Frequency Properties of Polymer Bonded Compacts Obtained from Ball Milled Permalloy Powders with Mo and Cu Additions.

Nanocrystalline powders from the Permalloy family, Ni75Fe25, Ni79Fe16Mo5, and Ni77Fe14Cu5Mo4, were obtained by mechanical alloying starting from elemental powders. All compositions were milled for up to 24 h in a high-energy planetary ball mill. The powders were single phase and nanocrystalline as determined by X-ray diffraction studies, with larger flatted particle sizes for Ni75Fe25 (about 400 µm) and Ni77Fe14Cu5Mo4 (about 470 µm), and smaller particle sizes for Ni79Fe16Mo5 (about 170 µm). The homogeneity of the samples was verified by energy-dispersive X-ray spectroscopy (EDX). Soft magnetic composites were obtained by adding 3% of Araldite to the powders, followed by compaction at 700 MPa, and then polymerization. A very good powder covering by the polymer layer was proven by EDX elementals maps. The influence of composition change on the electrical resistivity of the compacts was studied. Hysteresis measurements in static and dynamic fields of up to 10 kHz were recorded, showing the influence of composition and particle size on the compact properties.

Crystal, electronic and magnetic structures of a novel series of intergrowth carbometalates R4Co2C3 (R = Y, Gd, Tb).

A series of new ternary isostructural R4Co2C3 (R = Y, Gd, Tb) carbides was synthesized by annealing of arc-melted stoichiometric samples. The crystal structure of Tb4Co2C3 [space group P2/m, Pearson symbol mP18, a = 12.754(2) Å, b = 3.6251(4) Å, c = 7.0731(9) Å, ß = 105.601(6)°] was solved by direct methods from neutron powder diffraction data collected at 100 K. The room temperature unit cell parameters of the new phases were determined by X-ray powder diffraction technique. The crystal structure of Tb4Co2C3 is characterized as an intergrowth structure resulting from the stacking of alternating TbCoC (YCoC-type) and Tb2C (anti-CdCl2 type) fragments with a 2 : 1 ratio. Tb4Co2C3 orders ferromagnetically at TC = 35(1) K, whereas the isostructural Gd4Co2C3 reveals two magnetic transitions at TC1 = 82(3) K and TC2 = 13(2) K. Density functional theory (DFT) calculations confirm that the magnetic moments of the R4Co2C3 (R = Gd, Tb) carbides are exclusively due to the rare-earth elements. Y4Co2C3 is shown to be a Pauli-paramagnet by experimental and theoretical studies.

Hydrogen Insertion in the Intermetallic GdScGe: A Drastic Reduction of the Dimensionality of the Magnetic and Transport Properties.

Intermetallic phases have been investigated with respect to their ability to accept small atoms in interstitial sites without changing the host structure. Among those, the intermetallic compounds crystallizing in the tetragonal CeScSi-type structure are able to absorb hydrogen atoms. These compounds are of particular interest because they can show electride-like character and, therefore, can be exploited as new catalysts. Here we report the case of GdScGe which uptakes hydrogen at 623 K and under a H2 gas pressure between 0.5 and 4 MPa. The formation of the hydride GdScGeH, with H atoms entering into the [Gd4] tetrahedra, preserves the host structure but induces an anisotropic volume expansion with a strong increase of the c-parameter and a slight decrease of the a-parameter. Interestingly, we show for the first time for this family of materials that hydrogen insertion reduces the dimensionality of the magnetic and transport properties from 3D to quasi-2D which results in a vanishing of the ferromagnetic order ( TC = 350 K for GdScGe) and a change of the metallic conduction behavior to a nonmetallic one. As evidenced by density functional theory calculations, such drastic effects are accounted for through the Gd-H chemical bonding effect and the oxidizing effect of H whereas the volume expansion plays only a minor role.

Bond-valence model for metal cluster compounds. I. Common lattice strains.

The bond-valence model was commonly considered as inappropriate to metal cluster compounds, but recently it was shown that the model provides unique information on the lattice strains and stabilization mechanisms in (TM)6-chalcohalides (TM = transition metal in the cluster). The previous study was mainly devoted to the non-uniform distribution of the anion valences (bond-valence sums) around clusters. This and the following paper focuses on two additional phenomena: (i) a steric conflict between counter-cations and the cluster-ligand framework resulting in `common' lattice strains (this paper), and (ii) steric conflict between the small (TM)6-cluster and the large coordination polyhedron around the cluster or so-called matrix effect [the next paper; Levi et al. (2013), Acta Cryst. B69, 426-438]. It was shown that both phenomena can be well described by changes in the bond-valence parameters. The calculations were based on the structural data known to date for a variety of (TM)6-cluster compounds, Mx(TM)6Ly (TM = Nb, Mo, W and Re; M = various additional cations, L = the chalcogen and/or halogen ligands). The results were used to explain the structural peculiarities of these compounds with remarkable physical properties and the mechanisms of their stabilization.

Bond-valence model for metal cluster compounds. II. Matrix effect.

The bond-valence model was commonly considered as inappropriate to metal cluster compounds, but recently it was shown that the model provides unique information on the lattice strains and stabilization mechanisms in (TM)6-chalcohalides, Mx(TM)6Ly (TM = transition metal, L = the chalcogen and/or halogen ligands; M = counter-cation). The previous study was mainly devoted to the non-uniform distribution of the anion valences (bond-valence sums) around clusters. This and the previous paper are focused on two additional phenomena: (i) a steric conflict between counter-cations and the cluster-ligand framework resulting in `common' lattice strains [previous paper: Levi et al. (2013). Acta Cryst. B69, 419-425], and (ii) steric conflict between the small (TM)6-cluster and the large coordination polyhedron around the cluster or so-called matrix effect (this paper). It was shown that both phenomena can be well described by changes in the bond-valence parameters. This paper demonstrates that the matrix effect results in high strains in the TM-L bonds in most of the (TM)6-chalcohalides (TM = Nb, Mo, W and Re). In spite of this, the violations for the total TM valence are minimal, because the cluster stretching is fully or partially compensated by compression of the TM-L bonds. As a result, the influence of the matrix effect on the material stability is rather positive: it decreases the volume of the structural units and in many cases ensures a more favorable distribution of the bond valences around TM atoms, stabilizing the cluster compound.

Magnetic properties of the RbMnPO4 zeolite-ABW-type material: a frustrated zigzag spin chain.

The crystal structure and magnetic properties of the RbMnPO4 zeolite-ABW-type material have been studied by temperature-dependent neutron powder diffraction, low-temperature magnetometry, and heat capacity measurements. RbMnPO4 represents a rare example of a weak ferromagnetic polar material, containing Mn(2+) ions with TN = 4.7 K. The neutron powder diffraction pattern recorded at T = 10 K shows that the compound crystallizes in the chiral and polar monoclinic space group P2(1) (No. 4) with the unit cell parameters: a = 8.94635(9), b = 5.43415(5), and c = 9.10250(8) Å and ß = 90.4209(6)°. A close inspection of the crystal structure of RbMnPO4 shows that this material presents two different types of zigzag chains running along the b axis. This is a unique feature among the zeolite-ABW-type materials exhibiting the P2(1) symmetry. At low temperature, RbMnPO4 exhibits a canted antiferromagnetic structure characterized by the propagation vector k1 = 0, resulting in the magnetic symmetry P2(1)'. The magnetic moments lie mostly along the b axis with the ferromagnetic component being in the ac plane. Due to the geometrical frustration present in this system, an intermediate phase appears within the temperature range 4.7-5.1 K characterized by the propagation vector k2 = (kx, 0, kz) with kx/kz ≈ 2. This ratio is reminiscent of the multiferroic phase of the orthorhombic RMnO3 phases (R = rare earth), suggesting that RbMnPO4 could present some multiferroic properties at low temperature. Our density functional calculations confirm the presence of magnetic frustration, which explains this intermediate incommensurate phase. Taking into account the strongest magnetic interactions, we are able to reproduce the magnetic structure observed experimentally at low temperature.

A study of the high temperature spin reorientation in YCoFe(3)B.

The iron-57 Mössbauer spectra of YCoFe(3)B have been measured between 4.2 and 480 K and reveal that YCoFe(3)B exhibits an axial orientation of the iron magnetic moments below 450 K and a basal orientation above 450 K. This spin reorientation, also observed in the thermomagnetic curves, results from the different signs of the contributions to the magnetic anisotropy of the 2c and 6i sites that are occupied by iron. The neutron diffraction patterns of YCoFe(3)B have been measured at 2 K and between 290 and 770 K and have been successfully analyzed with a model compatible with the magnetic orientation obtained from the Mössbauer spectra. The hybridization between the cobalt or iron 3d orbitals and the boron 2p orbitals leads to a larger magnetic moment and hyperfine field on the 2c site as compared to the 6i site.

Data assimilation for short range atmospheric dispersion of radionuclides: a case study of second-order sensitivity.

This article presents the application of variational data assimilation to a simple Gaussian plume model for radionuclides. Adjoint modeling is applied to the model in order to minimize discrepancies between contamination observations and model outputs. The interest of such an approach is to get a better estimation of some parameters such as emissions or dispersion parameters. A second-order analysis is also performed to assess the sensitivity of the optimized parameters to some poorly known parameters. Sensitivity with respect to network design is also done.