Showing papers on "Orthorhombic crystal system published in 1971"

The Shape-Memory Effect in a Titanium-35 wt.-% Niobium Alloy

Abstract: The shape-memory effect has been studied in a titanium −35 wt.-% niobium alloy. The effect is observed under two different sets of conditions. First, when the fully martensitic alloy is deformed below Ms it recovers its original shape when heated above As Secondly, if the alloy is deformed between Ms and As (with Ms artificially depressed by a fine dispersion of ω-phase precipitates, which prevents the martensite forming on cooling through the expected Ms , a strain-induced martensite is formed and on heating above As the deformation is recovered. When the orthorhombic martensite forms from the cubic β phase, there is an expansion of the unit cell along two axes of the orthorhombic and a contraction along the third. The orientation relationship between the two phases predicts 12 possible variants and on stressing that variant will be formed which results in a relief of the stress, e.g. under a tensile stress those lattice planes with an increased spacing in the orthorhombic phase will tend to lie ...

Refinement of the Structure of Mg3(VO4)2

Abstract: Magnesium orthovanadate crystallizes in the orthorhombic space group Cmca with a = 6.053(3), b = 11.442(6), c = 8.330(3) A, and Z = 4. The structure was refined by full matrix least square methods ...

Ferroelectric Ferroelastic Paramagnetic Beta‐Gd2(MoO4)3 Crystal Structure of the Transition‐Metal Molybdates and Tungstates. VI

Abstract: Beta‐gadolinium molybdate, β‐Gd2(MoO4)3, is ferroelectric and ferroelastic with a Curie temperature of about 160°C and is paramagnetic with a moment of 7.98 μB. β‐Gd2(MoO4)3 crystallizes in the orthorhombic system with space group Pba2 and four formulas in the unit cell. The lattice constants at 298°K are a = 10.38582 ± 0.00014, b = 10.41861 ± 0.00010, and c = 10.70039 ± 0.00004 A. The integrated intensities of 14 490 reflections were measured with PEXRAD, within a hemisphere of reciprocal space of radius (sinθ) / λ = 1.02 A−1. The crystal structure was solved by interpretation of Patterson and Fourier series and refined by the method of least squares, using a total of 2545 symmetry independent Fmeas. The final agreement factor R is 0.089, based on isotropic thermal vibrations for all atoms. X‐radiation exposure introduces appreciable nonrandom error in the Fmeas. The structure consists of three crystallographically independent discrete sets of (MoO4)2− tetrahedra; one set is oriented with apex oxygen ato...

Untersuchungen mit einer neuen Röntgenkamera: Phasenumwandlungen in festem Bromwasserstoff

Abstract: For X-ray investigations of structural changes in solids (including extremely air sensitive substances, frozen liquids or condensed gases) as a function of temperature a moving film camera has been developed on the basis of a modified Guinier technique. Temperatures can be maintained to within ±0.05°C from −1190° to 400 °C. The phase transitions occurring in solid hydrogen bromide have been investigated with the new camera. HBr is shown to crystallize in two orthorhombic low temperature and two face-centered cubic high temperature phases.

Crystal Structure of Low Chalcocite

Abstract: CHALCOCITE, Cu2S, is hexagonal above 104° C, and has a small, hexagonal unit cell containing 2 Cu2S, in which the sulphur atoms are arranged in nearly perfect hexagonal clOsest packing. Buerger and Wuensch1 and Sadanaga, Ohmasa, and Morimoto 2 have shown that the copper atoms are distributed throughout the interstices between the sulphur atoms, almost in a fluid state, with only partial concentration at certain sites. Below the transition point a large superstructure is formed based on the hexagonal cell, as was demonstrated in 1944 by Buerger and Buerger3. This phase, commonly known as low chalcocite, was found by them to have an orthorhombic unit cell with a=11.92 A, b=27.34 A, c=13.44 A, containing 96 Cu2S. These authors showed that the only orthorhombic space group compatible with the hexagonal close-packed sulphur lattice and the observed extinctions was the noncentrosymmetric group Ab2m(C152v). All attempts since then to solve the structure on this basis have been fruitless. I have discovered that the true symmetry of low chalcocite is mono-clinic, and with this knowledge have been able to find a complete solution of the structure problem. Following a brief preliminary report of this work4, refinement of the structure has proceeded slowly, with interruptions, but is now complete. In this article the essential structural results are reported, and a full description of the structure problem and its crystal chemical implications will be given elsewhere.

Crystallographic data for an orthorhombic form of rubrene

Abstract: Orthorhombic rubrene crystallizes with a = 14.44, b = 7.18, c = 26.97 A; Dmeas = 1.26, Dcalc = 1.263 g.cm−3, Z = 4. The space group is Aba2.

Crystal Structure of α-Zn3(VO4)2

Abstract: α-Zn3(VO4)2, the low temperature form of this compound, crystallizes in the orthorhombic space group Cmca with a = 6.088(3), b = 11.489(3), and c = 8.280(3) A and Z = 4. A total of 726 unique refle...

Crystal Structure of the Antibiotic Monensin Similarities and Differences between Free Acid and Metal Complex

Abstract: The structure of monensin, C36H62O11, has been determined by X-ray analysis of its crystalline monohydrate (orthorhombic, a = 15.15, b = 23.61, c = 10.65 A, Z= 4, space group P212121). Phases were assigned by direct methods, making use of the ‘tangent formula’. Although the conformation of the free acid resembles that of the silver salt in being cyclic, there are differences in the hydrogen bonding pattern. These features are discussed in relation to the complexation of metal ions by monensin.

Dissolution rates of polycrystalline samples of gypsum and orthorhombic forms of calcium sulphate by a rotating disc method

Abstract: The dissolution into water of anhydrite and gypsum was studied at 25°C using a rotating disc of polycrystalline material, and the kinetics were followed with a calcium selective electrode. The dissolution rate of gypsum was transport controlled while that of anhydrite was intermediate between chemical and transport control.

High-resolution X-ray diffraction patterns of crystalline transfer RNA that show helical regions

Abstract: Yeast phenylalanyl transfer RNA crystallizes in a simple orthorhombic unit cell (a = 33.2, b = 56.1, c = 161 A), and the crystal yields an x-ray diffraction pattern with a resolution of 2.3 A. From an analysis of the packing in the unit cell it is concluded that the molecular dimensions are approximately 80 by 33 by 28 A. The diffraction pattern viewed along the a-axis has a distribution characteristic of double-helical nucleic acids. However, this distribution is not found when the pattern is viewed along the b-axis. This has been interpreted as indicating that the double-helical portions of the transfer RNA molecule are approximately half a helical turn in length, and therefore can contain 4-7 base pairs. These results are consistent with the cloverleaf formulation of transfer RNA secondary structure.

The Magnetic and Structural Properties of Bulk Amorphous and Crystalline Co‐P Alloys

Abstract: Bulk samples, of the order 0.2 mm thick, of cobalt-phosphorus alloys have been prepared by prolonged electroless deposition with phosphorus contents varying from 5 to 9 at%. Alloys with less than 8.8 at% were found to be crystalline with a single-phase h.c.p. structure, and those with 9 at% were amorphous. The saturation magnetization, magnetostriction, coercivity, and susceptibility of the crystalline alloys varied uniformly with phosphorus content. The amorphous alloys had significantly different magnetic properties from the crystalline ones of similar composition; in particular the susceptibility was nearly two orders of magnitude greater. This greater susceptibility was predominantly associated with the absence of magnetocrystalline anisotropy in the non-crystalline material. On annealing, the amorphous material first transformed to a single-phase h.c.p. structure similar to that of the crystalline alloys, and at a higher temperature to the equilibrium two-phase structure of a cobalt-rich h.c.p. phase and an orthorhombic Co2P phase. The susceptibility of the amorphous alloy fell by a factor of 15 times on annealing. This was attributed to the introduction of crystalline order and hence magneto-crystalline anisotropy. Massive Proben von Kobalt-Phosphorlegierungen mit einer Dicke von der Grosenordnung 0,2 mm wurden durch fortgesetzte elektrodenlose Auftragung hergestellt, wobei der Phosphorgehalt zwischen 5 und 9 % variierte. Es wurde gefunden, das Legierungen mit weniger als 8,8 at% in einer einzigen kristallinen Phase mit h.d.p.-Struktur kristallisieren und die mit 9 at % amorph sind. Die Sattigungsmagnetisierung, Magnetostriktion, Koerzitivkraft und Suszeptibilitat der kristallinen Legierungen andern sich gleichmasig mit dem Phosphorgehalt. Die amorphen Legierungen zeigten betrachtlich unter-schiedliche magnetische Eigenschaften gegenuber kristallinen Legierungen ahnlicher Zusammensetzung; insbesondere war die Suszeptibilitat um nahezu zwei Grosenordnungen groser. Diese grosere Suszeptibilitat war vorwiegend mit dem Fehlen magnetokristalliner Anisotropie im nichtkristallinen Material verbunden. Bei einer Temperung ging das amorphe Material zuerst in eine einphasige h.d.p.-Struktur uber, die der der kristallinen Legierung ahnlich ist, und bei einer hoheren Temperatur in die Gleichgewichts-Zwei-Phasen-Struktur, eine Kobalt angereicherte h.d.p.-Phase und eine orthorhombische Co2P-Phase. Die Suszeptibilitat der amorphen Legierung ging um den Faktor funfzehn nach der Temperung zuruck. Dies wurde der Einfuhrung einer kristallinen Ordnung und damit einer magneto-kristallinen Anisotropie zugeschrieben.

Structure of Concanavalin A at 4.25-Ångström Resolution

Abstract: An electron density map produced by x-ray diffraction analysis of concanavalin A has been calculated to 4.25 A from data of three isomorphous heavy atom derivatives. The crystals are orthorhombic, with unit-cell dimensions of 63.1, 87.0, and 89.2 A for a, b, and c, respectively. The space group is I222, with eight asymmetric units per unit cell. The crystal asymmetric unit contains 27,000 daltons of protein and reflects the chemically unique component (protomer) within the oligomer. Separate chemical studies indicate that the protomer consists of two different polypeptide chains. Four protomers cluster around the intersection of three mutually perpendicular two-fold rotation axes to form a molecule of 108,000 daltons. The molecule can also be subdivided into two-protomer units of 54,000 daltons. Within the two-protomer unit, there are significantly more contacts joining the protomers than there are between adjacent two-protomer units that form the total molecule. These results provide a possible explanation for disagreement in molecular weights obtained in previous ultracentrifugal studies.

A thermodynamic theory of the cubic-orthorhombic phase transition in boracites

Abstract: A thermodynamic theory of the cubic-orthorhombic phase transition in boracites is developed. The temperature dependences of dielectric, piezoelectric and elastic constants near the phase transition temperature are derived. Since the parameter of the phase transition is not the polarization, the spontaneous strains and polarization are not related by electromechanical constants. Magnetic properties of boracites are not considered.

The Crystal and Molecular Structure of 2,5-Distyrylpyrazine

Abstract: 2,5-Distyrylpyrazine(C20H16N2), which has photo-polymerizability in the crystalline state, crystallizes in the orthorhombic system with cell dimensions of a=20.638, b=9.599, and c=7.655 A, including four molecules in the unit cell. The space group is Pbca. The structure has been determined by the direct method and refined by the block-diagonal-matrix least-squares method using three-dimensional data. The molecules is not planar; the average plane of the pyrazine ring makes a dihedral angle of 12.09° with that of the benzene ring. The molecules are spaced by the c-translation forming an almost parallel plane-to-plane stack. In this stack, the ethylenic double bonds approach each other at the shortest intermolecular distances of 3.939 A, since the long axis of the molecule makes an angle of about 30° with the c-axis. Therefore, the polymerization may occur between these double bonds to form a cyclobutane ring.

Pyroelectric Ammonium Iodate, a Potential Ferroelastic: Crystal Structure

Abstract: Ammonium iodate, NH4IO3, is pyroelectric at room temperature and crystallizes in the orthorhombic system with space group Pc21n and four formulas in the unit cell. The lattice constants at 297°K are a = 6.4115 ± 0.0005, b = 9.1706 ± 0.0005, and c = 6.3740 ± 0.0005 A. The integrated intensities of 4591 reflections within a hemisphere of reciprocal space of radius (sinθ) / λ = 1.02 A−1 were measured using PEXRAD. The crystal structure was solved by Patterson and Fourier series and refined by the method of least‐squares using 1458 symmetry independent Fmeas. The final agreement factor R is 0.054, based on a model with anisotropic iodine and oxygen and isotropic nitrogen thermal vibrations, and with a spherical distribution of hydrogen around the nitrogen atom. The structure consists of discrete IO3− pyramidal ions arranged to give a highly distorted octahedral environment about the iodine. The IO3− ions and NH4− ions form chains parallel to the polar axis. Three I–O distances are in the range 1.765–1.836 A a...

The Crystal Structure of Bis-(L-prolinato)palladium(II)

Abstract: The crystal structure of bis-(L-prolinato)palladium(II) has been determined from three-dimensional X-ray data collected by a photographic method. The crystals are orthorhombic with a = 10"31, b = 12.17, c= 9"71/~; space group B2212 with four molecules in the unit cell. The structure has been refined by block-diagonal least-squares methods to an R value of 0.085 for the 694 observed reflexions. This is a cis complex, and the palladium atom has square planar coordination (Pd-O = 2.02/~, Pd-N = 2.03/~). The complex molecule has a twofold axis of rotation. The five-membered chelate ring is bent at the two coordinating atoms. Bond lengths are substantially normal. A1J the imino hydrogen atoms are involved in hydrogen bonds which form layers of the molecules parallel to (010).