Which intermetallic compounds of transition metals form stable hydrides ?

Flux Growth of an Intermetallic with Interstitial Fluorides via Decomposition of a Fluorocarbon

La₁₅(FeC₆)₄F₂ was grown as large crystals by reacting iron in a La/Ni eutectic flux in the presence of decafluorobiphenyl (C₁₂F₁₀) which acts as both a carbon and fluoride source. This mild fluorinating technique enables the isolation of an intermetallic product containing fluoride interstitials, as opposed to forming ionic metal fluorides. The compound adopts a structure in the hexagonal crystal system with space group P6̅ which features FeC₆ units composed of a central iron atom coordinated by three ethylenide units in a trigonal planar configuration. The structure is related to the previously reported La₁₅(FeC₆)₄H, but with fluoride fully occupying the interstitial hydride positions, which induces partial occupancies and site splitting disorder in the adjacent layers of lanthanide ions. No supercell formation is observed.

Structural and In Situ X-ray Diffraction Study of Hydrogenation of CaxMg1−xNi2 (0 ≤ x ≤ 1)

In the quasi-binary system CaNi2-MgNi2 solid-solutions CaxMg1−xNi2 (0 ≤ x ≤ 1) were prepared from the elements. They crystallize in the hexagonal Laves phase type (MgNi2, C36) for x ≤ 0.33 (P63/mmc, a = 482.51(7) pm, c = 1582.1(3) pm for x = 0, a = 482.59(3), c = 1583.1(1) for x = 0.33) and in the cubic Laves phase type (MgCu2, C15) for 0.33 < x (Fd3¯m, a = 697.12(3) pm for x = 0.5, a = 705.11(2) pm for x = 0.67, a = 724.80(2) pm for x = 1). After hydrogenation in an autoclave the X-ray diffraction patterns changed completely. Reflections assigned to CaNiH3, and Ni and Rietveld refinement confirmed this. The hydrogenation properties of CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds were also studied in situ by X-ray powder diffraction. In situ X-ray powder diffraction of CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds under 0.3 MPa hydrogen gas flow (15 sccm), data collected on a Rigaku SmartLab diffractometer in an Anton Paar XRK 900 Reactor Chamber using Cu-Kα1 radiation. Scanning electron microscopy and EDX spectroscopy confirmed the entitled materials and elemental composition, respectively. From the Transmission electron microscopy and Selected area electron diffraction concluded that the CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds were crystalline.

Hydrogen order in hydrides of Laves phases

Many Laves phases AM 2 takes up hydrogen to form interstitial hydrides in which hydrogen atoms partially occupy A 2 M 2, AM 3, and/or M 4 tetrahedral interstices. They often exhibit temperature-driven order-disorder phase transitions, which are triggered by repulsion of hydrogen atoms occupying neighboring tetrahedral interstices. Because of the phase widths with respect to hydrogen a complete ordering, i.e., full occupation of all hydrogen positions is usually not achieved. Order-disorder transitions in Laves phase hydrides are thus phase transitions between crystal structures with different degrees of hydrogen order. Comparing the crystal structures of ordered and disordered phases reveals close symmetry relationships in all known cases. This allows new insights into the crystal chemical description of such phases and into the nature of the phase transitions. Structural relationships for over 40 hydrides of cubic and hexagonal Laves phases ZrV2, HfV2, ZrCr2, ZrCo2, LaMg2, CeMg2, PrMg2, NdMg2, SmMg2, YMn2, ErMn2, TmMn2, LuMn2, Lu0.4Y0.6Mn2 YFe2, and ErFe2 are concisely described in terms of crystallographic group-subgroup schemes (Bärnighausen trees) covering 32 different crystal structure types, 26 of which represent hydrogen-ordered crystal structures.

Structure and properties of a novel boride: ThNi12B6

Characterization of the crystal structure and physico-chemical behaviour of ThNi₁₂B₆clearly documents the two-dimensional character of Fermi surfaces originating from 3³4²-Ni-nets.

High resolution electron microscopy of amorphous interlayers between metal thin films and silicon

High-resolution electron microscopy of amorphous interlayers (a-interlayer) formed by solid-state diffusion between metal thin films and silicon is reviewed. In this paper, an overview of the development is presented. Pertinent data obtained on the growth kinetics and structure of a-interlayers in polycrystalline metal thin films on single-crystal silicon are reported. For the Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si and Ta/Si systems, the growth of a-interlayer was found to follow a linear law in the initial stage. Si atoms were found to be the dominant diffusing species in the solid phase amorphization in the Ti/Si, Zr/Si, and Hf/Si systems. For the Y/Si system, the stability of amorphous interlayer depends critically on the composition of the amorphous films. Auto-correlation function analysis was utilized to determine the structure of the amorphous interlayers. HRTEM in conjunction with the fast Fourier transform were applied to determine the first nucleated crystalline phase. Simultaneous presence of multiphases was observed to occur in a number of refractory metal/Si systems.