Small-angle neutron scattering study of high-pressure sintered detonation nanodiamonds

The structure of detonation diamonds sintered at a high pressure (7 GPa) and temperatures of 1200–1700°C has been investigated by small-angle neutron scattering. It is shown that sintering leads to an increase in the particle size from 6 to 30 nm and established that this increase is due to the chainlike oriented attachment of particles. This study supplements the oriented-attachment model, which was suggested based on the X-ray diffraction spectra of detonation nanodiamonds (DNDs) sintered under the same conditions.     

X-ray diffraction and X-ray standing-wave study of the lead stearate film structure

A new approach to the study of the structural quality of crystals is proposed. It is based on the use of X-ray standing-wave method without measuring secondary processes and considers the multiwave interaction of diffraction reflections corresponding to different harmonics of the same crystallographic reflection. A theory of multiwave X-ray diffraction is developed to calculate the rocking curves in the X-ray diffraction scheme under consideration for a long-period quasi-one-dimensional crystal. This phase-sensitive method is used to study the structure of a multilayer lead stearate film on a silicon substrate. Some specific structural features are revealed for the surface layer of the thin film, which are most likely due to the tilt of the upper layer molecules with respect to the external normal to the film surface.     

An X-ray diffraction study of the structure of barium fluorozirconate and fluorohafnate glasses

Two binary glasses have been prepared in the ZrF₄BaF₂ and HfF₄BaF₂ systems, containing 64 mol.% ZrF₄ and 66 mol.% HfF₄, respectively. The two plate-like glass samples were studied by X-ray diffraction, and the resulting data were subjected to a Fourier analysis. The pair correlation functions thus obtained yield a ZrF (or HfF) nearest neighbor coordination number of ～ 7.6 for both glasses. The second peak in the correlation functions has been assigned to a combination of FF and BaF interactions, while the third peak is probably due in part to ZrZr (Hf-Hf) interactions. The diffraction data have been compared with the predictions of structural models for barium fluorozirconate and fluorohafnate glasses.     

X-ray diffraction study of the morphology and structure of pulse-anodized porous Si multilayers

Porous Si layers, obtained by pulsed electrochemical etching of n-Si(001) substrates (resistivity 0.01 Ω cm) in a 1: 1 mixture of hydrofluoric acid and ethanol, have been investigated by high-resolution X-ray diffraction and electron microscopy. The average structural parameters of the layers grown (thickness, strain, porosity, pore size) are determined. It is found that pulsed anodic oxidation leads to a decrease in the average strain of layers. It is established that, at frequencies up to 1 Hz, anodic oxidation makes obtaining porous silicon multilayers with layer thicknesses of 20–300 nm containing Si nanocrystallites possible. It is shown that X-ray diffuse scattering from pores yields information about their ordering and can be used to monitor the processes of electrochemical etching used to form porous layers.     

An X-ray diffraction study of the amorphous structure of chemically vapor-deposited silicon nitride

X-ray diffraction measurements have been carried out for two kinds of samples of CVD-amorphous silicon nitride with different values of the density. From radial distribution analysis, it became clear that the short range structure in these samples resembles that of the crystalline β-phase. Coloring in CVD-amorphous Si₃N₄ was found to be induced by X-ray irradiation. Based on the deficient numbers of the first nearest neighbours in SiN bondings, it was proposed that CVD-amorphous Si₃N₄ consists of small clusters including vacancies in their matrix.     

Synchrotron X-ray diffraction study of the structure of oral stratum corneum model lipid membranes

Membranes modeling the lipid fraction of the mucous membrane of the human oral cavity have been studied by X-ray synchrotron diffraction. Ternary systems, composed of a mixture of sphingomyelin, dipalmitoylphosphatidylcholine, and dipalmitoylphosphatidylethanolamine with component weight ratios of 1: 2: 2 and 1: 2: 1, have lamellar structures in the gel phase at the physiological temperature (37°C). An inverted hexagonal phase is formed in the system with 40% dipalmitoylphosphatidylethanolamine in the temperature range of 60–70°C. This phase coexists with the liquid-crystalline lamellar phase in a narrow temperature range and completely surpresses the lamellar phase with an increase in temperature to 80°C. Multi-component oral stratum corneum membranes are characterized by several lamellar phases at 20–37°C and the coexistence of one or several lamellar phases with inverted hexagonal phase at 80–90°C.     

X-ray diffraction study of thiocarbamide hydrochloride

Thiocarbamide hydrochloride single crystals forming a salt with the composition [(NH₂)₂CSH]⁺Cl^? are studied by the X-ray diffraction method. The tetragonal crystals have the unit-cell parameters a = 7.556(1) Å and c = 18.329(3) Å, V = 1046.5 ų, Z = 8, ρ_calcd = 1.436 g/cm³, sp. gr. P4₁2₁2, and R = 0.050. Each chloride anion in the crystal forms five hydrogen bonds with thee different cations.     

X-ray diffraction study of some indolinones

X-ray diffraction studies are reported for six indolinones. The results show that two different types of associates are present in the crystal state of the compounds having an NH group. The indolinone molecules without any substituents form cyclic dimers while those with one or two methyl groups attached to the C3 atom of the lactam ring form infinite hydrogen bonded chains. It is confirmed that the lactam moiety forms a typical planar amide structure with no sign of the enol form present.     

X-ray powder diffraction study of polytetrafluoroethylene

Samples of polytetrafluoroethylene were studied by X-ray diffraction. A quantitative X-ray powder diffraction analysis of three components of the polymer was performed for the first time. All samples of polytetrafluoroethylene were found to be three-phase and consist of one crystalline and two amorphous phases. One of the amorphous phases is composed of low-molecular-weight products. The structure of the latter phase was established for the first time by X-ray diffraction methods and computer simulation.     

An X-ray diffraction refinement of the structure of natural thomsonite

Thomsonite Na₂Ca₄Al₁₀Si₁₀O₄₀, orthorhombic Pncn, a = 1.3124 nm, b = 1.3078 nm, c = 0.662 nm, V = 1.1369 nm³, Z = 2, D_m = 2.30, D_c = 2.26, μ = 12.4 cm⁻¹. The average length of the bond T–O = 0.1685 nm. The final R-value for 456 independent observed reflections is 0.085.     

X-ray diffraction study on amorphous gallium

Amorphous films of some μm in thickness, prepared by low temperature condensation in an ultra-high vacuum onto liquid helium cooled substrates, have been studied in situ by using an X-ray diffractometer operating in a symmetrical reflection mode. The structure factor of gallium has been obtained over the wavevector range 1.3 to 16.1 Å⁻ by means of two wavelengths, Cr_K and Mo_K, monochromatized by balanced filters. The average number of nearest neighbours deduced from the well-resolved first maximum in the radial distribution function is equal to 9.3 atoms. The results are compared to those previously found by electron diffraction measurements on thin films and also to the structure of supercooled liquid.     

X-ray diffraction study of mesophase bitemplate-containing silicas

The effect of the composition of a bitemplate (solubilization) mixture on the formation of a spatially organized structure of mesophase siliceous systems has been investigated by X-ray diffraction. Alkylpyridinium halides (C _n PyHal with n = 8?16 (counterion Cl) and 18 (counterion Br)) were used a solubilizator, and monoethanolamides (EAC _m ) of saturated n-aliphatic acids (m = 10?16) and trialkylphosphine oxides (H_{2e + 1}C _e )₃PO, where e = 8, 10, or 12, were used as solubilizates. Differences in the mesophase structure of template-containing silicas synthesized in aqueous and aqueous salt (NaCl) solutions have been revealed. It is shown that the type and degree of ordering of the silica mesophase structure depends on the nature, spatial organization, and the solubilizator and solubilizate concentrations.     

An X-ray diffraction refinement of the crystal structure of natural apophyllite

The crystal structure of apophyllite (Andersberg — GDR), with a = 8.966(2), c = = 15.767(1) Å, P4/mnc, z = 2, Q = 2.35 gm · cm⁻³, has been refined (R = 0.035) by least squares with 976 reflections collected with a diffractometer. Apophyllite is a sheet structure with the bringing Si O bond lengths 1.6236 Å, while the non-bridging bond length Si O(3) is 1.5843 Å. The two independent Si O Si angles are 140.09, 140.76 degrees. The mean Si O bond length is 1.6138 Å. As it was not possible to locate unequivocally the H-atoms with the X-ray data, the interpretation of the of water in the structure was based on the charge balance approach of DONNAY and ALLMAN . Assuming that the fluorine ion is bonded to calcium ion (Ca-F = = 2.416 Å) and to H3 atom (F-H3 = 0.9145(1) Å) and water molecule hydrogen bonded to silicate framework. The average bond distances O-H are 0.962(2) Å and angle 105.24(1)°.     

Neutron and X-ray diffraction studies of the structure of non-crystalline materials

Neutron and X-ray diffraction experiments have provided useful information about the topological and chemical short-range order in non-crystalline materials. The availability of new sources and detectors for X-rays and neutrons has greatly improved the statistical accuracy of the scattered intensity and extended its range in momentum (Q) space, yielding high-resolution atomic distribution functions. The methods of isotopic and isomorphous substitution have been used to determine the partial atomic structure factors and their corresponding atomic pair distribution functions in binary metallic systems, and to evaluate the nearest-neighbor interactions in more complicated inorganic glasses. Recent results of structural investigations on Ni-based amorphous alloys and on halide glasses are discussed.     

X-ray diffraction study of gadolinium stearate Langmuir-Blodgett films

The comparative structural studies of gadolinium stearate Langmuir-Blodgett films prepared with the use of two different subphases (aqueous solutions of gadolinium chloride and gadolinium acetate) have been performed by the methods of X-ray diffractometry. The films were applied onto single-crystal silicon substrates coated with a native-oxide layer either with the use of buffer molecular stearic-acid layers or without such layers. It was established that the films obtained with the use of gadolinium acetate and the preliminarily formed buffer layers are, in fact, single-phase Y-type films with highly ordered molecular structure, and the thickness of the bimolecular layer d = (51.3 ± 0.5) Å.     

X-ray diffraction study of highly purified human ceruloplasmin

The three-dimensional structure of ceruloplasmin (CP) with unoccupied labile metal-binding sites and the structure of CP containing Ni²⁺ in the labile sites were solved for the first time at 2.6 and 2.95 Å resolution, respectively. Crystallization was performed with the use of storage-stable CP, which was prepared in the presence of proteinase inhibitors and purified from (pre)proteinases. Ceruloplasmin with Ni²⁺ crystallized in the orthorhombic space group, which had been earlier unknown for CP. Ceruloplasmin with the unoccupied labile sites crystallized in the trigonal crystal form. The differences in intermolecular contacts observed in the trigonal and orthorhombic crystal structures of CP are considered. The conformational changes attendant upon Ni²⁺ binding are described. It was suggested that the labile sites are multifunctional and can both bind metal ions potentially toxic to organisms and be involved in electron transfer from substrates to the active site.     

X-ray diffraction study of defects in zinc-diffusion-doped silicon

Samples of CZ n-Si〈Zn〉(111) are prepared by high-temperature zinc-diffusion annealing followed by quenching and are studied by X-ray diffraction. The silicon contains an initial phosphorus impurity and zinc-compensating admixture at concentrations N _P = 1.5 × 10¹⁴ cm^?3 and N _Zn = 1 × 10¹⁴ cm^?3; i.e., the relation N _P/2 < N _Zn < N _P is fulfilled. Microdefects are studied by double- and triple-crystal X-ray diffraction in the dispersion free modes (n, ?n) and (n, ?n, +n). The samples are found to contain microdefects with two characteristic sizes (average sizes of about 1 μm and 70 nm). The interplanar distance in the near-surface layer with a thickness of 0.1 μm is smaller than this parameter in the remaining matrix, the difference being equal to d ₀ Δd/d ₀ ≈ 2 × 10^?5. This layer contains mainly vacancy-type microdefects. The angle between the reflecting planes and the local surface relief is Δψ = (7 ± 1) arcmin.     

An X-ray diffraction study of sodium borate glasses

X-ray diffraction measurements have been performed on two pairs of sodium borate glasses of differing compositions. Glasses of given composition were obtained with two different thermal histories. The radial distribution functions of samples of the same composition appear similar. The short range order is not affected by the range of quenching rates explored.     

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al₂O₃ substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10^?2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.     

X-ray diffraction study of the complexes of SAICAR synthase with adenosinetriphosphate

The three-dimensional structures of two enzyme-substrate complexes of SAICAR synthase from the yeast Saccharomyces cerevisiae with adenosinetriphosphate (ATP) prepared under different conditions were studied by X-ray diffraction analysis and then refined. An enzyme molecule was shown to contain two binding sites of ATP. One of these sites is located in the central cavity of the enzyme molecule and apparently binds the ATP molecule directly involved in the enzymatic reaction. In the complexes, the phosphate groups of ATP occupying this site adopt different conformations depending on the Mg²⁺ concentration. The functional role of the second binding site located at a distance of approximately 15 Å from the first site away from the central enzyme cavity has not been understood as yet. It might be that the second site perform the regulatory role in enzyme functioning.