Showing papers in "Crystallography Reports in 2000"
TL;DR: In this article, the crystal structures of two forms of the enzyme dimanganese catalase from Thermus Thermophilus (native and inhibited by chloride) were studied by X-ray diffraction analysis at 1.05 and 0.98 A resolution, respectively.
Abstract: The crystal structures of two forms of the enzyme dimanganese catalase from Thermus Thermophilus (native and inhibited by chloride) were studied by X-ray diffraction analysis at 1.05 and 0.98 A resolution, respectively. The atomic models of the molecules were refined to the R factors 9.8 and 10%, respectively. The three-dimensional molecular structures are characterized in detail. The analysis of electron-density distributions in the active centers of the native and inhibited enzyme forms revealed that the most flexible side chains of the amino acid residues Lys162 and Glu36 exist in two interrelated conformations. This allowed us to obtain the structural data necessary for understanding the mechanism of enzymatic activity of the dimanganese catalase.
131 citations
TL;DR: In this paper, the Ruddlesden-Popper phase was analyzed in terms of crystal chemistry and the geometrical boundaries of the existence ranges of these compounds with multilayer stacks were determined and new phases of this type were predicted.
Abstract: The layer perovskite-like structures of the so-called Ruddlesden-Popper phases have been analyzed in terms of crystal chemistry. The geometrical boundaries of the existence ranges of these compounds with multilayer stacks are determined and new phases of this type are predicted. The possible variants of related structures are also considered.
123 citations
TL;DR: In this article, the Rietveld method was used to study double calcium and alkali metal phosphates described by the general formula Ca10M(PO4)7(M = Li, Na, K) and the lattice parameters were a = 10.4203(1) and c = 37.279(2) A (for M = K); sp. gr. R3c, Z = 6.
Abstract: Crystal structures of double calcium and alkali metal phosphates described by the general formula Ca10
M(PO4)7(M = Li, Na, K) have been studied by the Rietveld method. The lattice parameters are a = 10.4203(1) and c = 37.389(1) A (for M = Li), a = 10.4391(1) and c = 37.310(1) A (for M = Na), and a = 10.4229(1) and c = 37.279(2) A (for M = K); sp. gr. R3c, Z = 6. The specific features of the distribution of alkali metal cations over the structure positions are discussed.
61 citations
TL;DR: In this article, the effect of an applied electric field on cholesteric structures is studied, and the bistability of the cholesterics-nematic transition, electric field-induced color textures of cholesterol mixtures, and electro-optics of amorphous cholesterolic structures are considered.
Abstract: The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.
28 citations
TL;DR: In this article, the authors studied the crystal structures of the compounds Ca9R(VO4)7 (R = Tb (I), Dy (II), Ho (III), and Y (IV).
Abstract: Crystal structures of the compounds Ca9R(VO4)7 (R = Tb (I), Dy (II), Ho (III), and Y (IV) have been studied by the method of the full-profile analysis. All the compounds are crystallized in the trigonal system (sp. gr. R3c, Z = 6) with the unit-cell parameters (I) a = 10.8592(1), c = 38.035(1), V = 3884.2(2) A3; (II) a = 10.8564(1), c = 38.009(1) A, V = 3879.6(2) A3, (III) a = 10.8565(1) and c = 37.995(1) A, V = 3878.3(2) A3, and (IV) a = 10.8588(1), c = 37.995(1) A, V = 3879.9(2) A3. In structures I–IV, rare earth and calcium cations occupy three positions—M(1), M(2), and M(5). Rare earth cations occupy the R3+ positions almost in the same way: 2.7–2.6(2) cations in the M(1) position; 2.7–2.3(2) cations in the M(2) position, and 0.6–1.0(1) cation in the M(5) position. At the same time, the occupancy of the M(5) position regularly increases with a decrease of the R3+ radius.
28 citations
27 citations
TL;DR: In this article, the "indicative" surfaces of the constants of the piezooptic effect and their stereographic projections have been constructed with due regard for crystal elasticity for lithium niobate crystals described by the symmetry class 3m.
Abstract: For the first time, the “indicative” surfaces of the constants of the piezooptic effect and their stereographic projections have been constructed with due regard for crystal elasticity for lithium niobate crystals described by the symmetry class 3m. The comparative analysis of these surfaces and the corresponding surfaces of the true piezooptic effect has been performed.
25 citations
TL;DR: In this article, the elastic, piezoelectric, and dielectric properties of α-GeO2 crystals were studied at room temperature by the resonance method on crystals grown on α-quartz seeds from solutions in the recirculating mode.
Abstract: The elastic, piezoelectric, and dielectric characteristics of α-GeO2 crystals are studied at room temperature. The measurements were performed by the resonance method on crystals grown on α-quartz seeds from solutions in the recirculating mode. In germanium dioxide crystals, the piezoelectric moduli have high values. The characteristic features of variations of the electromechanical and elastic properties in crystals with the quartzlike structure observed earlier are confirmed.
20 citations
TL;DR: In this article, the results of the study of the potassium laurate-1-decanol-water lyotropic composition are pre- sented, and the procedure of preparation of micellar solutions provides the good reproducibility of the phase dia- grams.
Abstract: The results of the study of the potassium laurate-1-decanol-water lyotropic composition are pre- sented. The procedure of preparation of micellar solutions provides the good reproducibility of the phase dia- grams. The configurations of the domains of the liquid crystal order are constructed on the concentration plane at the constant temperature as well as the temperature sections ( T , C 1-decanol ) (at the constant ratio of potassium laurate to water) and ( T , C water ) (at the constant ratio of potassium laurate to 1-decanol). The liquid-crystal phases observed are described. © 2000 MAIK "Nauka/Interperiodica".
20 citations
TL;DR: In this article, the crystal structures of Ca9R(VO4)7 (R = Nd (I), Sm (II), or Gd (III)) were studied by the Rietveld method.
Abstract: The crystal structures of Ca9R(VO4)7 (R = Nd (I), Sm (II), or Gd (III)) were studied by the Rietveld method. The compounds are isostructural to Ca3(VO4)2 and are crystallized in the trigonal system (sp. gr. R3c, Z = 6). The unit-cell parameters are as follows: for I, a = 10.8720(5) A, c = 38.121(1) A; for II, a = 10.8652(5) A, c = 38.098(1) A; and for III, a = 10.8631(5) A, c = 38.072(1) A. In the structures of I and II, the M(1), M(2), and M(3) positions are statistically occupied by the rare-earth cations and calcium anions. In the structure of III, the Gd3+ cations occupy the M(1) and M(2) positions. The distributions of the R3+ cations over the positions are characteristic of each structure. The composition of the cerium-ontaining compound Ca9.81Ce0.42(VO4)7 (a = 10.8552(5) A, c = 38.037(1) A) was refined and its crystal structure was solved from the X-ray powder data. In this compound, cerium atoms are in the oxidation states +3 and +4.
19 citations
TL;DR: In this article, the phase diagram of the ternary Li2O-CuO-B2O3 system was used to grow large high-quality single crystals of copper metaborate.
Abstract: Large high-quality single crystals of copper metaborate are grown on the basis of the phase diagram of the ternary Li2O-CuO-B2O3 system. Bright blue crystals with a volume of about 1 cm3 were grown by the method of spontaneous crystallization while slowly cooling the melt. The magnetic susceptibility and electron spin resonance were measured. It is shown that the effective magnetic moment of a Cu2+ ion is equal to 1.6 μB and the g-factor, to 2.170 and 2.133 for the magnetic field oriented parallel and perpendicular to the fourfold axis, respectively. At 21 and 10 K, sharp anomalies of magnetic susceptibility are observed.
TL;DR: In this article, it was shown that most of the structural types are determined by a small set of planar nets densely filled with the heaviest atoms of the structure, which are combined into atomic sublattices (matrices of the structures) by the operations of pseudo symmetry, thus reducing the number of degrees of freedom in the structure.
Abstract: On the basis of an original interpretation of the crystal state, it is shown that most of the structural types are determined by a small set of planar nets densely filled with the heaviest atoms of the structure. These nets are combined into atomic sublattices (matrices of the structure types) by the operations of (pseudo) symmetry, thus reducing the number of the degrees of freedom in the structure, which, in turn, provides structure stability. It is shown on examples of heavy-cation oxides, fluorides, and sulfides that the number of the types of such matrices (F-, I-, etc.) is limited. Each type is characterized by a certain structure-determining complex of mutually intersecting crystallographic planes parallel to the most densely filled atomic nets. Most of the atomic nets considered in crystal chemistry are, in fact, various sections of the sublattices-matrices formed by a limited number of densely filled atomic nets.
TL;DR: The crystal structures of K[M(Dtpa)] ⋅ 3H2O (M = Zr or Hf) and NH4[Sn(dtpa) as discussed by the authors were studied by X-ray diffraction and the coordination number of the metal atom in all the compounds is eight.
Abstract: The crystal structures of K[M(Dtpa)] ⋅ 3H2O (M = Zr or Hf) and NH4[Sn(Dtpa)] ⋅ H2O are studied by X-ray diffraction. The coordination number of the metal atom in all the compounds is eight. The NH4[Sn(Dtpa)] ⋅ H2O complex is isostructural to the (H5O2)[M(Dtpa)] ⋅ H2O (M = Sn or Hf) compounds studied earlier.
TL;DR: A series of potassium titanyl phosphate crystals, KTiOPO4, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied as discussed by the authors.
Abstract: A series of potassium titanyl phosphate crystals, KTiOPO4, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas T
C decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K2TiNb2P2O3 is formed.
TL;DR: In this article, a new orthorhombic polar variety of Pb-hilgardite containing Na+ and HOCl2− ions in the cavities of the framework has been prepared by hydrothermal synthesis.
Abstract: A new orthorhombic polar variety of Pb-hilgardite containing Na+ and HOCl2− ions in the cavities of the framework has been prepared by hydrothermal synthesis. A correlation between the structural features and nonlinear-optical properties of the crystals is discussed. Similar to the hexaborate group, pentaborates with different degrees of condensation of the boron—oxygen radical are characterized by close structural similarity, which is described within the OD theory with distinguishing structural units (blocks or modules) of different dimensionality—bricks, rods, and layers. Consideration of the variants of their connection allows us to describe the diversity of the pentaborate family including hypothetical structures.
TL;DR: In this article, the crystal structure of the synthetic Ge-modification of the mineral natisite, Na2{ TiOGeO4}, has been refined by X-ray diffraction method (a four-circle diffractometer, 2θ/θ scanning, MoK α-radiation, θmax = 50°: a = 6.658(1), c = 5.161(1) A, sp. gr. P4/nmm, Z = 2, ρcalcd = 3.58 g/cm3, R1
Abstract: The crystal structure of the synthetic Ge-modification of the mineral natisite, Na2{ TiOGeO4}, has been refined by X-ray diffraction method (a four-circle diffractometer, 2θ/θ scanning, MoK α-radiation, θmax = 50°: a = 6.658(1), c = 5.161(1) A, sp. gr. P4/nmm, Z = 2, ρcalcd = 3.58 g/cm3, R1 = 0.030, s = 1.131, wR(F 2) = 0.058 (352 reflections with I≥2σI). The comparative crystallochemical analysis of the related (including hypothetical) phases with the anionic {MTO 5} radicals (M = Ti, V, Ge; T = Ge, Si, P) is performed with the aim of revealing a possible relationship between the composition and the structure type of the compounds.
TL;DR: In this article, the valence states and coordination of titanium ions in beryl crystals grown from flux and by gastransport reactions have been studied by the methods of electron paramagnetic resonance (EPR) and optical spectroscopy.
Abstract: The valence states and coordination of titanium ions in beryl crystals grown from flux and by gastransport reactions have been studied by the methods of electron paramagnetic resonance (EPR) and optical spectroscopy. The parameters of the EPR spectra for Ti3+ in the Al3+ and Si4+ sites are determined. It is found that at concentrations exceeding 0.1–0.15 wt %, the EPR-spectra have additional lines due to exchange-bonded pairs of Ti3+ ions. It is shown that the two-humped absorption band with the maxima at 495 (π), 545 (π), and 495 (π) nm are formed due to Ti3+ ions located in the octahedral position.
TL;DR: In this paper, the crystal structure of α-RbB3O5 was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms.
Abstract: The crystal structure of α-RbB3O5 was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms to R
p
= 2.93, R
wp
= 3.80, R
B
= 2.53, R
F
= 2.84, and s = 1.54. The compound is isostructural to CsB3O5: it is orthorhombic, sp. gr. P212121, a = 8.209(1), b = 10.092(1), c = 5.382(1) A, and V = 445.9 A 3. The framework structure is formed by the boron-oxygen [B
2
III
BIVO5] − rings consisting of two [BO3]-triangles and a [BO4]-tetrahedron. The rings are linked to form systems of helical chains running along the twofold screw axes parallel 21 to the a-and b-axes and infinite channels parallel to the a-and c-axes, which accommodate Rb atoms. The data were collected on an ADP-2 diffractometer [CuK
α radiation, Ni-filter, 12.00° < 2θ < 110.00°, a step in 2θ equal to 0.02°, count time 8 s per step, and 711 reflections α1 + α2)]. All the calculations were performed using version 3.3 of the WYRIET program. The comparison of the structures of α-and β-RbB3O5 and CsB3O5 revealed that the type of deformations in the framework structures of alkali-metal borates due to the changes of the temperature or the substitution of cations is determined by the role played by metal atoms, and especially, by large and heavy ions.
TL;DR: The crystal structure of a new representative of the eudialyte family was studied in this article, which is characterized by a low calcium content and by ordering the elements that isomorphically replace calcium, which lowers the symmetry from R3m to R3.
Abstract: The crystal structure of a new representative of the eudialyte family was studied. This mineral is characterized by a low calcium content and by ordering the elements that isomorphically replace calcium, which lowers the symmetry from R3m to R3. The structure of the sample under study consists of the six-membered rings with two octahedra of substantially different volumes, one occupied mainly by manganese and the other, mainly by sodium and characterized by the average distances of 2.22 and 2.45 A, respectively. The formation of such octahedra is the most characteristic structural feature of the third low-symmetry mineral of the eudialyte family.
TL;DR: In this paper, the propagation of electromagnetic waves in stratified bianisotropic chiral structures is described by the 4 × 4 matrix method and the amplitude and the polarization characteristics (intensity, polarization azimuth, and ellipticity) of reflected and transmitted electromagnetic waves are studied as functions of the angle of the wave incidence onto the structure.
Abstract: Propagation of electromagnetic waves in stratified bianisotropic chiral structures is described by the 4 × 4 matrix method. At arbitrary layer parameters, the amplitude and the polarization characteristics (intensity, polarization azimuth, and ellipticity) of reflected and transmitted electromagnetic waves are studied as functions of the angle of the wave incidence onto the structure.
TL;DR: The crystal structure of the cubic modification of the natural mineral loparite has been studied for the first time by the methods of the X-ray diffraction analysis (γMoKα radiation, 105 independent reflections with I > 3σ(I), R = 0.041 in the anisotropic approximation) as discussed by the authors.
Abstract: The crystal structure of the cubic modification of the natural mineral loparite has been studied for the first time by the methods of the X-ray diffraction analysis (γMoK
α radiation, 105 independent reflections with I > 3σ(I), R = 0.041 in the anisotropic approximation). The structure belongs to the perovskite type (ABO
3) with the double period of the cubic unit cell, a = 7.767(1) A (sp. gr. Pn3m; Z = 2 for the composition (Ca,Na,Ce)(Na,Ce)3(Ti,Nb)2Ti2O12. Period doubling is explained by ordering of cations both in the A and the B positions.
TL;DR: In this article, an X-ray diffraction study of a new representative of the eudialyte family (R = 4.5, 2347 independent reflections) provided the establishment of a number of characteristic features of its structure associated with a high zirconium content and some other characteristics of the composition.
Abstract: X-ray diffraction study of a new representative of the eudialyte family (R = 4.5%, 2347 independent reflections) provided the establishment of a number of characteristic features of its structure associated with a high zirconium content and some other characteristics of the composition. The deficiency of Ca is compensated with Mn, Fe, rare-earth elements, Sr, and Na. The excessive Zr atoms are located in octahedra participating in the formation of a new type of a TiZr3 cluster.
TL;DR: The crystal structure of strontium hilgardite, CaSr[B5O9]Cl ⋅ H2O, was established by the method of X-ray diffraction analysis.
Abstract: The crystal structure of strontium hilgardite, CaSr[B5O9]Cl ⋅ H2O, was established by the method of X-ray diffraction analysis (synchrotron radiation; diffractometer equipped with a position-sensitive detector; λ = 0.688 A; 3191 reflections with F < 4σ(F); R = 0.045 in the anisotropic approximation). The triclinic unit-cell parameters are as follows: a = 6.5732(6) A, b = 6.4445(6) A, c = 6.3693(6) A, α = 60.995(2)°, β = 61.257(2)°, γ = 77.191(2)°, sp. gr. P1; Z = 1. The Ca and Sr atoms were found to be disordered over two positions. The structures of strontium hilgardite and hilgardite-1A differ in the configurations of the seven-vertex Sr-and Ca(2)-polyhedra. The structure solved in this work is consistent with two series of borates studied previously. One of these groups involves pentaborates with different degrees of hydration of borate complexes, and the second group includes Sr-containing borates.
TL;DR: The concept of claggregation in chlorine-containing organic compounds and the principles of their classification are briefly discussed in this article. And the method for singling out claggregates in substances with high chlorine content is considered, using the examples of hexachlorobenzene and pentachloroberenzene.
Abstract: The concept of Cl-aggregation in chlorine-containing organic compounds and the principles of their classification are briefly discussed. The method for singling out Cl-aggregates in substances with high chlorine content is considered, using the examples of hexachlorobenzene and pentachlorobenzene. It is established that Cl-aggregates in these compounds are three dimensional (i
3-aggregates) and contain subaggregates of lower dimensionalities. The results obtained agree with the earlier assumptions about an important role of the chlorophobic effect in Cl-aggregation.
TL;DR: In this article, it was shown that hydroxyapatite grows from aqueous solutions in the form of very thin (with the thickness of the order of several lattice parameters) plate-like crystals.
Abstract: Hydroxyapatite crystals grown from aqueous solutions have been studied by the methods of high-resolution electron microscopy and transmission electron diffraction. Processing of the experimental electron micrographs with the use of the Digital Micrograph program and the study of the corresponding Fourier trans-forms showed that the submicron microcrystals grow mainly along the [0001] direction. The (0001) and the
$$(01\bar 10)$$
planes are perpendicular and parallel to the long edge of the crystals, respectively. The good accord between the experimental electron-microscopy images and the electron microscopy images calculated by the EMS program was attained only for crystals with the thicknesses ranging from one to five lattice periods. This allows us to state that hydroxyapatite grows from aqueous solutions in the form of very thin (with the thickness of the order of several lattice parameters) platelike crystals.
TL;DR: Possible types of orientational relationships for hexagonal and cubic gallium nitride layers on aluminum-magnesium spinel substrates with various crystallographic orientations have been determined as a result of the theoretical and experimental studies of this system.
Abstract: Possible types of orientational relationships for hexagonal and cubic gallium nitride layers on aluminum-magnesium spinel substrates with various crystallographic orientations have been determined as a result of the theoretical and the experimental studies of this system. Most of the experimental data and the data obtained in the present study are consistent with the theoretical considerations. The causes of some apparent inconsistencies are discussed.
TL;DR: In this paper, the linear and the circular bulk photovoltaic and photorefractive effects in a piezoelectric La3Ga5SiO14:Fe crystal have been studied.
Abstract: The linear and the circular bulk photovoltaic and photorefractive effects in a piezoelectric La3Ga5SiO14: Fe crystal have been studied. It is shown that the photorefractive effect is induced by generation of the photovoltaic voltage.
TL;DR: The data obtained from the analysis of the solubility phase diagrams have been used in growth of K2Ni(SO4)2 ⋅ 6H2O crystals as discussed by the authors.
Abstract: The data obtained from the analysis of the solubility phase diagrams have been used in growth of K2Ni(SO4)2 ⋅ 6H2O crystals. The X-ray diffraction studies of the crystals grown showed that, at room temperature, the crystals belong to the diffraction class 2/m. The crystal structure is similar to the structures of the Tutton salts. The study of the absorption spectra and the differential scanning calorimetry (DSC) revealed a pronounced anomaly in the vicinity of 45–46°C, which seems to be associated with the structural changes in the crystal.
TL;DR: The crystal structure of the new mineral urusovite Cu[AlAsO5] has been determined by direct methods and refined to R = 0.048 (wR= 0.103) as discussed by the authors.
Abstract: The crystal structure of the new mineral urusovite Cu[AlAsO5] has been determined (monoclinic system, sp. gr. P21/c, a = 7.335(1) A, b = 10.255(1) A, c = 5.599(1) A, β = 99.79(1)°, V = 415.0(1) A3, Z = 4). The structure is solved by direct methods and refined to R = 0.048 (wR = 0.103). The structure is built by open-branched {uB, ∞
2
} [AlAsO5]2− layers parallel to the (100) plane. The layers consist of two-link chains of ∞
1
[Al2O6] tetrahedra, which are parallel to the z-axis and are connected through the [AsO4]-groups. The sharing-edges [CuO5] tetragonal pyramids link the aluminoarsenate layers to form a three-dimensional framework. The established layer is the second example of an tetrahedral aluminoarsenate polyanion.