S. Obbade

Hydrogen in hard magnetic materials

Abstract The rare-earth/transition metal alloys exhibiting high permanent magnet properties are able to reversibly absorb significant amounts of hydrogen. This affects markedly the fundamental characteristics of such intermetallics, and can be used as a probe to understand better the basis of the magnetic interactions. In addition, structural, mechanical and chemical aspects of the hydrogenation process are used to improve the synthesis of appropriate powders for the magnet technologies.

Precise crystal and magnetic structure determinations. Part I: a neutron diffraction study of Nd2Fe14B at 20 K

Nd2Fe14B is the well known basis of the best performing hard magnets. It exhibits a spin reorientation transition (SRT) at 140 K, where the easy axis magnetisation turns to a canted magnetic structure at low temperature. An accurate neutron diffraction experiment performed on a single crystal, allows us to precisely determine the low temperature non-collinear magnetic structure and to compare it with the Ho parent compound. Moreover, a lowering of the crystal symmetry (space groupCm) is shown to take place in addition to the SRT. High magnetostrictive forces and negative NdNd interactions make possible such a crystal and magnetic symmetry lowering, which is complicated by the SRT induced by high order crystal electric fields terms.

A precise crystal structure determination. Part II: an X-ray four-circle study of Nd2Fe14B at 20 and 290 K

The crystal structure of Nd2Fe14B has been analysed at 20 and 290 K by using the four-circle X-ray diffraction technique. The room temperature tetragonal structure (space groupP42tmum) has been accurately determined. At low temperature, the monoclinic distortion (space groupCm) proposed from a recent neutron diffraction experiment has been fully confirmed.

About hydrogen insertion in ThMn12 type alloys

Abstract The absorption of hydrogen in ternary RFe 12− x M x (R=Y or rare earth, M=Ti, Mo) alloys has been investigated by neutron diffraction. These compounds crystallise in the tetragonal (I4/mmm) structure of ThMn 12 type. In this work we report structural and magnetic investigations undertaken by neutron diffraction for some hydrides with high hydrogen contents and we focus on the location of hydrogen in Ti and Mo containing materials. The effects of hydrogenation on the intrinsic properties such as enhancement of Curie temperature, lattice parameters, Fe moment as determined by neutron diffraction, magnetic and Mossbauer measurements, are discussed.

Ultrafast Hydro-Micromechanical Synthesis of Calcium Zincate: Structural and Morphological Characterizations

Calcium zincate is a compound with a large panel of application: mainly known as an advantageous replacement of zinc oxide in negative electrodes for air-zinc or nickel-zinc batteries, it is also used as precursor catalyst in biodiesel synthesis and as antifungal compound for the protection of limestone monuments. However, its synthesis is not optimized yet. In this study, it was elaborated using an ultrafast synthesis protocol: Hydro-Micromechanical Synthesis. Two other synthesis methods, Hydrochemical Synthesis and Hydrothermal Synthesis, were used for comparison. In all cases, the as-synthesized samples were analyzed by X-ray diffraction, scanning electron microscopy, and LASER diffraction particle size analysis. Rietveld method was used to refine various structural parameters and obtain an average crystallite size, on a Hydro-Micromechanical submicronic sample. X-ray single crystal structure determination was performed on a crystal obtained by Hydrochemical Synthesis. It has been shown that regardless of the synthesis protocol, the prepared samples always crystallize in the same crystal lattice, with space group and only differ from their macroscopic textural parameters. Nevertheless, only the Hydro-Micromechanical method is industrially scalable and enables a precise control of the textural parameters of the obtained calcium zincate.

Synthesis and Structural, Electrical, and Magnetic Properties of New Iron–Aluminum Alluaudite Phases β-Na2Ni2M(PO4)3 (M = Fe and Al)

Herein we report the studies of different physical properties (structural, magnetic, thermal, morphologic, electrical, and electrochemical) of two new allotropic β-Na2Ni2M(PO4)3 (NNMP) phosphates, with M = Fe and Al. Pure orthorhombic single-phase powders were prepared under air, using an autocombustion synthesis method. They crystallize in the orthorhombic Imma space group with similar unit cell parameters [a = 10.1592(2), b = 13.0321(3), c = 6.4864(2) Å] and [a = 10.3993(1), b = 13.1966(1), c = 6.4955(1) Å] for β-Na2Ni2M(PO4)3 (NNAP) and β-Na2Ni2Fe(PO4)3 (NNFP), respectively. Crystal structures of both compounds were determined using X-ray powder diffraction and Rietveld method refinements, which indicate the occurrence of Ni2+ in the 8g site, and of M3+ in the 4a site of the structure. The structure consists of a three-dimensional anionic framework obtained by the association on MO6, NiO6, and PO4 polyhedra, sharing edges and corners. The resulting three-dimensional structure creates monodimensional channels along the [100] and [010] directions formed by face-shared oxygen polyhedra and occupied by Na+ cations. This nondisordered cationic distribution is confirmed by a significant change of magnetic properties. Thus, both NNAP and NNFP samples show paramagnetic to ferromagnetic transition at 14 and 19 K, respectively. For the two compounds, thermal stability, electrical conductivity, and electrochemical properties have been also investigated. The intercalation/desintercalation properties of NNMP compounds as positive electrode were tested in sodium-ion batteries. The first cycling curves exhibit a significant polarization for both prepared samples.

Structural chemistry of uranium vanadates: from 2-D to 3-D networks

Publisher Summary This chapter deals with the crystal chemistry of uranium vanadates. V5+cation often has tetrahedral coordination and forms compounds that are isotypic to phosphate and arsenate analogs. In its association with the (UO2)2+uranyl ion, vanadium adopts different coordination environments resulting in interesting compounds that do not have equivalents among phosphates and arsenates. Uranium, the heaviest natural element, is also the most widely studied actinide because depleted uranium is easy to manipulate; its production is essential for nuclear industry; it is an important constituent of nuclear waste; in material science, U compounds have possible applications as ion exchangers, ionic conductors, selective oxidation catalysts or storage materials for radionuclides; it occurs in several oxidation states; and hexavalent uranium has a propensity to display several coordination environments.