Showing papers in "Crystallography Reports in 2006"

Development of methods of EXAFS spectroscopy on synchrotron radiation beams: Review

Abstract: The theoretical fundamentals of extended X-ray absorption fine-structure (EXAFS) spectroscopy are discussed. This method makes it possible to obtain short-range order parameters for multicomponent amorphous and quasicrystalline media with a high accuracy. Methods of analysis of EXAFS spectra with allowance for the effects of multiple scattering are described. Experimental setups implementing EXAFS spectroscopy on synchrotron radiation beams are considered and the requirements on monochromatization of radiation beams are given. A brief description of the energy-dispersive EXAFS spectrometer at the Kurchatov Synchrotron Radiation Source is given. The results of experimental investigations of tungsten and molybdenum oxides and fullerides are reported.

Potassium bromo-borate K3[B6O10]Br—A new nonlinear optical material

Abstract: A new polar hexaborate, K{sub 3}[B{sub 6}O{sub 10}]Br (space group R3m), is synthesized under hydrothermal conditions. The radical anion is composed of macallisterite hexaborate blocks 6[3{delta} + 3T] formed by boron triangles and boron tetrahedra. The blocks are joined together through vertices into a [B{sub 6}O{sub 10}]{sub {infinity}}{sub {infinity}}{sub {infinity}}{sup 2-} zeolite-like framework of a new type. Large K{sup +} cations and Br{sup -} anions are located in extended channels that are aligned parallel to rhombohedral translations of the crystal lattice. The structure of the sublattice formed by large ions consists of BrK{sub 6} bromo-centered octahedra, which are linked together through vertices into a hexagonally distorted perovskite framework. The perovskite framework is inserted into the boron-oxygen framework so that these two frameworks do not intersect. The nonlinear optical properties of powdered samples of the K{sub 3}[B{sub 6}O{sub 10}]Br compound are investigated using the second harmonic generation method. It is found that the K{sub 3}[B{sub 6}O{sub 10}]Br compound exhibits a high nonlinear optical activity that exceeds the activities of the majority of borate compounds and is characteristic of haloid borates of the hilgardite family.

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

Abstract: This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.

In situ atomic force microscopy of layer-by-layer crystal growth and key growth concepts

Abstract: Contradictions that have been found recently between the representations of classical theory and experiments on crystal growth from solutions are considered. Experimental data show that the density of kinks is low in many cases as a result of the low rate of their fluctuation generation, the Gibbs-Thomson law is not always applicable in these cases, and there is inconsistency with the Cabrera-Vermilyea model. The theory of growth of non-Kossel crystals, which is to be developed, is illustrated by the analysis of the experimental dependence of the growth rate on the solution stoichiometry.

Cs2Ni(SO4)2 · 6H2O (CNSH) crystal: Growth and some properties

Abstract: Large single crystals of cesium-nickel sulfate hexahydrate Cs2Ni(SO4)2 · 6H2O of optical quality were grown for the first time. The crystal structure and the optical and thermogravimetric properties of this compound were investigated.

Preparation of polyelectrolyte microcapsules with silver and gold nanoparticles in a shell and the remote destruction of microcapsules under laser irradiation

Abstract: A number of methods are proposed for encapsulating silver and gold nanoparticles into shells of polyelectrolyte microcapsules for the purpose of increasing the sensitivity of microcapsules to laser radiation. It is shown that capsules with nanocomposite shells can be remotely damaged under laser radiation with different powers and wavelengths. The sensitivity of capsules with silver and gold nanoparticles in shells to this radiation can be controlled by varying the conditions used for the preparation of the capsules. The release of the encapsulated material under laser radiation makes these systems promising for use as microcontainers intended for the target delivery of drugs in an organism.

Crystal structure of the new barium borate Ba5(BO3)2(B2O5)

Abstract: The crystal structure of the new barium borate Ba5(BO3)2(B2O5) is established (R = 0.0436). Single crystals were grown by spontaneous crystallization in the BaO-B2O3-Na2O system using the flux method. This compound crystallizes in the orthorhombic system, sp. gr. P212121; the unit-cell parameters are a = 9.590(2) A, b = 16.659(3) A, c = 22.919(6) A, and Z = 12. The structure consists of coordination polyhedra of barium cations and the anionic groups [BO3] (planar triangles) and [B2O5] (vertex-sharing double [BO3] triangles), which form a pseudohexagonal framework. Melting of barium borate occurs by a peritectic reaction at 1170 ± 10°C.

Bistable and multistable states in ferroelectric liquid crystals

Abstract: The effect of memorizing the structural states in a ferroelectric liquid crystal (FLC), known as biand multistability, is based on the existence of a domain structure that changes its properties under the action of an applied voltage. The criterion for the bistability steadiness of the optical-transmission states of FLC electrooptic cells is developed. This criterion is related to the hysteresis-loop parameters of the FLC layer. The conditions for the existence and steadiness of the hysteresis-free switching of the memorized optical-transmission states of multistable electrooptic cells are formulated.

Investigation of the structure of nanocrystalline refractory oxides by X-ray diffraction, electron microscopy, and atomic force microscopy

Abstract: The structures of nanocrystalline fibrous powders of refractory oxides have been investigated by different methods: determination of coherent-scattering regions, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic-force microscopy (AFM). The sizes of nanograins of different crystalline phases of refractory metal oxides have been determined during the formation of these nanograins and the dynamics of their growth during heat treatment in the temperature range 600–1600°C has been studied. The data on the structure of nanocrystalline refractory oxide powders, obtained by different methods, are in good agreement. According to the data on coherent-scattering regions, the sizes of the ZrO2 (Y2O3) and Al2O3 grains formed are in the range 4–6 nm, and the particle sizes determined according to the TEM and AFM data are in the ranges 5–7 and 2–10 nm, respectively. SEM analysis made it possible to investigate the dynamics of nanoparticle growth at temperatures above 1000°C and establish the limiting temperatures of their consolidation in fibers.

Piezo-optic surfaces of lithium niobate crystals

Abstract: A method of construction of the spatial distribution of the piezo-optic effect in crystals is proposed. A particular case of this method is the known technique of construction of indicator surfaces of the piezo-optic effect. The essence of the proposed method consists in determining the difference in the radius vectors of the optical indicatrix perturbed by stress and the free optical indicatrix. It is shown that this difference in the radius vectors is mathematically identical to the law of transformation of the piezo-optic tensor during the rotation of the coordinate system.

Structural state of expanded graphite prepared from intercalation compounds

Abstract: The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P63/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.

Three-Dimensional Structure of Laccase from Coriolus zonatus at 2.6 Å Resolution

Abstract: Laccase (oxygen oxidoreductase, EC 1.14.18.1) belongs to the copper-containing oxidases. This enzyme catalyzes reduction of molecular oxygen by different organic and inorganic compounds to water without the formation of hydrogen peroxide. The three-dimensional structure of native laccase from Coriolus zonatus was solved and refined at 2.6 A resolution (Rfactor = 21.23%, Rfree = 23.82%, rms deviations for the bond lengths and bond angles are 0.008 A and 1.19°, respectively). The primary structure of the polypeptide chain and the architecture of the active site were refined. The carbohydrate component of the enzyme was identified. The access and exit water channels providing the access of molecular oxygen to the active site and release of water, which is the reduction product of molecular oxygen, from the protein molecule were found in the structure.

Potassium and ammonium dihydrogen phosphates activated with thallium: Growth and luminescence and scintillation properties

Abstract: The conditions for the growth of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals with concentrations of the thallium activator in the initial solution of 0.01, 0.1, and 1.0 wt % are investigated. It is shown that the character of incorporation and distribution of thallium in the KDP and ADP lattices is limited, apparently, by the difference in the ionic radii of K+, NH 4 + , and Tl+ cations and the charge state of prismatic {100} and pyramidal {101} growth planes. Doping of KDP and ADP with thallium (to 0.1 and 1.0 wt %, respectively) does not deteriorate the structural quality of these crystals. The dependence of the lattice parameters a and c on the thallium impurity concentration is investigated. The absorption bands of thallium in the KDP:Tl+ and ADP:Tl+ crystals peak at 218 and 215 nm, respectively, while the photoluminescence band peaks at 280 nm for both types of crystals. The relative light yield upon excitation of scintillations by α particles (Pu239) and β particles (Bi207) is measured.

Dynamic variation in the lattice parameter of a crystal under ultrasonic treatment in X-ray diffraction experiments

Abstract: The X-ray diffraction in the Laue geometry is investigated in germanium and silicon single crystals upon excitation of long-wavelength ultrasonic elastic strain waves traveling along the sample surface. The X-ray diffraction beam is bounded by a slit 0.2 mm in size, which is considerably less than the wavelength of the ultrasonic wave. The use of this slit makes it possible to separate crystal regions with a nearly homogeneous strain. As a consequence, the rocking curves stroboscopically measured in a double-crystal dispersionless scheme at different instants of time almost coincide with those for a perfect crystal with a lattice parameter varying in time. The rocking curves measured in a time-integrated mode turn out to be broadened, but their integrated intensities remain unchanged. Possible applications of the developed method are discussed.

Molecular and crystal structure of mesomorphic para-n-hexyloxybenzoic acid

Abstract: The triclinic modification of para-n-hexyloxybenzoic acid is studied using X-ray diffraction analysis (MoK α radiation) at 120 K. The crystallographic parameters of the compound are as follows: space group, $$P\overline 1 $$ a = 7.8854(4) A, b = 14.3021(7) A, c = 33.0513(16) A, α = 88.738(1)°, β = 85.701(1)°, γ = 78.331(1)°, and Z = 12. The asymmetric unit of the crystal contains six molecules of the acid with different orientations of the aliphatic chain. In the crystal, the molecules are joined into dimers by hydrogen bonds between the carboxyl groups. The crystal packing is built on the principle of the separation of aromatic and aliphatic regions.

Crystal structure of neptunium(V) sulfate hexahydrate, (NpO2)2SO4 · 6H2O

Abstract: Single crystals of (NpO2)2SO4 · 6H2O are obtained, and their structure is determined. The structure is built of NpO 2 + dioxo cations, SO 4 2− anions, and molecules of coordination and crystallization water. The NpO 2 + ions are linked into cationic ribbons stretched along the [001] direction. In the ribbons, neptunoyl ions of one type act as monodentate ligands, whereas neptunoyl ions of the other type coordinate the neighboring neptunoyl groups by two oxygen atoms. The Np(1) and Np(2) atoms have oxygen environments in the shape of a pentagonal bipyramid whose equatorial plane consists of oxygen atoms of the neighboring dioxo cations, sulfate ions, and water molecules. The sulfate ion acts as a bidentate ligand bridging the two neighboring atoms Np(1) and Np(2). Six water molecules are revealed in the structure; one of them is a crystallization water molecule. Hydrogen bonds link cationic ribbons into a three-dimensional network.

Electron microscopy of biomaterials based on hydroxyapatite

Abstract: Three types of biomaterials based on hydroxyapatite are synthesized and investigated. Hydroxyapatite nanocrystals or microcrystals precipitated from low-temperature aqueous solutions serve as the initial material used for preparing spherical porous granules approximately 300–500 μm in diameter. Sintering of hydroxyapatite crystals at a temperature of 870°C for 2 h or at 1000°C (for 3 h) + 1200°C (for 2 h) brings about the formation of solid ceramics with different internal structures. According to the electron microscopic data, the ceramic material prepared at 870°C is formed by agglomerated hydroxyapatite nanocrystals, whereas the ceramics sintered at 1200°C (with a bending strength of the order of 100 MPa) are composed of crystal blocks as large as 2 μm. It is established that all the biomaterials have a single-phase composition and consist of the hydroxyapatite with a structure retained up to a temperature of 1200°C.

Structural transformations of minerals in deep geospheres: A review

Abstract: The structure and composition of inner geospheres are considered in light of new data on the structural transformations of minerals under high pressure. More than 100 tetrahedral complexes in silicates of the Earth’s crust give way to no more than 20 structural types of minerals of this class in the Earth’s mantle. The main difference in their structures is associated with the transformation of Si tetrahedra into Si octahedra. New data on the structural transformations of minerals in deep geospheres indicate that the mineralogical diversity of the Earth’s crust is substantially richer than that of deep geospheres; however, mantle mineralogy is not as primitive as was supposed even twenty or thirty years ago. The results of recent seismological investigations and quantum-mechanical calculations allow the assumption that there exists a new previously unknown phase transformation under the conditions in the Earth’s inner core.

Growth of KDP crystals from solutions with mechanical impurities

Abstract: Growth of KDP crystals from aqueous solutions with SiO2 particles whose size ranges from 10–2 to 400 μm in the static and dynamic modes has been studied. The effect of mother-solution supersaturation and particle size and concentration on the process of particle capture by a growing crystal is considered as well as types of inhomogeneities formed in the crystal under the influence of these factors. It is shown that the larger the particle size, the higher the probability of particle capture by a crystal. The influence of supersaturation, growth rate, face morphology, and particle concentration on particle capture and defect formation in crystals is also discussed.

Growth of R 1 − y Sr y F3 − y crystals with rare earth elements of the cerium subgroup (R = La, Ce, Pr, or Nd; 0 ≤ y ≤ 0.16) and the dependence of their density and optical characteristics on composition

Abstract: Crystals of the R 1 − y Sr y F3 − y phases (R = La, Ce, Pr, or Nd; 0 ≤ y ≤ 0.16) with tysonite (LaF3) structure are grown by the Bridgman method. A linear dependence of the crystal density on the SrF2 concentration has been revealed for each series of solid solutions. A scheme of heterovalent isomorphous substitution in the tysonite structure, R 3+ + F1− → M 2+ + V F, with compensation of the difference in the cation charges due to the formation of anion vacancies, is confirmed experimentally. The optical transmission spectra are measured in the wavelength range λ = 2.18–22.22 μm. It is shown that the 50%-transmission edge in the IR range is 10.5 μm for all crystals. The refractive index and the polarizability of crystals decrease with decreasing SrF2 content according to a linear law. The melting curves of the R 1 − y Sr y F3 − y phases studied have a maximum; therefore, these phases can be recommended for use as crystals with a high homogeneity and different optical properties in comparison with the corresponding RF3 crystals.

Structure of unilamellar vesicles: Numerical analysis based on small-angle neutron scattering data

Abstract: The structure of polydispersed populations of unilamellar vesicles is studied by small-angle neutron scattering for three types of lipid systems, namely, single-, two-and four-component vesicular systems. Results of the numerical analysis based on the separated-form-factor model are reported.

Neutron standing waves in layered systems

Abstract: A theory of propagation of neutron waves in multilayer magnetic and nonmagnetic systems is presented. It is shown how a system forms the wave function of a neutron, how the wave function formed can be controlled, and how this function can be used to investigate materials. The results of experimental investigations of the neutron reflection from multilayer systems with simultaneous measurement of different types of secondary radiation are reported.

Structural changes in ferroelectric polymers under the action of strong electric fields by the example of polyvinylidene fluoride

Abstract: The regularities of solid-phase polymorphic transformations under the action of strong electric fields in polyvinylidene fluoride (PVDF) films have been analyzed. For the homopolymer films crystallized in a mixture of nonpolar α and polar β phases, two transitions are revealed: α → αp and α → β. In contrast to predictions, these transitions occur simultaneously. For the isotropic PVDF films crystallized in the α phase, the critical fields corresponding to their transition to the ferroelectric β phase may differ by almost an order of magnitude. It is suggested that nucleation of a new phase occurs through the fluctuation mechanism of formation of conformational defects (of the kink-bond type) in the initial crystals. Such processes are controlled to a great extent by the dynamics of through chains in the amorphous-phase regions adjacent to crystals when a decrease in their activation energy in the processes of micro-Brownian motion increases the probability of forming conformational defects in a crystal. If PVDF is crystallized in the ferroelectric phase, the action of strong fields leads to two consequences. First, according to the X-ray diffraction data, rotation of the polar b axes of a crystal through the mechanism of 60° reorientations is observed. Second, the degree of crystallinity can increase (partially irreversibly) through an increase in the crystal size.

Main regularities of crystallization of calcium oxalate in the presence of amino acids

Abstract: Processes of crystallization of calcium oxalate from aqueous solutions are investigated. Main regularities of the nucleation and crystal growth of calcium oxalate during mass crystallization without admixtures and in the presence of amino acids are established. The inhibiting action of the aspartic and glutamic amino acids on the growth of calcium oxalate crystals is demonstrated. Addition of amino acids does not affect the nucleation kinetics.

Investigation of multicomponent fluoride optical materials in the UV spectral region: I. Single crystals of Ca 1−x R x F 2+x ( R = Sc, Y, La, Yb, Lu) solid solutions

Abstract: Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca1−x R xF2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region.

Distribution of aluminum and indium impurities in crystals of Ge-Si solid solutions grown from the melt

Abstract: The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge1−x Si x (0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge1−x Si x (0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals.

Mn, Mg, and Zn ilmenite group titanates: A reconnaissance rietveld study

Abstract: Ecandrewsite-geikielite (Zn1/2Mg1/2TiO3), ecandrewsite-pyrophanite (Zn1/2Mn1/2TiO3), pyrophanite-geikielite (Mg1/2Mn1/2TiO3), and ecandrewsite-pyrophanite-geikielite (Zn1/3Mn1/3Mg1/3TiO3) titanates were synthesized at 900–1000°C at ambient pressure. All of the compounds adopt R— ilmenite-type structures. These structures were refined by the Rietveld method from X-ray powder diffraction data. Unit cell parameters and unit cell volumes decrease with decreasing average radii of the A2+ cation. All structures consist of distorted AO6 and TiO6 octahedra. In common with pyrophanite and ilmenite, the distortion of the AO6 octahedra is less than that of the TiO6 octahedra. Data are given for the volumes and distortion indexes of all coordination polyhedra.

Isotropic lazurite: A cubic single crystal with an incommensurate three-dimensional modulation of the structure

Abstract: The incommensurately modulated structure of the isotropic lazurite Na6.41Ca1.36K0.04(Si6.09Al5.91)O24(SO4)1.73Cl0.17 from the Baĭkal region is solved in the (3 + 3)-dimensional symmetry group P23(γγ0) [where γ = 0.2154(1)] with the basic unit cell parameter a = 9.077(1) A. The structure is refined to R0 = 0.04 for the main reflections and R1 = 0.15 for the satellite reflections. Six modulation waves are directed along the side diagonals of the unit cell. The structural model under investigation is compared with the twin model previously proposed for the same crystal. The structural data obtained are used to construct fragments of the modulated structure. Considering the constructed fragments of the modulated structure, it has become possible for the first time to reproduce and describe the three-dimensional modulation of the framework and to reveal its relation to modulations of intraframework atoms.

Hydrothermal crystallization of Na2Ti6O13, Na2Ti3O7, and Na16Ti10O28 in the NaOH-TiO2-H2O system at a temperature of 500°C and a pressure of 0.1 GPa: The structural mechanism of self-assembly of titanates from suprapolyhedral clusters

Abstract: An increase in the NaOH concentration in the NaOH-TiO2 (rutile)-H2O system at a temperature of 500°C and a pressure of 0.1 GPa leads to the crystallization R-TiO2 + Na2Ti6O13 → Na2Ti3O7 → Na16Ti10O28. Crystals of the Na2Ti6O13 titanate (space group C2/m) have the three-dimensional framework structure Ti6O13. The structure of the Na2Ti3O7 titanate (space group P21/m) contains the two-dimensional layers Ti3O7. The structure of the Na16Ti10O28 titanate (space group P-1) is composed of the isolated ten-polyhedron cluster precursors Ti10O28. In all the structures, the titanium atoms have an octahedral coordination (MTiO6). The matrix self-assembly of the Na2Ti6O13 and Na2Ti3O7 (Na4Ti6O14) crystal structures from Na4M12 invariant precursors is modeled. These precursors are clusters consisting of twelve M polyhedra linked through the edges. It is demonstrated that the structurally rigid precursors Na4M12 control all processes of the subsequent evolution of the crystal-forming titanate clusters. The specific features of the self-assembly of the Na2Ti3O7 structure that result from the additional incorporation of twice the number of sodium atoms into the composition of the high-level clusters are considered.

Structural infection and anomalous phase sequences in complex oxides prepared from nanodispersed components

Abstract: The complex oxides prepared by solid-phase synthesis from nanoscopic components are studied using X-ray diffraction. It is demonstrated that the use of nanoscopic components in the solid-phase synthesis of lutetium borate LuBO3 and europium molybdate Eu2(MoO4)3 leads to anomalous sequences of phase transformations in these compounds: the vaterite ⇒ calcite ⇔ vaterite sequence for LuBO3 and the β ⇒ α ⇔ β sequence for Eu2(MoO4)3 are observed instead of the previously known sequences, namely, the calcite ⇔ vaterite sequence for LuBO3 and the α ⇔ β sequence for Eu2(MoO4)3. The revealed anomalous sequences do not depend on the procedure used for preparing reactants and are determined by the nanoscopic sizes of the initial components. It is found that microscopic additions of a number of simple oxides can suppress the kinetics of solid-phase synthesis of particular complex oxides and initiate the formation of new phases in the synthesis of other complex oxides (the so-called structural infection effect).