5d Transition Elements Research Articles

Roles of the hydride forming and non-forming elements in hydrogen storage alloys

Abstract The electronic structures of small octahedral model clusters containing hydrogen, 3d, 4d, 5d transition and non-transition elements are investigated by the DV-Xα molecular orbital method. It is found that hydrogen makes a strong chemical bond with the hydride non-forming elements, B, as long as the hydride forming elements, A, exist in the neighborhood. This is a reason why hydrogen is located preferentially near the hydride non-forming elements in many hydrides. Also it is suggested that the ratio of the A–B bond strength to the A–A bond strength lies in a certain range for conventional hydrogen storage A–B alloys.

An electronic approach to the hydrogen overpotential for Zr alloys

Abstract A theory proposed for understanding the alloying effect on the hydrogen overpotential for transition-metal based alloys was further confirmed in Zr alloys from a series of experiments and electronic structure calculations. The hydrogen overpotential was measured of Zr binary alloys containing a constant amount (3 mol%) of 3d, 4d and 5d transition elements. It changed largely with alloying transition elements, M. Such changes in the hydrogen overpotential resembled the characteristic changes observed for the elements, M, to be in the pure metal state. These experimental results could be explained using the calculated ionicity of M in Zr by the DV–Xα cluster method. All the alloying elements except for Hf, had the negative ionicity due to the electron transfer from the base Zr metal to M. The negatively-charged alloying elements acted as the preferable site for the hydrogen evolution reaction, resulting in the corresponding changes in the hydrogen overpotential with alloying elements. Thus, the proposed theory was found to be valid for Zr alloys.

Charge-transfer transitions of 4d- and 5d-transition elements in garnets

Electronic state calculations of the (M 4+O 6 2−) 8− octahedral cluster ( M = 4d- and 5d-transition elements) were performed by an unrestricted SCF SW-Xα method. In order to verify the calculations, single crystalline films of (Gd, Ca 2+) 3(Ga, M 4−) 5O 12 were prepared by a liquid-phase epitaxy (LPE) method onto (1 1 1)-oriented Gd 3Ga 5O 12 (GGG) substrates, and their optical absorption spectra were measured in the range 0.2–0.9 μm. The absorption spectra are assigned to the charge-transfer transitions from O 2− 2p to M 4− d-orbitals or t 1u(2p) → t 2g *(d) and t 2u(2p) → t 2g *(d) transitions.

New transuranium intermetallic compounds with ThCr 2Si 2- and CaBe 2Ge 2-type structures

The crystallographic data for 24 new RMt 2× 2 compounds (R  Th, Np, Pu; Mt  3d, 4d or 5d transition element from the VIa-IIb families; X  Si, Ge) crystallizing with the ThCr 2Si 2- or CaBe 2Ge 2-type structure, are presented. The variation in the crystallographic parameters through this family of actinide compounds is discussed. The average atomic volume V at and the c/a ratio present a quite singular variation in the cases of Ru and Pt compounds. These parameters seem to be useful as a rough indication in the selection of compounds with unusual physical properties.

ELASTIC CONSTANTS OF d TRANSITION ELEMENTS AND d TRANSITION ALLOYS

The shear elastic constant, C′, is calculated from first principles for the cubic 4d and 5d transition elements. This study also includes calculations for selected alloys using the virtual crystal approximation. The tetragonal shear constant for these elements and alloys is found to follow a trend which can be related to the calculated crystal structure stabilities. In fact, the trend of C′ behaves roughly as the the trend displayed by the energy difference between the fcc and bcc crystal structures. The theoretical results are generally in ~90% agreement with experiment for the tetragonal shear constant and this implies indirectly that the discrepancy between theory and experiment found for the crystal energies do not lie in the theoretical data.

5d transition and 4f rare earth elements in II–VI semiconductors as luminescent centres and probes in diagnostics of implanted layers

Luminescence of implanted Ta, W transition (5d) and Pr, Nd, Er and Tm rare earth (4f) element centres was studied in the II–VI semiconductors CdTe, CdSe, CdS and ZnSe. The radiative transitions were identified in Tanabe-Sugano crystal field theory diagrams as electron transitions between levels with different spins. Impurity-defect complexing was examined in thermal and laser annealing processes.

Deep Levels due to 4d- and 5d-Transition Element Impurties in III-V Semiconductors

Deep Levels due to 4d- and 5d-Transition Element Impurties in III-V Semiconductors

Lattice relaxation in silicon doped with 4d and 5d transition metals

Photoionization cross-section spectra from deep centers in silicon doped with technologically important 4d and 5d transition elements were analyzed by the Ridley and Amato lattice coupling model to determine threshold energy and lattice relaxation parameters corresponding to optically induced transitions involving either band. The average optic phonon energy is 50 meV. Electron transitions to the conduction band from the silver, platinum, and gold acceptor centers have, respectively, threshold energies (in meV) ET0 =550,226, and 570. For silver and gold, the Huang–Rhys parameter S could not be determined because of a mixture of both allowed and forbidden transitions; for platinum, S=0.3. Hole transitions from the valence band to the same centers have, respectively, ET0 =580, 905, 590, and S=1.3, 0.5, 0.8. Hole transitions from the valence band to the donor centers of these elements are, respectively, ET0 =340, 320, 335 and S=1.2, 1.4, 0.4. ET0, and S values are uncertain to within ±5 meV and ±0.05, respectively. Electron transition data from the donor centers of these elements to the conduction band are not available or insufficient to allow analysis of the threshold region.

Resistance to oxidation of NiAl alloys at high temperatures

The heat resistance of NiAl alloys with 4d- or 5d-transition element additives (zirconium, hafnium, niobium, tantalum or tungsten) has been studied at high temperatures: 1373, 1473 and 1573 K. Some regularities for the effect of 3d-, 4d- and 5d-transition element additives on NiAl oxidation have been found. The oxidation kinetics are described by linear and square laws. The boundary concentrations of additives stabilizing the resistance to oxidation of NiAl alloys have been found. The relation between alloy substructures and change in heat resistance is discussed.

Photoionization cross-section studies of the platinum-donor center in silicon

The relative photoionization cross section of the platinum donor center in silicon was measured over the wavelength range of 2.4 to 3.9 μm by electrical deep-level optical spectroscopy on an n+p junction at 80 K. The data were analyzed in terms of the lattice-coupling model proposed by Ridley and Amato which was modified for valence-band nonparabolicity. Good agreement was obtained between the experimental results and the model calculations of the cross section with the energy level of the donor at 0.320±0.005 eV above the valence-band edge and a Huang–Rhys factor S of approximately 1.4. This S value corresponds to a Franck–Condon energy shift of 70 meV with a phonon energy of 50 meV. Previously reported photoionization data of the gold donor were also fit by the same model, yielding S≂0.4, a surprisingly small value. Estimates were made of the majority-carrier capture cross section for these two levels and for the platinum acceptor center in silicon, which was measured previously. These estimates, based on Ridley’s quantum-defect model and our measured S values, are several orders of magnitude smaller than the corresponding measured values, indicative of the complex nature of these 5d-transition elements in silicon. More detailed models, perhaps including anharmonicity of the defect vibrations, are required to understand these large capture cross sections.

Residual resistivities of 4d and 5d impurities in aluminium

Systematic experiments are reported on residual resistivities of Al-based dilute alloys of 4d and 5d transition elements. For all the dilute alloys investigated, there is no temperature dependence of the residual resistivities over the range 1.5-4.2K. It is found that the residual resistivities of 4d impurities agree with those of 3d and that the residual resistivities of 5d impurities are smaller than those of 3d. The experimental results are explained by incorporating the spin-orbit coupling on the impurity atom into Friedel's resonance model.

Magnetic hyperfine fields at Ta in Co and Fe hosts measured by the e?-? time-differential perturbedangular-correlation method

The radioactivity181Hf was implanted into pure Co and Fe hosts with the help of an isotope separator. The ferromagnetic hosts produce very strong magnetic hyperfine interactions at the nuclear site of Ta atoms. These hyperfine interactions were studied by time-differential measurements of the 133 keV conversion electron — 482 keV γ angular correlation. It is found that in both hosts an appreciable fraction of Hf atoms occupies regular lattice sites after the implantation. Using the knowng-factor of the 482 keV state of181Ta the magnetic hyperfine fieldsHhf=±362.4(5.0) and ±596(18) kG for the Co and Fe hosts, respectively, were deduced These fields fit nicely into the systematics of the hyperfine fields for the 5d transition elements but are not well accounted by the existing theoretical models.

Contributions to the Chemistry of the Binary Compounds of the Transition Elements

AbstractPhase and structural relationships of the sulfur, selenium, and tellurium compounds of the 4d and 5d transition elements of groups IV to VII of the periodic system are discussed. Homologous elements behave very similarly with respect to the chalcogens, and this is particularly the case for niobium and tantalum, and for molybdenum and tungsten. However, zirconium, niobium, and molybdenum have a greater tendency towards formation of chalcogen‐poor phases than their homologues hafnium, tantalum, and tungsten. Subchalcogenides are known only for zirconium and niobium. The number of phases and the tendency towards formation of solid solutions are considerably smaller among the tellurides than among the sulfides and selenides. The crystal structures of the telluride phases also differ from those of the sulfide and selenide phases of analogous composition. In addition, a review of the phase and structural relationships of the arsenic and antimony compounds of the 4d and 5d transition elements of groups V to VII is given.

On the magnetic properties of covalentXY6complexes

An account is given of present ideas about the electron orbits in octahedral complexes containing 4dand 5dtransition elements, with particular reference to paramagnetic resonance. The complex is treated by the method of molecular orbitals with the assumption that the magnetic carriers are partly in centrald-orbits and partly inpπ-orbits round the outer nuclei. This is shown to lead to a reduction in the orbitalg-factor and to the possibility of observing the hyperfine structure from the outer nuclei, thus testing the initial assumptions.

Paramagnetic Resonance and Covalent Bonding

IN the course of a programme of investigating paramagnetic resonance in compounds containing 4d and 5d transition elements, we have recently found an unusual hyperfine structure in ammonium chloroiridate, which gives information about the bonding in this compound. The essential experimental result is that in a crystal of (NH4)2 [IrCl6], diluted with the isomorphous diamagnetic platinum salt, the resonance spectrum at a temperature of 20° K. and wave-length of 1 cm. shows a complex hyperfine structure consisting of many lines, of which sixteen equally spaced lines can be resolved in the most favourable directions, with overall separation of about 150 gauss. At first sight one would expect only four lines corresponding to the known nuclear spins (I = 3/2) of iridium-191 and 193. (The nuclear magnetic moments are approximately equal.) It is possible to explain the presence of the additional lines as arising from the chlorine-35, 37 (I = 3/2) nuclei, and the detail of the spectrum by assuming that the hyperfine structure of chlorine is one-third of that of the iridium.