Showing papers on "Orthorhombic crystal system published in 1997"

Anisotropic parameters and P-wave velocity for orthorhombic media

Abstract: Although orthorhombic (or orthotropic) symmetry is believed to be common for fractured reservoirs, the difficulties in dealing with nine independent elastic constants have precluded this model from being used in seismology. A notation introduced in this work is designed to help make seismic inversion and processing for orthorhombic media more practical by simplifying the description of a wide range of seismic signatures. Taking advantage of the fact that the Christoffel equation has the same form in the symmetry planes of orthorhombic and transversely isotropic (TI) media, we can replace the stiffness coefficients by two vertical (P and S) velocities and seven dimensionless parameters that represent an extension of Thomsen's anisotropy coefficients to orthorhombic models. By design, this notation provides a uniform description of anisotropic media with both orthorhombic and TI symmetry. The dimensionless anisotropic parameters introduced here preserve all attractive features of Thomsen notation in treatin...

Orthorhombic media: Modeling elastic wave behavior in a vertically fractured earth

Abstract: Vertical fractures and horizontal fine layering combine to form a long-wavelength equivalent orthorhombic medium. Such media constitute a subset of the set of all orthorhombic media. Orthorhombic elastic symmetry is the lowest symmetry for which the slowness surface (the solution of the Christoffel equation) is bicubic rather than sextic. Various properties of orthorhombic media, such as the number and location of conical points and longitudinal directions, may be derived from the slowness surface or, because of its bicubic character, the squared slowness surface, which is a cubic surface. From the occurrence and angular orientation of some of these distinctive features, conclusions can be drawn with respect to the properties of the medium and to the parameters of the assumed underlying causes of the anisotropy. The estimation of these more subtle properties gains greater importance with the proliferation of multiazimuthal seismic surveys and the ability to drill along ever-more complicated 3-D well trajectories.

Compressibility, Phase Transitions, and Oxygen Migration in Zirconium Tungstate, ZrW2O8

Abstract: In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW2O8) undergoes a quenchable phase transition to an orthorhombic phase, the structure of which has been solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the migration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms can be explained in terms of the “cross-bracing” between polyhedra that occurs as a result of the phase transition.

Molecular and Crystal Structure of Hydrated Chitosan

Abstract: The molecular and crystal structure of the hydrated form of chitosan, which was obtained by deacetylation of chitin from crab tendon, was determined by the X-ray fiber diffraction method and the linked-atom least-squares method. The chitosan chains crystallize in an orthorhombic unit cell with dimensions a = 8.95(4), b = 16.97(6), c (fiber axis) = 10.34(4) A and a space group P212121. The chain conformation is a 2-fold helix stabilized by O3---O5 hydrogen bond with the gt orientation of O6. The unit cell contains four chains and eight water molecules. There are direct hydrogen bonds (N2---O6) between adjacent chains along the b-axis, which makes a sheet structure parallel to the bc-plane. These sheets stack along the a-axis. Each sheet is related to its neighboring sheet by 21-symmetry along the b-axis. Water molecules form columns between these sheets and contribute to stabilize the structure by making water-bridges between polymer chains.

Structure and magnetic order in undoped lanthanum manganite

Abstract: The undoped lanthanum manganite system of nominal composition ${\mathrm{LaMnO}}_{3}$ has been analyzed by neutron powder diffraction for different sample heat treatment methods. Four distinct crystallographic phases have been identified: (i) an orthorhombic phase of space group Pnma and lattice parameters (at 300 K) a=5.7385(3), b=7.7024(3), c=5.5378(2) \AA{}, produced by annealing in a reducing atmosphere. The system develops long-range antiferromagnetic order below ${\mathrm{T}}_{\mathrm{N}}$=140 K with the ${\mathrm{Mn}}^{3+}$ spins coupled ferromagnetically in the a-c plane and antiferromagnetically along b, with the spin direction along a. The volume of this phase increases monotonically with increasing temperature, but both the a and c lattice parameters exhibit negative thermal expansion in alternate temperature regimes. (ii) A second (previously unreported) orthorhombic phase that exhibits a smaller splitting, also of space group Pnma and lattice parameters (at 300 K) a=5.4954(3), b=7.7854(4), c=5.5355(3) \AA{}, produced by annealing in an oxygen (or air) atmosphere. This system orders with a simple ferromagnetic structure at ${\mathrm{T}}_{\mathrm{c}}$=140 K, with the spin direction along c. Phases (i) and (ii) can be transformed reversibly by suitable heat treatment of the same sample, and exist with a range of lattice parameters and compositions. The unit-cell volume for the antiferromagnetic phase is considerably larger than for the ferromagnetic phase, which agrees with the double-exchange model proposed for this system. (iii) A monoclinic phase of space group ${\mathrm{P}112}_{1}$/a and lattice parameters (at 200 K) a=5.4660(4), b=7.7616(7), c=5.5241(5) \AA{}, \ensuremath{\gamma}=90.909\ifmmode^\circ\else\textdegree\fi{}(5) that orders ferromagnetically below 140 K. (iv) A rhombohedral phase of space group R3-barc and hexagonal lattice parameters (at 300 K) a=5.5259(2), c=13.3240(4) \AA{}, that is observed only above room temperature. Occupancy refinements show that phase (i) ideally has the stoichiometric composition ${\mathrm{LaMnO}}_{3}$, while the results for the Mn-O bond distances suggest that phases (ii), (iii), and (iv) are progressively richer in oxygen (and thus ${\mathrm{Mn}}^{4+}$). The results of our study strongly suggest the progressive development of cation vacancies in equal numbers on the La and Mn sites as the oxygen content is increased by heat treatment. In the monoclinic phase the Mn ions occupy two crystallographically independent sites, but no evidence of ordering of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ was observed. The structures of the four phases are closely related to that of perovskite. The ${\mathrm{MnO}}_{6}$ octahedra are tilted from the undistorted configuration, the tilt system being ${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$ in the rhombohedral structure and ${\mathrm{a}}^{+}$${\mathrm{b}}^{\mathrm{\ensuremath{-}}}$${\mathrm{b}}^{\mathrm{\ensuremath{-}}}$ in both orthorhombic modifications.

Cation disorder in ferroelectric Aurivillius phases of the typeBi2ANb2O9(A=Ba, Sr, Ca)

Abstract: The structures of the ferroelectric two-layer Aurivillius phases Bi 2 ANb 2 O 9 (A=Ba, Sr, Ca) have been refined using a combination of X-ray and neutron powder diffraction data. Bi 2 BaNb 2 O 9 is not significantly distorted from idealised symmetry and has been refined in tetragonal space group I4/mmm, a=3.9362(1) and c=25.6582(7) A. The Sr and Ca compounds have been refined in orthorhombic space group A2 1 am, with a=5.5193(3), b=5.5148(3), c=25.0857(6) A and a=5.4833(1), b=5.4423(1), c=24.8984(6) A, respectively. The orthorhombic distortion increases with decreasing A 2+ cation size and originates from bonding requirements at the perovskite A site, in agreement with previous work. However, in contrast to earlier work, we find a partial mixing of Bi and A cations on their respective sites, which increases in the order Ca

Temperature dependence of elastic, dielectric, and piezoelectric properties of “single crystalline’’ films of vinylidene fluoride trifluoroethylene copolymer

Abstract: Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

Two-dimensional melting of an anisotropic crystal observed at the molecular level

Abstract: A distinctive two-dimensional (2D) melting transition occurring at nearly 100 degrees Celsius ( degrees C) has been observed in Langmuir-Blodgett films by in situ atomic force microscopy (AFM). A 2D orthorhombic crystal phase melted to a 2D smectic phase at about 91 degrees C. The smectic phase was characterized by 1D molecular periodicity with short-range correlations (about 40 angstroms). At 95 degrees C, the smectic order melted to form a hexatic phase. Infrared spectroscopy measurements were consistent with the AFM observations. These observations support the dislocation-mediated melting scenario for an anisotropic 2D crystal predicted by Ostlund and Halperin. A longer wavelength height modulation was also observed in the smectic and hexatic phases.

The Orthorhombic Structure of Iron: An in Situ Study at High-Temperature and High-Pressure

Abstract: An in situ angle-dispersive x-ray diffraction study was undertaken of iron in a laser-heated, diamond-anvil cell up to 2375 kelvin and between 30 and 100 gigapascals in Al2O3- and SiO2-pressure media. The resolution and reliability of diffraction peak intensities allow quantitative assessment of a structural model. The results confirm that iron undergoes a phase transformation at high pressures and temperatures. The space group isPbcm for an orthorhombic lattice, and the atomic topology is close to that of ɛ hexagonal close-packed iron.

A novel class of phenol–pyridine co-crystals for secondharmonic generation

Abstract: To test the approach of combining both ionic and hydrogen-bonding interactions for the design of non-linear optical (NLO) materials, a number of phenol–pyridine co-crystals have been synthesized and their NLO properties investigated. The co-crystals are characterized by second harmonic generation measurements as well as the more conventional methods of melting point measurements, infrared and nuclear magnetic resonance spectroscopy. To investigate whether the phenol co-crystals are organic salts, the 2-methoxy-4-nitrophenol–4-(dimethylamino)pyridine (2:1) co-crystal 6 and the 2-methoxy-4-nitrophenol–4-pyrrolidinylpyridine–water (1:1:1) co-crystal 8 are further characterized by X-ray single-crystal diffraction. Crystal structure analyses reveal that both 6 and 8 are ionic co-crystals (or organic salts) composed of a phenoxide anion, a pyridinium cation and a neutral molecule. In the two co-crystals, the phenoxide, pyridinium and neutral molecules are held together by ionic attractions as well as hydrogen-bonding interactions. Both 6 and 8 crystallize in non-centrosymmetric structures [Pna2 1 (orthorhombic), a=6.880(4), b=38.40(1), c=8.454(3) A, Z=4, D c =1.369 g cm -3 and R=0.051 for 6 and Cc (monoclinic), a=7.302(3), b=23.518(2), c=9.940(1) A, β=107.12(2)°, Z=4, D c =1.365 g cm -3 and R=0.036 for 8]. In addition to X-ray structure determination, it is possible to predict whether phenol–pyridine co-crystals are organic salts based on the ΔpK a [pK a (pyridine)-pK a (phenol)] and stoichiometric ratio of the co-crystals. Preliminary results suggest that this type of co-crystals, particularly for the ionic co-crystals, may have a higher chance of forming non-centrosymmetric structures than the normal achiral organic compounds.

Crystal structure of κ-alumina: an X-ray powder diffraction,TEM and NMR study

Abstract: The crystal structure of κ-alumina (κ-Al 2 O 3 ) has been determined ab initio from an X-ray powder diffraction pattern (reliability factors: R Bragg =0.046, R p =0.090, R wp =0.115, χ 2 =11.7). The acentric structure (orthorhombic system, space group Pna2 1 , a=4.8437(2) A, b=8.3300(3) A, c=8.9547(4) A, Z=8) is built up from a pseudo-close-packed stacking ABAC of oxygen atoms, with aluminium in octahedral and tetrahedral environments in a 3:1 ratio, which form zigzag ribbons of edge-sharing octahedra and corner-sharing tetrahedra. κ-Alumina was characterized using magic angle spinning (MAS) 27 Al NMR at two fields and multiple quantum magic angle spinning (3Q MQ MAS) 27 Al NMR. A high-resolution electron microscopy study confirmed the structure and showed the presence of two types of defects: antiphase boundaries and 120 ° disorientations. A model is proposed for these two types of defects, which leaves unchanged the pseudo-close-packed arrangement of the oxygen atoms and assumes a shift or 120 ° twinning of aluminium ions.

New Inorganic Spin-Peierls Compound NaV 2O 5 Evidenced by X-Ray and Neutron Scattering

Abstract: In this letter we report lattice dimerization and spin-gap formation of NaV 2 O 5 below its spin-Peierls transition temperature T sp =35.3 K, in which there are V 4+ ( S =1/2) linear chains along the b axis of its orthorhombic structure. A single-crystal X-ray scattering experiment shows superlattice reflections with a lattice modulation vector q =(1/2, 1/2, 1/4) . Inelastic neutron scattering from a powder sample at T =7 K clearly demonstrates a gap energy of 9.8 meV at a scattering vector | Q |=1.0 A -1 corresponding to the reciprocal lattice point (1, 1/2, 0).

Structure of Dehydrated Zeolite Li−LSX by Neutron Diffraction: Evidence for a Low-Temperature Orthorhombic Faujasite

Abstract: Li−LSX (Li−faujasite with Si/Al = 1), cubic at room temperature, is orthorhombic (Fddd) at 10 K. In the room-temperature Fd3 phase, Li cations in the supercage are evenly distributed between 2-fold-coordinated sites III and III‘, in close agreement with the cation distribution in Na−X. The cubic−orthorhombic phase transition implies extensive redistribution of strain through the lattice and displacement of Li cations. In the low-temperature Fddd phase the occupancy of sites II is reduced, and nearly half the Li cations are in the supercage, most of them in a new site III‘, midway between neighboring sites II.

Characterisation, conductivity and mechanical properties of theoxygen-ion conductor La0.9Sr0.1Ga0.8Mg0.2O3-x

Abstract: The new oxygen-ion conductor La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-x has been prepared by conventional solid-state reaction at high temperatures and characterised by X-ray diffraction, scanning and transmission electron microscopy, and conductivity (four-probe dc and impedance) measurements. The room-temperature structure is orthorhombic, space groupPnma (no. 62), with a=5.5391(7) A, b=7.8236(12) A, c=5.5224(7) A. The material undergoes a phase transition at 445 K to a rhombohedral structure. Mechanical property measurements at room temperature and at 1173 K give average strength measurements of 162±14 MPa and 55±11 MPa respectively. Conductivity and ionic transport number measurements confirm predominantly ionic conduction. The contribution from the grain boundary conductivity is extremely small at temperatures below 673 K. At 1073 K, an ionic conductivity value of 0.12 S cm -1 was recorded in air.

Electrochemical Cyclability of Orthorhombic LiMnO2 Characterization of Cycled Materials

Abstract: The electrochemical removal of lithium from orthorhombic LiMnO{sub 2} (o-LiMnO{sub 2}) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li{sub x}MnO{sub 2} phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for {approx}0.3 {micro}m crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended X-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO{sub 2}.

Structure and Dielectric Properties of (Ca1-xNd2x/3)TiO3

Abstract: (Ca1-x Nd2x /3)TiO3 sintered samples are fabricated with 0x1 and the crystal structure and dielectric properties are investigated. It is shown that (Ca1-x Nd2x /3)TiO3 has an orthorhombic GdFeO3-type structure for 0x0.69 and an La2/3TiO3-type double-layered perovskite structure for 0.78x0.93. High e and high fQ values are obtained for the GdFeO3 phase of (Ca1-x Nd2x /3)TiO3, e.g., e=108 and fQ=17200 GHz for x=0.39. On the other hand, for the La2/3TiO3 phase, e is between 80 and 100 and fQ is below 1000 GHz.

Hydrothermal Syntheses and Single-Crystal Structures of Some Novel Guanidinium-Zinc-Phosphates

Abstract: The mild-condition hydrothermal syntheses and single-crystal structures of three new guanidinium zinc phosphates are reported. CN3H6·Zn2(HPO4)2H2PO4 and (CN3H6)2·Zn(HPO4)2 are built up from three-dimensional Zn/P/O frameworks encapsulating guanidinium cations in 12-ring channel systems. The latter material has the lowest nodal-framework-atom density per unit volume of any open-framework phase characterized so far. (CN3H6)6·Zn2(OH)(PO4)3·H2O is a one-dimensional phase with respect to Zn/P/O connectivity and has a unique triply bridged tetrahedral chain connectivity for simple phosphates. Crystal data: CN3H6·Zn2(HPO4)2H2PO4, Mr = 478.78, monoclinic, space group P21 (No. 4), a = 5.1318(4) A, b = 7.8411(7) A, c = 16.510(1) A, β = 90.110(7)°, V = 664.36(8) A3, Z = 2, R = 2.43%, Rw = 2.67% [2608 observed reflections with I > 3σ(I)]. (CN3H6)2·Zn(HPO4)2, Mr = 377.50, orthorhombic, space group Pna21 (No. 33), a = 10.447(2) A, b = 12.349(2) A, c = 10.225(2) A, V = 1319.2(5) A3, Z = 4, R = 3.26%, Rw = 3.32% [2252 o...

Spin delocalization in the Copper(I) Complexes of Bis(verdazyl) Diradicals

Abstract: 1,1‘,5,5‘-tetramethyl-6,6‘-dioxobis(verdazyl) (BVD) reacts with copper(I) halides in acetonitrile and copper(II) halides in methanol to give copper(I) coordination polymers of composition [Cu2X2(BVD)]x. When X = Cl and X = Br, these polymers crystallize in orthorhombic unit cells with dimensions a = 6.684(1) A, b = 12.524(3) A, and c = 8.717(2) A (X = Cl) and a = 12.680(2) A, b = 6.744(1) A, and c = 8.822(2) A (X = Br). With X = I, powder diffraction indicates a monoclinic unit cell with dimensions a = 12.669 A, b = 8.461 A, c = 7.679 A, and β = 91.88°, although poor crystal quality prevented a full structure determination. Magnetic susceptibility measurements taken on the three polymers indicate that the spins couple in one-dimensional chains with alternating exchange parameters: J1 = −190 cm-1, J2 = −116 cm-1 (X = Cl); J1 = −200 cm-1, J2 = −110 cm-1 (X = Br); and J1 = −271 cm-1, J2 = −200 cm-1 (X = I). Variable temperature ESR measurements on the unstable monomeric complex resulting from reaction of BV...

Bulk Superconductivity in Single Crystals of PrBa 2Cu 3Ox

Abstract: We report the observation of bulk superconductivity in PrBa2Cu3Ox single crystals for the first time. These samples were grown by the travelling-solvent floating-zone (TSFZ) method in oxygen-reduced atmosphere. The as-grown crystal has a longer c-axis lattice constant an than any values previously reported for oxygen-reduced PrBa2Cu3Ox . After oxygen annealing, resistivity becomes zero below T=80 K, and zero-field-cooled magnetization shows diamagnetism with full volume fraction. X-ray energy dispersion spectrum and X-ray diffraction analyses show that this superconducting phase is orthorhombic PrBa2Cu3Ox .

Hydrothermal synthesis, structure, and magnetic properties of a novel monodimensional iron phosphate : fef(hpo4)2,n2c3h12,(h2o)x (x 0.20) (ulm-14)

Abstract: ULM-14 or [FeF(HPO4)2,N2C3H12,(H2O)x] (x ≈ 0.20) was synthesized by the hydrothermal method (24 h, 453 K) under autogenous pressure. ULM-14 crystallizes in the orthorhombic system (space group Pmca...

Comparison of the structures of the cubic and tetragonal forms of horse-spleen apoferritin.

Abstract: Horse-spleen apoferritin is known to crystallize in three different space groups, cubic F432, tetragonal P4212 and orthorhombic P21212. A structure comparison of the cubic and tetragonal forms is presented here. Both crystal forms were obtained by the vapor-diffusion technique and data were collected at 2.26 A (cubic crystal) and 2.60 A (tetragonal crystal) resolution. Two main differences were observed between these crystal structures: (i) whereas intermolecular contacts only involve salt-bridge type interactions via cadmium ions in the cubic structure, two types of interactions are observed in the tetragonal crystal (cadmium-ion-mediated salt bridges and hydrogen-bonding interactions) and (ii) cadmium ions bound in the threefold axes of ferritin molecules exhibit lower site-occupation factors in the tetragonal structure than in the cubic one.

Active material for lithium battery

Abstract: PROBLEM TO BE SOLVED: To enhance capacity by constituting an active material of Li, Cu, P and O elements of the specific ratio. SOLUTION: An active material is expressed by a chemical formula (LiCu 1-x PO 4 ). Here, (0≤X≤1) is realized. In this active material, a crystal system is an orthorhombic system in (0.5