Structural features of 3d metal compounds with 1-hydroxyethylidenediphosphonic acid

Structural features of 3d metal complexes with anions of 1-hydroxyethylidenediphosphonic acid (HEDP, H₄ L), in which the M: HEDP ratios are equal to 1: 2, 1: 1, 3: 2, and 5: 2, are discussed. The Cu(II): HEDP = 1: 2 complexes are characterized by five types of structures: monomeric structures trans-[Cu(H_{4 ? n} L)₂(H₂O)₂]^{2 ? 2n} , cis-[Cu(H_{4 ? n} L)₂(H₂O)₂]^{2 ? 2n} , and [Cu(L)₂]^6?; the dimeric structure { [Cu(H₂ L)(H₂O)]₂(μ ₂-H₂ L)₂}^4? ; and the polymeric chain structure {[Cu(μ ₂-H₂ L)₂]^2?}_∞. Six coordination modes exhibited by HEDP in the Cu(II) compounds are described.     

Optical and chiro-optical properties of crystals with sillenite structure

The experimental data on the optical and chiro-optical properties of crystals with the sillenite structure Bi₁₂ M _xO_{20 ± δ} (M= elements of II–VIII groups) have been reviewed. The relations between the changes in the chemical composition, the crystal structure of Bi₁₂ M _xO_{20 ± δ} compounds, absorption, circular dichroism, and optical rotation spectra of sillenites have been established. A model taking into account the contribution of the electronic transitions of optically active tetrahedral [MO₄] ^n? complexes to the total optical rotation in sillenites is suggested. The data on the state (oxidation degree) and the recharging processes of 3d-elements in sillenites are systematized. The models of optical centers responsible for the photochromic effect in sillenites are critically analyzed.     

Structural conditionality of the piezoelectric properties of langasite family crystals

The atomic displacements upon isomorphic substitutions in crystals of the langasite family have been analyzed. The thermal parameters are determined and the probability density function of atoms is analyzed. Local potential energy minima are found which can be occupied by atoms under external effects. The contributions of cations in all four independent crystallographic positions and anions in all three such positions to the piezoelectric properties are established. One specific structural feature is the constant (at isomorphic substitutions) or possible (under external effects) but always opposite displacements of two cations along symmetry axis 2. Large cations in eight-vertex polyhedra make the main contribution to the piezoelectric properties. The cations in the tetrahedra on symmetry axis 2 weaken these properties. The cations in the octahedra in the origin of coordinates and in the tetrahedra on symmetry axes 3 only slightly affect the piezoelectricity.     

Structural features of hafnium iron phosphate glasses

Structural features and properties of a series of hafnium iron phosphate glasses have been investigated by Mössbauer spectroscopy and X-ray diffraction. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The isomer shift values obtained from the Mössbauer fits show that both Fe(II) and Fe(III) ions are in octahedral or distorted octahedral coordination. The crystalline HfP₂O₇ phase was detected in all the samples by powder X-ray diffraction but this did not degrade the chemical durability of the glasses as the dissolution rates of the glasses are comparable to that of base iron phosphate glass.     

Structural hierarchy and transformations in crystals of the perovskite family

The specific features of structural transformations in lanthanum manganite and lithium niobate are analyzed in terms of the macrosymmetry conservation principle discussed in the papers by L.A. Shuvalov. The possible mechanisms of phase transformations are proposed reasoning from a sequence of structural transitions, including the restoration of a virtual configuration of cubic perovskite with double the lattice parameter. The basic scheme of mutual transformations of two orthorhombic modifications is described for lanthanum manganite. A similar scheme proposed for lithium niobate is used to elucidate the nature of the mechanical twinning observed upon mutual switching of orientational domains.     

Some physical properties of boron sillenite: Bi24.5BO38.25

Large single crystals (≈1cm³) of the incongruently melting boron sillenite Bi_24.5BO_38.25 (space group I23 ) have been grown by the TSSG method with a high crystalline quality. Specimens from nearly inclusion‐free crystals were the basis for measurements of such physical properties like the dispersion of the refractive indices, the optical rotatory dispersion, elasticity as well as thermal expansion. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural energetics of the intermetallic compounds CaMg2 and CaAl2

The structures of the Laves phases CaMg₂ and CaAl₂ are investigated using the optimized model potential. Charge transfer, which is taken into account in an approximate and simple way, plays an important role in the determination of the stable crystal structures of both CaMg₂ and CaAl₂.     

Structural state of expanded graphite prepared from intercalation compounds

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.     

Structural features of Fab fragments of rheumatoid factor IgM-RF in solution

The structural features of the Fab fragments of monoclonal (Waldenström’s disease) immunoglobulin M (IgM) and rheumatoid immunoglobulin M (IgM-RF) were studied by a complex of methods, including small-angle X-ray scattering (SAXS), electron spin resonance (ESR), and mass spectrometry (MS). The Fab-RF fragment was demonstrated to be much more flexible in the region of interdomain contacts, the molecular weights and the shapes of the Fab and Fab-RF macromolecules in solution being only slightly different. According to the ESR data, the rotational correlation time for a spin label introduced into the peptide sequence for Fab is twice as large as that for Fab-RF (21±2 and 11±1 ns, respectively), whereas the molecular weights of these fragments differ by only 0.5% (mass-spectrometric data), which correlates with the results of molecular-shape modeling by small-angle X-ray scattering. The conclusion about the higher flexibility of the Fab-RF fragment contributes to an understanding of the specificity of interactions between the rheumatoid factor and the antigens of the own organism.     

Structural investigation of model compounds for an acceptor component of a new type of charge-transfer complexes based on viologen analogues. Characteristic features of the molecular and supramolecular structures

The crystal and molecular structures of six perchlorates (viologen analogues) are studied. These compounds serve as models of the acceptor component of new charge-transfer complexes containing bis(18-crown-6)stilbene as the donor. The polycyclic aromatic system of divalent cations is demonstrated to be virtually planar. In all cations, the side chains at the nitrogen atoms are oriented in opposite directions almost perpendicular to the plane of the cyclic system. This orientation of the spacers of these carbocations is indicative of their preorganization for the formation of 1: 2 charge-transfer complexes. Analysis of the crystal packings provides evidence that two positive charges on the conjugated systems of the organic cations and the perchlorate anions play a destructive role in the formation of stacking motifs. An increase in the size of the conjugated system and the involvement of an aromatic solvent molecule as an additional building block in a supramolecular system are favorable for the formation of a stacking supramolecular architecture.     

Structural features of low-dimensional molecular conductors—Representatives of new hybrid polyfunctional materials: Review

The crystal structures of the family of low-dimensional molecular conductors based on radical cation salts of different organic π donors with photochromic and magnetic metal complexes as anions have been considered. This class of supramolecular systems demonstrates a large variety of structural types and a wide range of transport properties. The specificity of the structure and properties of such hybrid materials is illustrated by several examples. The crystallochemical analysis of the conductors considered indicates the possibility of purposeful control of their transport properties via changing of the charge, sizes, shape, and symmetry of the anionic block components. The specificity of the crystal structure and properties of some organic conductors shows that such systems can be used as model systems in the study of new physical phenomena related to electron correlation and effects of charge ordering.     

Growth and optical characterization of cerium and lead‐doped Bi12TiO20 sillenite single crystals

Bi₁₂TiO₂₀ (BTO) single crystals doped with PbO and CeO₂ were grown by the Top Seeded Solution Growth (TSSG) technique from the liquid phase with nominal compositions of 10Bi₂O₃ : (1–x)TiO₂ : x PbO and 10Bi₂O₃ : (1–x)TiO₂ : xCeO₂ with x = 0.25 and 0.10. No growth‐related difficulties were encountered other than those typical of sillenite crystals. Samples with good optical quality were obtained and were characterized by optical absorption, dark current, spectral photocurrent dependence, optical activity and electro‐optic coefficient measurements. A comparison is made of the results of the optical measurements of doped and undoped BTO crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of phases of the sillenite family in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript> system

X-ray diffraction studies of sillenite Bi₂₄V₂O₄₀ single crystals grown by the hydrothermal method are performed for a separate crystal and powdered crystals. It is found that the composition of the two specimens is described by the (Bi_{24 − x} ▭ _x )[Bi _y ³⁺V_1−y ⁵⁺]₂ O₄₀ general formula with completely populated oxygen sites but differs in the content of vacancies at the bismuth site (this was established for the first time) and the Bi: V ratio at the tetrahedral site. The structural models of all the vanadium-containing sillenites reported in the literature are considered, and the possibility that Bi atoms are located at the centers of BiO₄ tetrahedra is established.     

Structural features and isomorphous substitutions in chromium- and magnesium-containing beryls and chromium-containing beryllian indialite grown by the flux method

Beryls and beryllian indialite {the general formula M ^VI ₂ T(2)^IV ₃ T(1)^IV ₆O₁₈} synthesized in magnesium-containing flux systems saturated with chromium are investigated using X-ray diffraction. The isovalent schemes of the isomorphous incorporation of chromium into Moctahedra of these compounds and the simultaneously realized heterovalent schemes with the participation of other components are revealed from the occupancies of the positions. It is demonstrated that an increase in the average bond lengths in the M positions leads predominantly to an increase in the parameter α. In the beryllian indialites, the T(1) tetrahedra are substantially closer to perfect tetrahedra, the T(2) tetrahedra are distorted to a lesser extent, and the M octahedra are distorted to a greater extent than those in beryls. The structural indications of the ability of compounds with a beryl structure to congruently melt are distinguished.     

Structural features of single crystals of LuB<Subscript>12</Subscript> upon a transition to the cage-glass phase

The unit-cell parameters of dodecaboride LuB₁₂, which undergoes a transition to the cage-glass phase, have been determined for the first time in the temperature range of 50–75 K by X-ray diffraction, and the single-crystal structure of this compound is established at 50 K. Nonlinear changes in the unit-cell parameters correspond to anomalies in the physical properties near the glass-transition temperature T* ~ 50–70 K. This compound has cubic symmetry at room temperature, and it is reduced to tetragonal symmetry at lower temperatures. Based on the X-ray diffraction data and relying on the physical properties of the crystals, the structure model, in which a small part (~15%) of Lu atoms are displaced from the 2a sites at the centers of the B₂₄ cuboctahedra to the 16n sites of sp. gr. I4/mmm, seems preferable.     

Synthesis and crystal growth of sillenite phases in the Bi2O3 ‐ TiO2 ‐ Nb2O5 system

The synthesis of Bi₂O₃‐Nb₂O₅ sillenite phase (BNbO) and the solubility of this phase with Bi₁₂TiO₂₀ was investigated by solid‐state reaction synthesis and niobium doped Bi₁₂TiO₂₀ (BTO:Nb) crystals were grown by the Top Seeded Solution Growth (TSSG) technique. The structures of polycrystalline compounds were checked by X‐ray powder diffraction method at room temperature. The correct composition of the sillenite phase stabilized with niobium was determined as Bi₁₂[Nb_0.17Bi_0.83]O_19.7 (BNbO) with unit cell parameter a = 10.261(2) Å. The system BTO‐BNbO is poorly soluble, but niobium doped BTO crystals were grown from the liquid composition 10Bi₂O₃ : xTiO₂ : (1‐x)/2 Nb₂O₅, with x = 0.95 and 0.90. A niobium concentration limit in the liquid phase is established in order to grow BTO:Nb with good crystalline quality. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural chemistry of peroxo compounds of group VI transition metals: I. Peroxo complexes of chromium (a review)

The specific features revealed in the structure of the d ³ Cr(III), d ² Cr(IV), d ¹Cr(V), and d ⁰ Cr(VI) peroxo complexes with the ratios M:O₂ = 1:1, 1:2, and 1:4 are considered. It is noted that, in eleven compounds of the general formula Cr(O₂)_nO_m A _p (n = 1, 2, 4; m = 0, 1; p = 0–4), the metal atoms can be in four oxidations states: +3 (d ³), +4 (d ⁴), +5 (d ¹), and +6 (d ⁰). This property distinguishes chromium peroxo compounds from molybdenum and tungsten dioxygen complexes, which, with one exception, are represented by the d ⁰ M(VI) compounds.     

Structural chemistry of peroxo compounds of group VI transition metals: II. Peroxo complexes of molybdenum and tungsten: A review

The specific features revealed in the structure of the molybdenum and tungsten peroxo complexes with the ratios M: O₂ = 1: 1, 1: 2, and 1: 4 are considered. It is demonstrated that the geometry of the coordination polyhedron of the metal atom is primarily determined by the “metal: peroxo ligand” ratio. Formally, the pentagonal bipyramidal coordination polyhedra of the Mo(VI) and W(VI) oxo monoperoxo and oxo diperoxo complexes (the coordination numbers of the metal atoms are equal to seven) have different geometries, namely, the MO(O₂)A ₄ pseudooctahedral and MO(O₂)₂ A ₂ pseudotrigonal bipyramidal configurations.     

Structural features of epitaxial NiFe2O4 thin films grown on different substrates by direct liquid injection chemical vapor deposition

NiFe₂O₄ (NFO) thin films are grown on four different substrates, i.e., Lead Zinc Niobate–Lead Titanate (PZN–PT), Lead Magnesium Niobate–Lead Titanate (PMN–PT), MgAl₂O₄ (MAO) and SrTiO₃ (STO), by a direct liquid injection chemical vapor deposition technique (DLI-CVD) under optimum growth conditions where relatively high growth rate (～20 nm/min), smooth surface morphology and high saturation magnetization values in the range of 260–290 emu/ cm³ are obtained. The NFO films with correct stoichiometry (Ni:Fe=1:2) grow epitaxially on all four substrates, as confirmed by energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. While the films on PMN–PT and PZN–PT substrates are partially strained, essentially complete strain relaxation occurs for films grown on MAO and STO. The formations of threading dislocations along with dark diffused contrast areas related to antiphase domains having a different cation ordering are observed on all four substrates. These crystal defects are correlated with lattice mismatch between the film and substrate and result in changes in magnetic properties of the films. Atomic resolution HAADF imaging and EDX line profiles show formation of a sharp interface between the film and the substrate with no inter-diffusion of Pb or other elements across the interface. Antiphase domains are observed to originate at the film–substrate interface.