Showing papers on "Orthorhombic crystal system published in 1969"

Ferroelectric Tungsten Bronze‐Type Crystal Structures. II. Barium Sodium Niobate Ba(4 + x)Na(2 − 2x)Nb10O30

Abstract: Ferroelectric Ba(4+x)Na(2 − 2x)Nb10O30, with a Curie temperature of 833°K, has a tungsten bronze‐type structure and crystallizes in the orthorhombic system, with subcell lattice constants a = 17.59182 ± 0.00001, b = 17.62560 ± 0.00005, and c = 3.994915 ± 0.000004 A at 298°K. The space group is Cmm2, and there are two formulas in the subcell. The c axis of the true cell is double that of the subcell. The integrated intensities of 6911 reflections within a reciprocal hemisphere of radius (sinθ)/λ = 1.02 A−1 were measured with PEXRAD, 1873 symmetry‐independent structure factors being significantly above background. The metal atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from metal‐phased Fourier series. The final agreement index between measured and calculated structure factors is 0.0579. The structure differs only in detail from previously determined tetragonal tungsten bronze structures. In the general formula (A1)2(A2)4C4(B1)2(B2)8O30, the B1 and B2 sites are fully occupied by Nb, the A2 sites by Ba and the A1 site by 87.0% Na and 6.5% Ba. Evidence from chemical analysis, x‐ray density calculations and the present determination suggests that the best approximation to the formula of the crystal studied is Ba4.13Na1.74Nb10O30. The Ba and O atoms at z ≃ 12 are disordered in a manner similar to the O atoms in the Ba layer in barium strontium niobate. The four crystallographically independent Nb atoms, each in octahedral coordination, are linked to O atoms by distances ranging from 1.765 ± 0.021 to 2.270 ± 0.021 A, with a mean value of 1.989 A. Ba is 10 coordinated, with Ba–O distances ranging upward from 2.671 ± 0.013 A. Na is 12 coordinated, with Na–O distances ranging from 2.660 ± 0.014 to 2.990 ± 0.015 A, with a mean of 2.788 A. The Nb‐atom displacements from the mean oxygen planes lie in the range 0.171–0.205 A; the parabolic relation with Curie temperature predicts a displacement of 0.204 A. The measured value of Ps at room temperature is 40 μC cm−2: the linear relation between displacement and polarization predicts a saturation value of 44–53 μC cm−2. All metal atoms are displaced from the oxygen planes in the sense given by the macroscopic positive polarity.

Crystallographic and Magnetic Properties of Solid Solutions of the Phosphides M2P, M = Cr, Mn, Fe, Co, and Ni

Abstract: Several phosphides with the chemical formula M2P crystallize in a hexagonal structure: Mn2P, Fe2P, and Ni2P. The structure of Co2P is orthorhombic. These two structures are related via an identical elementary subcell consisting of a tetrahedral‐site and a pyramidal‐site M‐atom pair. This investigation shows that solid solutions between two hexagonal end members, such as Fe2P or Ni2P with Mn2P, may exhibit orthorhombic structures at intermediate compositions. Fe2P shows complete solid solubility with Ni2P. Curiously, hexagonal and orthorhombic symmetries alternate with decreasing number of 3d electrons, Ni2P(hex) ‐Co2P(ortho) ‐Fe2P(hex) ‐FeMnP(ortho) ‐Mn2P(hex). Lattice parameter variations with composition and Mossbauer studies reveal atomic ordering in the mixed systems, Mn and Cr substituting for pyramidal‐site iron in FeMnP and FeCrP while Ni and Co substitute preferentially for tetrahedralsite iron in FeCoP and FeNiP. While neither Co2P nor Mn2P are ferromagnetic, intermediate phases are, the Curie te...

Crystal Structure of Pyroelectric Paramagnetic Barium Manganese Fluoride, BaMnF4

Abstract: BaMnF4, pyroelectric at room temperature and paramagnetic above 30°K, crystallizes in the orthorhombic system with space group A21am and lattice constants a = 5.9845 ± 0.0003, b = 15.098 ± 0.002, and c = 4.2216 ± 0.0003 A at 298°K. The integrated intensities of 5085 reflections within a reciprocal lattice hemisphere of radius (sinθ) / λ = 1.02 A−1 were measured using pexrad. The crystal structure has been solved and refined by a combination of Patterson and Fourier series and the method of least squares, using 455 independent and significantly nonzero F(meas). The final agreement factor R, for atoms with anisotropic temperature coefficients, is 0.0466. The structure consists of MnF6 octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba2+ ions only, giving rise to excellent (010) cleavage. Within the sheets there are nearly linear –Mn–F–Mn–F– chains parallel to c. Parallel to a there is a second set of planar zigzag –Mn–F–Mn–F– chains, formed by adjacent shared ...

Crystal Growth and Properties of Mica‐Like Potassium Niobates

Abstract: Large single crystals of and can be grown by Czochralski pulling from the melt. In the case of a small amount of excess is used to avoid incongruent crystallization. Both materials show pronounced cleavage very similar to that of mica. The electrical properties of are also very similar to those of mica, making this material a possible mica substitute as well as being an interesting new dielectric material in its own right. is orthorhombic Pmmm, with , not the previously reported perovskite structure. Other characteristics determined include optical, mechanical, and chemical properties. When exposed to atmospheres between about 25 and 85% relative humidity, becomes a trihydrate, with higher hydration above about 85% relative humidity. Optical, electrical, and structural properties of all three forms of are described.

The crystal structure of orthoenstatite

Abstract: The crystal structure of a natural orthorhombic enstatite (orthoenstatite) from the Bishopville meteorite, has been refined by the least-squares method using 680 three-dimensional reflections. The space group and the cell dimensions of this orthoenstatite are Pbca and a = 18.210, b = 8.812 and c = 5.178 A. The crystal structure of orthoenstatite is compared with that of clinoenstatitc and the geometric relationship between the two polymorphs is described. The orthoenstatite structure is shown to be an exact twin of clinoenstatite on a unitcell scale. The orthorhombic cell is composed of two monoclinic cells of clinoenstatite joined on (100) through a b-glide plane. The orthoenstatite structure is also compared with the structures of hypersthene and orthoferrosilite. The corresponding interatomic distances show the following characteristics. \Vith increasing Fe replacement of Mg the interatomic distances M-O in the metal octahedra increase. The mean interatomic distances, M-O, of the octahedra can thus be used to determine the Mg:Fe

Polymorphic Behavior of Titania under Dynamic Loading

Abstract: Single‐crystal and polycrystalline specimens (including powder compacts of various densities) of titanium dioxide have been subjected to shock‐wave pressures in the 150–1000–kbar range. Hugoniot measurements have disclosed a phase transition commencing below 200 kbar, and x‐ray diffraction studies of specimens recovered after shocking to various pressures above 150 kbar have shown the presence of an orthorhombic phase with the α‐PbO2 structure. Calculated lattice parameters are a = 4.55 A, b = 5.46 A, and c = 4.92 A, which correspond to a crystal density of 4.34 g/cm3. The orthorhombic phase appears to result on release of pressure from a considerably denser phase of TiO2 (postulated to have approximately 8:4 oxygen‐titanium coordination) that dynamic measurements indicate is formed under shock. Yields of the α‐PbO2 phase as high as about 90% have been obtained from [001]‐ and [111]‐oriented rutile crystals shocked to 450 kbar. At atmospheric pressure this phase can be retained indefinitely at temperature...

The system niobium-sulfur

Abstract: The following phases have been found in the system niobium-sulfur: NbS3, 2s-NbS2, 3s-NbS2, 2s-Nb1 + xS2, 3s-Nb1 + xS2, Nb3S4, h.t.-Nb1 − xS, l.t.-Nb1 − xS, Nb21S8. While NbS3 is a diamagnetic semiconductor, the other niobium sulfides are metallic with nearly temperature-independent paramagnetism. H.t.-Nb1 − xS, stable above 780 °C, has an orthorhombic structure of the MnP type with infinite zigzag Nb-Nb chains. The low-temperature form of Nb1 − xS has a hexagonal superstructure of the NiAs type with triangular Nb3 clusters. The structures of the other niobium sulfides that had been reported previously, were confirmed.

Soft Phonon Modes in KMnF3

Abstract: Neutron inelastic scattering experiments show that the phase transition of KMnF 3 at 184°K is a result of the softening of the \(\varGamma_{25}\) phonon mode at the [111] zone boundary. The over-all characteristics of the \(\varGamma_{25}\) mode are similar to those that have been observed for this same mode in SrTiO 3 near its 110°K phase transition. We also observe that the M 3 mode at the [110] zone boundary has the very low energy of 3.1±0.2 MeV at room temperature, and condenses not at 184°K but rather in the vicinity of its Neel temperature, 88°K. The X-ray studies by Beckman and Knox indicate that the space group of KMnF 3 below 184°K is the orthorhombic P b n m D ( 2 h 16 ). This orthorhombic phase is consistent with our observation that the \(\varGamma_{25}\) mode is responsible for the 184°K transition in that the dominant contribution to the rotation of the fluorine octahedra is a superposition of two of the three triply degenerate modes of \(\varGamma_{25}\) condensing at the transition temper...

The Crystal and Molecular Structure of Carbazole

Abstract: The crystal structure of carbazole has been refined with three-dimensional X-ray data. Single crystals obtained from the melt are orthorhombic with four molecules in a unit cell of the dimensions a=7.772, b=19.182 and c=5.725 A; space group is Pnma. Positional and thermal parameters were obtained by the least-squares method. The final discrepancy index R is 10.9% excluding zero intensity data. The e.s.d.’s in bond lengths not involving hydrogen atoms are 0.006–0.010 A, and the corresponding errors in bond angles are about 0.5°. The molecule is planar within the deviation of 0.03 A. A crystallographic mirror plane normal to the b-axis passes through the nitrogen atom and is perpendicular to the molecular plane. Comparison of the observed and theoretical bond lengths is made with discussion thereof. Some physical properties related to the crystal structure are also discussed by comparing its structure with that of fluorene. With the help of thermodynamical data, it has been argued that the hydrogen bond of ...

The structure of a multilayer of lead stearate

Abstract: A study has been made of the molecular and crystallographic structures of Langmuir–Blodgett multilayer films of lead stearate. Electron microscopy and diffraction were used in the examination of such a film which had separated from the glass slide on which it had been formed. The films consist of a large number of three-dimensionally ordered lead stearate crystals arranged in near-perfect alignment, with the longest axes of their unit cell projecting from the plane of the supporting substrate. The unit cell of the lead stearate crystal is either monoclinic with the space group P21/a or orthorhombic with the space group P21212. The former is considered to be rather the more likely. The major lattice spacings are 4.96 A (100), 7.38 A (010) and 47 A (001). The β angle of the monoclinic unit cell is not known but is probably within a few degrees of 90°. The lead atoms are in special positions in the basal plane, regardless of which is the correct unit cell, and the hydrocarbon chains make an angle of 62±5 ° with the basal plane. Any free stearic acid in the films occurs as a separate phase.

ESR Study of γ-Irradiated K2SeO4

Abstract: Structural phase changes in K 2 SeO 4 have been found at 129.5°K and 93°K by introducing free radical SeO 4 - as a probe. The structure in the temperature range between 129.5°K and 93°K is expected to be orthorhombic with the crystallographic axes coincident with those of room temperature. The unit cell may contain twelve molecules located at three different sites. A superstructure, a tripled lattice parameter along the a axis, has been found by X-ray diffraction method and an expected space group is Pnam . It is ferroelectric along the c axis below 93°K. A possible unit cell contains twelve molecules located at six different sites and is monoclinic in the space group P 2 1 , in which the superstructure is maintained. The crystal probably consists of fine twins with the common c axis and they are related with each other by mirror planes. The higher transition is of second order and the lower of first order. Conformations of radicals SeO 2 - and SeO 3 - in the crystal lattice are illustrated.

Raman spectra of hydrogen and deuterium halide crystals.

Abstract: The crystal Raman Spectra of HCl, DCl, HBr, and DBr have been observed for the lattice and intramolecular vibrational regions. The results for the intramolecular region indicate that the crystal structure of the low‐temperature orthorhombic phase is C2v9 − Pn21a with four molecules in a unit cell, containing nonplanar hydrogen‐bonded zigzag chains. However, the nonplanarity of the chain is so small that it is not sensitively reflected to the spectra of lattice vibrational region. As a result, the lattice vibrational bands may be assigned adequately based on a planar structure C2v12 − Bb21m, which is the structure determined by neutron diffraction measurement. Normal coordinate calculations have been made by using several kinds of intermolecular potentials. A central force field which assumed dispersion and repulsion forces and dipole–dipole forces failed to explain the observed lattice frequencies. A potential which assumed hydrogen bonding explicitly gave a reasonable fit of the calculated frequencies wi...

Phase behaviour of n-alkanes and polyethylene. A thermodynamic study

Abstract: Free energies of fusion, and derived thermodynamic properties, are reported for a series of pure long chain n-alkanes, containing from 19 to 42 carbon atoms, in the temperature range –35°C → melting point. Orthorhombic, monoclinic (for which the sub-cell is orthorhombic), triclinic, and hexagonal phases are investigated and the properties of the corresponding polyethylene structures extrapolated using the Flory-Vrij equation. The orthorhombic form (extrapolated m.p., 141.6°C) is always stable under normal conditions but the triclinic modification is only 3-4 J g–1 less stable and, once formed, will remain indefinitely at all temperatures until these are such that there is considerable lattice mobility.

Structure of Pressure‐Induced Fibrous Sulfur

Abstract: The arrangement of the sulfur helices in the pressure‐induced fibrous form of sulfur has been determined from three‐dimensional x‐ray diffractometer data. The crystals show orthorhombic diffraction symmetry, but analysis of their structure has shown that the most probable space group is P2 and that the orthorhombic symmetry results from multiple twinning. The pseudo‐orthorhombic unit cell contains 16 ten‐atom helix segments, half of which are right‐handed and half left‐handed, in an essentially close‐packed arrangement. The monoclinic unit cell, which contains 8 ten‐atom segments, has lattice constants a = 17.6, b = 9.25, c = 13.8 A and β = 113°. There is strong evidence that the pressure‐induced fibrous form and the ψ form have the same structure.

NMR Study of Molecular Motions near the Solid–Solid Transition in the Metal Hexafluorides

Abstract: Fluorine‐19 magnetic resonance and relaxation measurements on orthorhombic MoF6 and on cubic MoF6 and WF6 are reported. In the cubic plastic phases the molecules have rapid rotational and slow translational motions. The theory of Resing and Torrey has been applied to interpret the relaxation data and yields a diffusion coefficient of 0.52 × 10−8 cm2sec−1 for MoF6 and 0.36 × 10−8 cm2 sec−1 for WF6 at the transition point. Slow rotations of the molecules already occur in the orthorhombic phases; this is shown by the study of the shape of the resonance lines in the presence of both motion and anisotropic chemical‐shift tensors. Accurate calculations of the contribution of these rotations to the longitudinal relaxation and to the second moments of the resonance line have been made. They show that in MoF6, which exhibits a solid–solid transition, the rotations of the molecules are quasi‐isotropic, whereas in UF6, where no transition occurs, the rotations are slower and take place preferentially about a single ...

Crystal Structure of (NH4)2MnF5

Abstract: (NH4)2MnF5 crystallizes from acid solution as orthorhombic crystals of space group Pnma with a = 6.20 ± 0.03, b = 7.94 ± 0.01, and c = 10.72 ± 0.01 A, and four formula units per cell. The stoichiometry is achieved by the growth parallel to b of infinite kinked anionic chains of composition (MnF5)n−2n, formed through sharing of opposite vertices of MnF6 octahedra. The shared fluorine atoms lie on mirror planes (y = 14, 34), as do the hexagonal networks of ammonium ions that bind the anionic chains together. Mn–F bond distances, corrected for the markedly anisotropic thermal motions, are 1.85 and 2.10 A. The observed elongation of the MnF6 octahedra in the chain direction is largely, but not wholly, interpretable as a Jahn–Teller distortion.

Thermally Induced Strains in Diamond Cubic, Tetragonal, Orthorhombic, and Hexagonal Films

Abstract: When thin films adhere to a substrate which has a different thermal expansion coefficient, changing the temperature of the composite will induce strains in both film and substrate. Calculations of strain and strain energy have been made for various polycrystalline diamond cubic, tetragonal, orthorhombic, and hexagonal films on substrates which are assumed to be undeformable and to have isotropic thermal expansion and elastic properties in directions parallel to the surface of the substrate. Computer programs were developed to calculate the above quantities for the diamond cubic semiconductors Ge and Si, the tetragonal metals Sn and In, the orthorhombic metal Ga, and the hexagonal metals Be, Cd, Co, Mg, Ti, Zn, and Zr. Each film was assumed to have been deposited on glass or on sodium chloride at room temperature and cooled to approximately liquid air temperature. The induced thermal strains and strain energies for many different crystallographically oriented grains were calculated. The particular (hkl) or...

The isomers of α-amino acids with copper(II). Part II. The crystal and molecular structure of cis-bis-[D-alaninato]copper(II)

Abstract: The crystal and molecular structure of a new crystalline modification of bis-[D-alaninato]copper(II), Cu(C3H6NO2)2′ has been determined from three-dimensional X-ray data. The new isomer crystallizes in the space group P212121(D24) of the orthorhombic system, with Z= 4 in a unit cell of dimensions a= 11·63, b= 8·77, and c= 8·43 A. 928 independent reflexions were estimated visually from Weissenberg and precession photographs and the structure was refined by least-squares methods to an unweighted R of 0·068. The inner co-ordination sphere around the copper atom is square pyramidal; the two bidentate α-amino acid rings are cis and form the basal plane of the pyramid, the apex being occupied by a carboxylate oxygen atom, from a neighbouring molecule, at a distance of 2·390(7)A. The molecular structure is built up from parallel chains of molecules, polymerised in the c direction. The chains are formed from molecules joined through the apical bond, and are separated by repulsion between the copper atom and a methyl group in a neighbouring chain, which occupies the sixth position of a severely distorted octahedron around the copper atom at a Cu ⋯ C distance of 3·50(1)A.

Structures of diethylenetriaminecopper(II) cations. Part I. Crystal structure of bis(diethylenetriamine)copper(II) nitrate

Abstract: The crystal structure of bis(diethylenetriamine)copper(II) nitrate has been determined by X-ray diffraction methods and refined by full-matrix least-squares procedure. The orthorhombic unit-cell (space group P212121) has dimensions a= 9·274, b= 12·192, c= 15·029 A, for Z= 4. Each diethylenetriamine ligand is tridentate, coordinating with the terminal nitrogen atoms trans to each other. The environment about the copper ion is a tetragonally distorted octahedron of a compressed form. The average lengths of the in-plane and tetragonal copper–nitrogen bonds are 2·22 and 2·01 A with an estimated standard deviation of 0·010–0·015 A. The symmetry of the cation is C2.

Metamagnetism in the perovskite compound Gd2CoMnO6

Abstract: The compound Gd2CoMnO6 crystallizes with the orthorhombic perovskite structure and is isomorphous with GdFeO3. A crystal structure refinement shows that the Mn3+-O2−-Mn3+ bond angles are ∼ 145° and that the distortion from cubic symmetry is considerable. A spontaneous moment of 5 e.m.u./g develops below 115°K due to the exchange interactions between near neighbouring Mn3+ ions. Below 115°K a field induced metamagnetic transition to a state with a moment of 20 e.m.u./g is also observed and this transition is depressed in temperature by 1°c for 360 oe applied field. The metamagnetism is accounted for on the basis of a transition between an uncompensated anti-ferromagnetic and a ferromagnetic state which is observed in Gd2CoMnO6 due to a change in sign of the Mn3+-O2−-Mn3+ exchange interaction originating from Jahn-Teller electronic ordering in Mn3+ ions. This ordering is favoured at low temperatures and introduces a negative contribution to the exchange interaction.