Orthorhombic Crystal Cell Research Articles

On the magnetic order of Gd5Ge3

We have investigated the magnetic structure of Gd5Ge3 by neutron powder diffraction down to 3.6 K. This compound presents three events in the heat capacity which we show are related to fundamental changes in the magnetic order. The primary antiferromagnetic ordering occurs at 82(2) K and produces a magnetic cell that is tripled with respect to the underlying orthorhombic crystal cell. The propagation vector is k1=[0 0 13]. At 74(2) K, the magnetic order becomes “anti-C” with a propagation vector k2 = [1 0 0]. A third change in the magnetic order occurs at 40(2) K, and the new magnetic structure is essentially the “anti-C” structure but with the addition of a tripled magnetic component corresponding to a propagation vector k3 = [13 0 0].

Real-Time Structure Changes during Uniaxial Stretching of Poly(ω-pentadecalactone) by in Situ Synchrotron WAXD/SAXS Techniques

Poly(ω-pentadecalactone) (PPDL), a model polymer in the poly(ω-hydroxyl fatty acids) family, is a new biopolymer with monomer synthesized by yeast-catalyzed ω-hydroxylation of fatty acids. In this study, deformation-induced structural changes in two PPDL samples with different molecular weights were studied by in situ wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The high molecular weight PPDL (PPDL-high) sample exhibited notable strain hardening, while the low molecular weight PPDL (PPDL-low) sample did not. The behavior can be explained by the entanglement density concept. The evolution of crystallinity (from WAXD) as a function of strain could be divided into four distinct regions, but their respective mechanisms differ slightly in each sample. During stretching, a mesomorphic phase formed in both samples, bridging between the amorphous and strain-induced crystal phases. The SAXS data verified the effect of molecular weight (or the entanglement density) on the deformation-induced structure of PPDL. The parameters of chain orientation factor (f) calculated from the orthorhombic crystal cell as well as the nonorthorhombic crystal cell proposed by Wilchinsky were used to follow the orientation process during stretching of PPDLs. It was found that the different molecular entanglement network (i.e., PPDL-low versus PPDL-high) led to different crystal orientation behavior, especially in the low strain range.

Nuclear momentum distribution in solid and liquid HF from ab initio calculation

A calculation of nuclear momentum distribution of liquid and solid hydrogen fluoride was performed. In both systems, density functional theory generalized gradient approximation functional of Perdew, Burke, and Ernzerhof was used for the calculation: for liquid hydrogen fluoride, using an atom centered basis set for an isolated molecule with optimized geometry, and for solid hydrogen fluoride using plane-wave basis sets on optimized orthorhombic crystal cell. For liquid hydrogen fluoride, a semiclassical approach was adopted with the vibrational contribution to momentum distribution obtained from the density functional theory calculation and translational and rotational contributions calculated classically. Nuclear momentum distribution in the solid hydrogen fluoride was calculated entirely quantum mechanically using phonon dispersion and vibrational density of states calculated in the framework of plane-wave density functional theory. Theoretical results were contrasted with recently obtained results of Compton (deep inelastic) neutron scattering on liquid and solid hydrogen fluoride. In case of liquid hydrogen fluoride, almost a perfect agreement between theory and experiment was achieved within the harmonic Born-Oppenheimer approximation. For the solid system under investigation, the harmonic approximation leads to small (4%) overestimation of the square root of the second moment indicating that neutron Compton scattering technique is sensitive to proton delocalization due to hydrogen bonding in solid hydrogen fluoride.

Redox and adsorption processes of hydrolyzed uranyl species on a mercury electrode from sodium chloride solutios

AbstractReduction and adsorption processes of hydrolyzed uranyl species on the mercury drop electrode, from 0.55 mol L−1 NaCl, were studied by electrochemical cathodic stripping techniques: cyclic voltammetry, alternating current voltammetry, chronoamperometry and double potential step chronocoulometry. The only uranyl species adsorbed on the Hg electrode (between pH 5 and 7) was found to be uncharged uranyl dihydroxide, as well as its reduction product. At bulk concentrations between 3 and 5 μmol L−1 that reactant forms an adsorbed compact film as a result of surface polymerization, nucleation or other condensation processes. The adsorption isotherm at pH 6.2 and −0.2 V was measured. It was fitted with a Frumkin type isotherm. At the plateau. a reduction current corresponding to 6.7 × 10−9 mol cm−2 of uranyl ions was measured. This represents on the average 7 adsorbed layers of UO2(OH), presuming an orthorhombic crystal cell structure. Multilayer adsorption is certain even if a polymeric form of uranyl dihydroxide was considered.

Neutron-diffraction study of the magnetic ordering in the insulating regime of the perovskites RNiO3 (R=Pr and Nd).

Neutron-diffraction experiments and polarization analysis techniques reveal that the metal-insulator transition in the orthorhombic perovskites ${\mathrm{PrNiO}}_{3}$ and ${\mathrm{NdNiO}}_{3}$ is accompanied by a sudden three-dimensional magnetic ordering of the Ni sites (${\mathrm{\ensuremath{\mu}}}_{\mathrm{Ni}}$\ensuremath{\approxeq}0.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$). The electronic localization is followed by a magnetic ground state consisting of an unusual antiferromagnetic structure, with k=(1/2,0,1/2) relative to the orthorhombic crystal cell. This propagation vector implies the symmetrical coexistence of ferro- and antiferromagnetic couplings along the three pseudocubic axes. This suggests the existence of a nonuniform orbital distribution of the single ${\mathit{e}}_{\mathit{g}}$ electron. The orbital superlattice may result from the breakdown of the degeneracy of the ${\mathrm{Ni}}^{\mathrm{III}}$(${\mathit{t}}_{2\mathit{g}}^{6}$${\mathit{e}}_{\mathit{g}}^{1}$) state due to electronic correlation. This type of spin arrangement has not been observed in other perovskite oxides. These findings are confirmed by the magnetic behavior of the single crystallographic Nd site: half of these ions become ordered and the other half remain disordered as in a simple paramagnet. This can be explained by the combination of polarization effects due to the exchange field from nickel moments and the breaking down of point symmetry at Ni site.

The crystal structures of aryl ether ketone polymers containing meta-phenyl links

The X-ray diffraction patterns have been examined from the crystalline aromatic ketone polymer PEEKmK (aryl ether ether ketone ketone polymer containing meta-phenyl links) with the formula: ▪ An orthorhombic crystal cell is proposed containing two chains with a = 7.71 A ̊ , b = 6.05 A ̊ and c = 39.9 A ̊ . The lateral dimensions of the cell and the relative arrangement of chains is very close to that of the related all- para-phenyl polymer PEEK (poly(aryl ether ether ketone)). The para-linked phenyls are torsionally rotated out from the bc plane as in PEEK but the meta-phenyl ring remains parallel to the bc plane. The conformation of the meta-phenyl rings imposes an energy penalty, which contributes to the reduced melting point of the polymer.

Infrared method for measuring orientation in polyethylene films

AbstractAn infrared method is described for measuring crystalline phase orientation in polyethylene films. The method uses the absorbances of the 730 and 719 cm−1 bands in the polarized spectra of an oriented film and in the spectra of an unoriented film made from the same polymer to estimate the orientation parameters of the orthorhombic crystal cell axes. © 1992 John Wiley & Sons, Inc.

Intercalation chemistry in layered transition metal oxide structures: Pyridine vanadium pentoxide

The reaction of pyridine, pyrazine, phenazine and 4-4′ dipyridine in m-xylene solutions on V 2O 5 yields new intercalation compounds. Only the pyridine compound corresponds to a definite formula: V 2O 5(C 5H 5N) 1.1. The EXAFS study at the vanadium K-edge is consistent with a pyridine molecule bonded to every second vanadium atom of the layer via coordination bond of length VN = 2.34 Å. From X-ray powder diffraction study, the orthorhombic crystal cell ( a = 11.543(8) Å, b = 7.111(4) Å, c = 21.56(1) Å) is built up from two V 2O 5 layers separated by 6.2 Å, leading to a value expected for pyridine intercalation. Temperature dependent susceptibility measurements show that 10% of the total vanadium is in oxidation state IV. An interpretation of the presence of the V 4+ is established on the basis of the mechanism involved in oxidation reactions catalyzed by V 2O 5.