gel structure on the gelator concentration

Glucofuranose-derivative based gels with various gelator concentration were studied using ultra-small angle X-ray scattering (USAXS) in the scattering vector range 0.0028–0.25 nm⁻¹. A complex method of data processing was applied to extract the structural parameters of the gelator, such as the mass fractal and surface fractal dimensions, d_m and d_s, respectively, the radius of gyration, R_g, the distance distribution function, p(r), as well as the volume distribution function, D(R), of particles forming aggregates. The results of these analyses show that the aggregate structure changes with the gelator concentration: when it increases, the size of the aggregates decreases, rough aggregates of the fractal type evolve to better developed smooth ones, and their shape changes from disk-like to rod-like. These results suggest that only two different types of aggregates are formed depending on the gelator concentration, and a gradual change between them is observed.     

Atomic structure and high anisotropy of thermal expansion in NiSi single crystals. II. Temperature-induced structural changes

X-ray diffraction analysis of the atomic structure of NiSi crystals was performed by diffraction data obtained at 295 and 418 K. The temperature-induced changes in the bond lengths and valence angles were analyzed and the high anisotropy of thermal expansion of these single crystals was interpreted in terms of crystal chemistry.     

Simulation studies of structural changes and relaxation processes in lysozyme under pressure

The paper describes preliminary results of a molecular dynamics simulation study on the influence of non-denaturing hydrostatic pressure on the structure and the relaxation dynamics of lysozyme. The overall compression and the structural changes are in agreement with results from recent nuclear magnetic resonance experiments. We find that moderate hydrostatic pressure reduces essentially the amplitudes of the atomic motions, but does not change the characteristics of the slow internal dynamics. The latter is well described by a fractional Ornstein–Uhlenbeck process, concerning both single particle and collective motions.     

Specific structural features of copper(II) complexonates with nitrilotriacetate anions: A review

The specific structural features of copper(II) complexes with nitrilotriacetate anions (H_n Nta, n = 0?2) are considered with due regard for the original results and data available in the literature. The structural features and the degree of distortion of copper coordination polyhedra (such as a (4 + 1) elongated square pyramid, a trigonal bipyramid, a (4 + 2) symmetric elongated tetragonal bipyramid, and a (4 + 1 + 1) asymmetric elongated tetragonal bipyramid) are discussed. Criteria for identification of five-vertex copper(II) polyhedra of the general formula Cu(H_n Nta)X (n = 0?2; X = Cl, Br, N, O) are proposed for the cases of a distorted square pyramid, a distorted trigonal bipyramid, and a polyhedron intermediate between them. The modes of coordination of H_n Nta ^{3 ? n} ligands with a mctal arc described in terms of crystal chemical formulas.     

Preparative and structural studies of halodimolybdates (II), III. Crystal and molecular structure of bis(4-methylpyridinium)hexaiododiaquo-dimolybdate(II)

The title compound (C₆H₈N)₂Mo₂I₆(H₂O)₂ crystallizes in theP¯1 space group, witha = 8.127(3),b = 8.337(3),c = 10.415(4) Å, = 73.99(2), = 84.35(2), = 73.91(2) ° andZ = 1. The structure was solved by direct methods and refined to the unweighted and weighted residuals of 0.044 and 0.052. The structure is characterized by the eclipsed configuration of the iodide ions and water molecules within Mo₂I₆(H₂O) ₂ ^2– ion and very short Mo-Mo bond distance 2.115(1) Å. Three independent Mo-I distances are 2.784(1), 2.760(2) and 2.797(1) Å. 4-Methylpyridinium ion is planar within 0.02° Å. The compound is isostructural with (C₆H₈N)₂Mo₂Br₆(H₂O)₂.Part II, Breni, J. V., Leban, I., and egedin, P. (1976).Z. Anorg. Allg. Chem. 427, 85.     

Thin layers deposited from V2O5 gels: II. An optical absorption study

Absorption spectra of V₂O₅ layers deposited from gels of various V⁴⁺ contents C were studied from near UV down to near IR. The high absorption region due to the charge transfer transition extends above ≈ 2 eV. Whatever C the optical gap is ≈ 2.25 eV. On the low energy side an Urbach tail is observed whose slope increases with C. In the near infrared absorption is related to the V⁴⁺ ions and increases linearly with C. The absorption band due to the optically induced polaron hopping is detected. The corresponding maximum E_max suffers a red shift when C increases. The experimental E_max data are very close to the values predicted from the polaron and disorder energies as deduced from the conductivity study in an earlier paper. The slight difference between the two sets of data allowed us to estimate the transfer integral J ≈ 0.015 eV.     

Synthetic and structural studies of the trans bis(2-N-H-pyrrolylcarbaldimine) complexes of nickel(II) and palladium(II)

The molecular bis(2-N-H-pyrrolylcarbaldimine)nickel(II) and palladium(II) complexes are isolated in moderate yield (30–35%) from in situ assembly of the Schiff base with metal(II) salt, base, and pyrrole-2-carbaldehyde in aqueous ammonia solution. The nickel(II) complex, 1, is monoclinic, space group P2₁/c, with a=11.289(6) ?, b=5.611(3) ?, c=8.287(5) ?, β=111.620(6)°, and V=488.0(5) ?³ with Z=2, for d _calc=1.667 Mg/m³. The palladium analog, 2, is isomorphous, space group P2₁/c, with a=11.481(3) ?, b=5.5738(10) ?, c=8.276(2) ?, β=110.923(12)°, and V=494.7(2) ?³ with Z=2, for d _calc=1.965 Mg/m³. In both crystal structures, the metal resides on an inversion center. In the IR spectra, ν(C=N) appears at 1561 cm⁻¹ in 1, and 1557 cm⁻¹ in 2, while ν(N–H) shows at 3345 cm⁻¹ and 3335 cm⁻¹, respectively. The ¹H-nmr spectra reveal the C–H and N–H protons of the imine group as sharp and broad doublets, respectively, at 7.62 and 8.38 δ in 1 and at 7.92 and 9.73 δ in 2.     

Electron diffraction study of the structural features of kaolinites

The structural variations of kaolinites have been investigated based on oblique-texture electron diffraction patterns. The features of manifestation of the 2D lattice of kaolinite layers in the geometry of the 20l and 13l reflections have been revealed. The manifestation of violations in the regular alternation of layers in kaolinites in the diffraction patterns has been analyzed using a numerical simulation of the diffraction profiles along the first (02l, 11l) and second (20l, 13l) ellipses of oblique-texture electron diffraction patterns. The simulation was performed for finite sequences of ten layers using the statistical Markov model in the quasihomogeneous approximation. It is shown that oblique-texture electron diffraction patterns can be used to reveal the coexistence of two phases with different structural perfections and particle morphologies in kaolinite samples.     

Dielectric properties and structural features of barium-iron phosphate glasses

The dielectric constant of barium-iron phosphate glasses with the general composition (40−x)BaO · xFe₂O₃ · (60−x)P₂O₅ has been investigated at two fixed frequencies (100 kHz and 9.0 GHz). The dielectric constant measured using microwave technique, and the ratio O/P of these glasses increase with increasing Fe₂O₃ content. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, infrared spectroscopy and differential thermal analysis. Both Fe(II) and Fe(III) ions present in these glasses in octahedral coordination act as permanent dipoles, and the increase of the iron concentration increase these permanent dipoles, contributing to the dielectric constant.     

ESR study of structural changes in chalcogenide glasses

Structural changes in chalcogenide glasses induced by various processes such as annealing, illumination or application of a high pressure were investigated from a microscopic viewpoint by measuring changes of the ESR spectrum from Mn doped as a probe. Here, Mn incorporated in the glass network exhibits a characteristic ESR signal with g = 4.3 having a hyperfine structure. It is found that the above-mentioned processes cause a change of the lineshape of the hyperfine line and a change of the hyperfine structure constant A, both of which reflect the bonding characteristics around Mn. One of the annealing effects for bulk glasses is the increase of the A-value. The change of the A-value for well-annealed As₂Se₃ and As₄Se₅Ge₁ films occurs reversibly in sequential illumination and annealing cycles, as does the shift of the optical gap. The reversible change of the lineshape is observed for well-annealed As₂Se₃ films in the illumination and annealing cycles. Application of a high pressure to bulk glasses causes changes similar to those by illumination for annealed films. From these results it is concluded that the randomness of amorphous structure decreases by annealing and increases by illumination or application of high pressure.     

Nuclear magnetic resonance studies of aluminosilicate gels prepared in high-alkaline and salt-concentrated solutions

Solid-state ²⁹Si, ²⁷Al, and ²³Na magic angle spinning (MAS) NMR techniques in combination with X-ray powder diffraction (XRD) are used to characterize aluminosilicate gels as a function of composition, pH, and reaction times. These gels were prepared at 80 °C using initial solutions with low Si/Al ratios, high alkaline and salt concentrations that are characteristic of nuclear tank wastes. XRD data show that cancrinite and sodalite are the main crystalline phases in the aluminosilicate gels produced. It is found that the pH and the salt content have significant effects on the nature of the aluminosilicate gels. Higher pH appears to increase the rate of crystallization, the degree of overall crystallinity and the percentage of cancrinite phases in aluminosilicate gels, whereas the high-salt concentration promotes the formation of cancrinite and sodalite and prohibits the formation of other zeolites. Complementary to XRD, NMR is extremely useful for providing the information on the structure of amorphous intermediate gels with no long-range order.     

Characteristic structural features of a tantalum-rich eudialyte variety from Brazil

The crystal structure of a new tantalum-rich variety of the mineral eudialyte discovered in Brazil was established by X-ray diffraction analysis (sp. gr. R3m, R = 0.038, 1092 independent reflections). The structural characteristic of this mineral is the presence of Ta atoms in the specific positions in the centers of planar “squares” with Ta-O distances ranging from 2.035(7) to 2.116(8) Å. Two additional oxygen atoms located at distances of 2.44 and 2.66 Å can complete these “squares” to strongly distorted octahedra.     

Invariants of reflectometry curves and the structural features of thin films

It is shown that the integral of the autocorrelation function of freestanding layered films determines the square of the mean electron density in the film cross section. Data-processing techniques are proposed to deal with the results of reflectometry measurements for films on substrates. Comparison of the autocorrelation functions calculated on the basis of the experimental curve and the model provides an opportunity to evaluate the sample quality.     

Crystal chemistry of germanates: Characteristic structural features of Li,Ge-germanates

Crystallochemical classification of eleven compounds from the Li-germanate family is suggested. Depending on the set of the primary building units (PBU) (M-octahedra of the composition [GeO₆] and T-tetrahedra of the composition [GeO₄]) and the type of their “condensation,” these germanates are divided into three crystallochemical groups: framework MT-structures (four phases), condensed MT-structures (two phases), and tetrahedral T-condensed structures (five phases). The structural characteristics of the framework Li,Ge-germanates are considered, i.e., their symmetry, crystallographically independent sets of the primary building units, framework architecture, and the types of chains and layers of the (Ge,O)-radicals.     

ESR and Mössbauer studies of dispersed states of Fe ions in silica gels

0.01Fe₂O₃---0.99SiO₂ gels were prepared by three different sol-gel methods and the effects of the heat-treatment temperature of the gels on the ESR and Mössbauer parameters were investigated to obtain information about the change in the state of iron ions in the gel. All the gels were found to be amorphous under 600 °C, and among these three methods, the best dispersed state of Fe³⁺ ions was obtained by the method where TEOS was partially hydrolyzed with the same mole content of water before the addition of Fe(OEt)₃. It was indicated that the reaction at the sol-stage plays an important role in the degree of dispersal of Fe³⁺ ions in this system.     

Scattering studies of photostructural changes in chalcogenide glasses

Structural changes which occur in chalcogenide glasses upon absorption of bandgap light have been investigated using neutron diffraction. The first part of this paper describes studies by small-angle neutron scattering of the irreversible photostructural changes which occur in obliquely evaporated amorphous chalcogenide films. The second part deals with the reversible photostructural effect observed in chalcogenide glasses, investigated by conventional neutron diffraction and EXAFS experiments. The various models which have been proposed for the photostructural effect are discussed in the light of these experimental results.     

Growth of KTiOPO<Subscript>4</Subscript> crystals doped with zinc and studies of their physical properties and specific structural features

A series of potassium titanyl phosphate single crystals doped with zinc (KTP: Zn) is grown by spontaneous flux crystallization. Their properties and the way the additive is implanted in the crystal lattice are studied. The inclusion of zinc atoms in the KTP structure is evidenced by the data of chemical analysis and the results of studies of electrophysical properties (the growth of conductivity and increase of relaxation anomalies). Precision X-ray diffraction studies of KTP: Zn single crystals reveal changes in the channel of the structure which correlate with the physical properties of this crystal series. No substitution of zinc atoms for titanium, phosphorus, or potassium atoms is found in the structure. Zinc atoms can be located at structural defects, domain walls, and the crystal surface.     

Structure analysis by reduced data. II. Increase of reproducibility of the results of diffraction studies

The second part of the paper shows the advantages of the method of interexperimental minimization (IEM) over the conventional least squares (LS) method in the crystal-structure refinement. The new IEM method minimizes the norm of the differences between the data measured in independent experiments. The parameters of the structure model refined by the LS and the IEM methods are compared for four experimental data sets from an alexandrite crystal Al₂BeO₄: Cr³⁺. The model parameters calculated by the LS method from the initial experimental data sets showed insufficient reproducibility (the number of admissible values of the statistical-test criterion based on the normal-probability plot equals 5.6%). The use of the IEM algorithm increases the number of admissible values up to 38.9%. However, the use of fitting and statistical methods cannot provide a 100% reproducibility of all the model parameters automatically, which requires the reconstruction of the lost information on the characteristics of the experiment.     

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol-gel route

The objective of this study is to understand the effect of low temperature sol-gel synthesis on the microstructural properties of lithium [xLi₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] and silver [xAg₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state ⁷Li and ²⁹Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, ⁷Li NMR shows quadrupolar satellite transitions attributed to LiNO₃, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with ²⁹Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag-L-III XANES spectral studies show a local environment similar to AgNO₃ for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li₂O-0.7SiO₂ composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li⁺ mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H⁺ mobility. Variation in conductivity is explained qualitatively using existing theoretical models.     

Synthesis and structural characterization of dihexamethylenetetraminetetraaquocobalt(II) hexaaquocobalt(II) sulfate hexahydrate

The synthesis and crystal structure of a new three dimensional cobalt(II) complex [Co(C₆H₁₂N₄)₂(H₂O)₄Co(H₂O)₆][SO₄]₂.6H₂O are reported. It crystallizes in the triclinic space group P-1 with a = 13.394(3), b = 16.077(3), c = 9.282(2) Å, = 89.71(2), = 90.42(2), = 114.02(2)°, V = 1825.6(7)ų and Z = 2. The structure is comprised of [Co(C₆H₁₂N₄)₂ (H₂O)₄Co(H₂O)₆][SO₄]₂ units and H₂O molecules. Two molecules of ligand and four molecules of water chelate one type of Co^II in an octahedral fashion through two N and four O atoms: the four O atoms form the equatorial plane and the two N atoms occupy the apical positions. The other type of Co^II cation is chelated by six water molecules.