STRUCTURE OF FARMING UNDER FREER TRADE AMONG NAFTA COUNTRIES

International Relations/Trade,

The magnetic and crystal structures of Sr2IrO4: A neutron diffraction study

We report a single-crystal neutron diffraction study of the layered $\rm Sr_2IrO_4$. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the $\rm IrO_6$ octahedra in $\rm Sr_2IrO_4$. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) $\mu_B$/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) $\mu_B$/site for the a axis and the b axis, respectively. It is intriguing that forbidden nuclear reflections of space group $I4_1/acd$ are also observed in a wide temperature range from 4 K to 600 K, which suggests a reduced crystal structure symmetry. This neutron-scattering work provides a direct, well-refined experimental characterization of the magnetic and crystal structures that are crucial to the understanding of the unconventional magnetism exhibited in this unusual magnetic insulator.Comment: the version appeared in PR

Formation and structural chemistry of the unusual cyanide-bridged dinuclear species [Ru-2(NN)(2)(CN)(7)](3-)(NN=2,2 '-bipyridine or 1,10-phenanthroline)

Crystallisation of simple cyanoruthenate complex anions [Ru(NN)(CN)(4)](2) (NN = 2,2'-bipyridine or 1,10-phenanthroline) in the presence of Lewis-acidic cations such as Ln(III) or guanidinium cations results, in addition to the expected [Ru(NN)(CN)(4)](2) salts, in the formation of small amounts of salts of the dinuclear species [Ru-2(NN)(2)(CN)(7)](3). These cyanide-bridged anions have arisen from the combination of two monomer units [Ru(NN)(CN)(4)](2) following the loss of one cyanide, presumably as HCN. The crystal structures of [Nd(H2O)(5.5)][Ru-2(bipy)(2)(CN)(7)] center dot 11H(2)O and [Pr(H2O)(6)][Ru-2(phen)(2)(CN)(7)] center dot 9H(2)O show that the cyanoruthenate anions form Ru-CN-Ln bridges to the Ln(III) cations, resulting in infinite coordination polymers consisting of fused Ru(2)Ln(2)(mu-CN)(4) squares and Ru(4)Ln(2)(mu-CN)(6) hexagons, which alternate to form a one-dimensional chain. In [CH6N3](3)[Ru-2(bipy)(2)(CN)(7)] center dot 2H(2)O in contrast the discrete complex anions are involved in an extensive network of hydrogen-bonding involving terminal cyanide ligands, water molecules, and guanidinium cations. In the [Ru-2(NN)(2)(CN)(7)](3) anions themselves the two NN ligands are approximately eclipsed, lying on the same side of the central Ru-CN-Ru axis, such that their peripheries are in close contact. Consequently, when NN = 4,4'-Bu-t(2)-2,2'-bipyridine the steric bulk of the t-butyl groups prevents the formation of the dinuclear anions, and the only product is the simple salt of the monomer, [CH6N3](2)[Ru((t)Bu(2)bipy)(CN)(4)] center dot 2H(2)O. We demonstrated by electrospray mass spectrometry that the dinuclear by-product [Ru-2(phen)(2)(CN)(7)](3) could be formed in significant amounts during the synthesis of monomeric [Ru(phen)(CN)(4)](2) if the reaction time was too long or the medium too acidic. In the solid state the luminescence properties of [Ru-2(bipy)(2)(CN)(7)](3) (as its guanidinium salt) are comparable to those of monomeric [Ru(bipy)(CN)(4)](2), with a (MLCT)-M-3 emission at 581 nm

Burial Records From New Mexico\u27s Rio Abajo: Tome and Associated Villages — Vol III

The purpose of this paper was to analyze and assess the prevalence of infectious disease in the predominant Hispanic population, derive infant mortality data, an important indicator of the general health status of a population, and to reveal the extent of hostility between the Indians and the colonists

Structure symmetry determination and magnetic evolution in Sr2Ir1−xRhxO4\rm Sr_2Ir_{1-x}Rh_{x}O_4

We use single-crystal neutron diffraction to determine the crystal structure symmetry and the magnetic evolution in the rhodium doped iridates $\rm Sr_2Ir_{1-x}Rh_{x}O_4$ ($0\leq x \leq 0.16$). Throughout this doping range, the crystal structure retains a tetragonal symmetry (space group $I4_1/a$) with two distinct magnetic Ir sites in the unit cell forming staggered $\rm IrO_6$ rotation. Upon Rh doping, the magnetic order is suppressed and the magnetic moment of Ir$^{4+}$ is reduced from 0.21 $\rm \mu_B$/Ir for $x=0$ to 0.18 $\rm \mu_B$/Ir for $x=0.12$. The magnetic structure at $x=0.12$ is different from that of the parent compound while the moments remain in the basal plane.Comment: Accepted for publication in Phys. Rev.

Direct evidence of a zigzag spin chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation on single crystal Na2IrO3\rm Na_2IrO_3

We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice $\rm Na_2IrO_3$. The system orders magnetically below $18.1(2)$ K with Ir$^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of $\rm 0.22(1) \mu_B$/Ir site. Such a configuration sharply contrasts the N{\'{e}}el or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the $\rm IrO_6$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5$d$-electron based honeycomb lattice.Comment: Revised version as that to appear in PR

Octanuclear heterometallic Fe-III-Ce-IV pivalate clusters: From a close {Fe4Ce4(mu(4)-O)(4)} cage to an open {Fe4Ce4(mu(4)-O)(2)(mu(3)-O)(2)} core

Ultrasonic irradiation of trinuclear [Fe3O(O2CCMe3)(6)(H2O)(3)]Me(3)CCO(2)2(Me3CCO2H) or hexanuclear [Fe6O2 (OH)(2)(O2CCMe3)(12)] pivalate precursors with Ce(NO3)(6)6H(2)O, NaN3 and triethanolamine (H3tea) in MeOH/ MeCN solution results in the synthesis of two new octanuclear Fe-III-Ce-IV clusters formulated as [Fe4Ce4O4 (O2CCMe3)(4)(tea)(4)(N-3)(4)(MeOH)(4)].MeOH (1) and [Fe4Ce4O4(O2CCMe3)(6)(tea)(4)(N-3)(2)(MeOH)(2)]center dot 3(MeOH) (2). The spectroscopic and thermal properties of these compounds corroborate oxidation states and formula. Single crystal X-ray diffraction analysis revealed that the metal atoms in clusters 1 and 2 are organized in unprecedented close {Fe4Ce4(mu(4)-O)(4)} and open {Fe4Ce4(mu(4)-O)(2)(mu(3)-O)(2)} cores, respectively. The topology of these cores has not been observed before in FeIII-CeIV chemistry

Zn-induced spin dynamics in overdoped La2−x_{2-x}Srx_xCu1−y_{1-y}Zny_yO4_4

Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_y$O$_4$ ($x=0.25$, $y\approx0.01$) are measured via inelastic neutron scattering techniques. As Zn$^{2+}$ is substituted onto the Cu$^{2+}$-sites, an anomalous enhancement of the local spin susceptibility $\chi^{\prime\prime}(\omega)$ appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector \textbf{Q}$=(\pi, \pi, 0)$ that coexists with the known incommensurate SDW excitations at \textbf{Q}$_{HK}=(\pi\pm\delta,\pi), (\pi,\pi\pm\delta)$. Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu$^{2+}$-spins, and the simultaneous suppression of T$_c$ as AF fluctuations are slowed in proximity to Zn-impurities suggests the continued importance of high energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.Comment: 10 pages, 8 figure

Applications of Parallel-Element, Embedded Mesh-Cap Acoustic Liner Concepts

This study explores progress achieved with 2DOF, 3DOF, and MDOF acoustic liners constructed with mesh caps embedded within a honeycomb core. These liner configurations offer potential for broadband noise reduction, and are suitable for conventional aircraft implementation. Samples for each configuration are tested in the NASA normal incidence tube and grazing flow impedance tube, with and without a wire mesh facesheet. Impedances based on these measured data compare favorably with those predicted using a transmission line impedance prediction model. Predicted impedances are then used as input for an aeroacoustic propagation code to compute axial acoustic pressure distributions in the grazing flow tube. These predicted distributions compare favorably with the corresponding measured distributions at frequencies away from the frequency of peak attenuation, but suffer slight degradation for frequencies very near the peak attenuation frequency, where the predicted results are sensitive to input impedance changes. As expected, the noise reduction frequency range increases as more degrees of freedom are included. Although the specific results achieved herein may differ from those that would be achieved with other 2DOF, 3DOF, and MDOF liners, this comparison highlights some of the key features that can be exploited in the design of parallel-element, embedded mesh-cap liners