T. Riesterer

Orthorhombic structure of γ-TiFeD≈2☆

Abstract The crystal structure of the hydrogen storage system γ-TiFeD1.94(3) was re-investigated by neutron powder diffraction profile analysis at room temperature and 113 ± 17 bar D2 pressure. In contrast with previous work which assumed a structure of monoclinic symmetry a structure of orthorhombic symmetry was assumed: space group Cmmm; lattice parameters a = 7.029(4) A , b = 6.233(2) A , c = 2.835(1) A ; titanium in equipoint 4h (x, 0, 1 2 ), x = 0.223(2); iron in 4i (0, y, 0), y = 0.2887(7); D1 in 4e ( 1 4 , 1 4 , 0); D2 in 2c ( 1 2 , 0, 1 2 ); D3 in 2a (0, 0, 0); agreement values after profile refinement Rwp = 0.160, RI = 0.085, RF = 0.060; ( sin θ λ ) max = 0.59 A − 1 ; 85 inequivalent contributing reflections hkl; 10 refined structural parameters. Refinements based on the less symmetric monoclinic model yield virtually the same fit and structure but require four more structural parameters. The structure is characterized by three types of octahedral metal atom interstices; two of type Ti4Fe2 occupied by deuterium atoms D1 and D3 and one of type Ti2Fe4 occupied by deuterium atom D2. The former are 100% occupied and the latter are 91% occupied. The shortest interatomic distances are Fe − D1 = 1.77 A , Ti − D2 = 1.95 A and D1 − D3 = 2.35 A .

Recent experimental results on the electronic structure of binary and ternary hydrides

Abstract Recent experimental results on the electronic structure of hydrides of palladium, the rare earths, niobium and magnesium, of the intermetallic compounds LaNi5, FeTi and Mg2Ni, of the zirconium-based Laves phase compounds and of a few glassy metals are reviewed and compared with results of band structure calculations. The review focuses on results obtained by photoelectron and X-ray spectroscopy methods. Emphasis is placed on missing data to stimulate further investigations.

Hydrogen induced changes of valency and hybridization in Ce intermetallic compounds

Abstract The cerium intermetallic compounds CeRu 2 and Ce 2 Fe 14 B both readily absorb hydrogen at normal conditions of temperature and pressure. The two systems exhibit fundamental changes in their physical properties corresponding to the transformation from non-magnetic α-type cerium to magnetic γ-type cerium with a localized magnetic moment. Magnetic measurements, neutron diffraction experiments and X-ray photoelectron spectroscopy give evidence of such transitions.

The comparison of the surface properties of FeTi and Fe2Ti4Ox in view of the different hydrogen sorption behaviours

Abstract FeTi and the oxygen-stabilized compound Fe 2 Ti 4 O x are known to form reversible metal hydrides. However, whereas samples of Fe 2 Ti 4 O x which have been exposed to air readily react with gaseous hydrogen at room temperature, FeTi samples have to be activated. We analysed the surface properties of Fe 2 Ti 4 O x and of FeTi before and after activation by means of photoelectron spectroscopy. A high pressure cell inside the spectrometer allowed us to analyse the samples immediately after the high pressure high temperature activation, thus avoiding a transfer in air. Elemental iron(0) and titanium(IV) are formed on the activation of FeTi; we have no evidence for the formation of TiH x , Fe 2 Ti 4 O x or FeTiO x . Significant differences between the oxidation behaviour of Fe 2 Ti 4 O x and that of FeTi fractured in an ultrahigh vacuum were found at room temperature.

The electronic structure of Zr(V1−xCrx)2 laves phase compounds and their hydrides studied by photoelectron spectroscopy

Abstract We report X-ray photoelectron spectroscopy core level and valence band spectra of the cubic C15 Laves phase compounds Zr(V 1− x Cr x ) 2 ( x = 0, 0.5, 1) and their hydrides together with UV photoelectron spectra of ZrV 2 and ZrCr 2 . Comparison of the UV photoelectron spectroscopy data with recent band structure calculations for ZrV 2 suggests that the difference in the stability of the hydrides of ZrV 2 and ZrCr 2 is due to an upward shift of the Fermi level by 0.65 eV. The metal core level shifts are proportional to the hydrogen content of the sample. Measured core level shifts normalized by the hydrogen content are found to have a characteristic value for each of the three elements in the system investigated.

On the hydrogen site occupation in hydrides of intermetallics

Abstract We present calculations for the site occupation in some ternary hydrides based on an extended version of the model of “imaginary binary hydrides”. We obtain reasonable agreement with structural data for the ZrV 2 H x system. However, the results obtained for other metal-hydrogen systems suggest that this agreement could be fortuitous. We attempt to find some reasons for the failure of the model applied here.

Photoemission study of the electronic structure of Zr-Pd compounds and their relation to hydride formation☆

Abstract Zirconium, palladium and Zr2Pd are known to form stable hydrides, whereas ZrPd2 does not. We have used photoelectron spectroscopy to study the electronic properties of Zr2Pd and ZrPd2 and have compared the results with spectra obtained from zirconium and palladium as well as with spectra of glassy and thin film Zr-Pd alloys. In ZrPd2, the Pd 4d band is full and is shifted away from EF so that EF falls into a region of low density of states. Accordingly, hydride formation requires a rather large shift of EF to accommodate part of the additional electron, which makes a strongly endothermic contribution to the heat of hydride formation.

UV inverse photoemission study of the empty density of states of the dihydrides of yttrium, lanthanum and cerium☆

Abstract We present UV-BIS spectra ( hv = 9.7 eV) of YH 1.9 , LaH 1.8 and CeH 2.1 . These compounds are metallic and they exhibit a non-vanishing density of states at E F . Two weak, d-derived structures around 1.0–1.5 eV and 2.5–3.0 eV above E F are observed as common features. In the LaH 1.8 spectrum a prominent peak is seen at 5.2 eV. Because the cross section for transitions into empty 4f states is very small at this low photon energy we believe that these are states with considerable 4f-valence band mixing. A relation to earlier core level results is briefly discussed.

Structure of the hydrogen arrangement around a positive muon in α′-PdHx☆

Abstract Positive muon μ+ spin rotation measurements have been performed in a single-crystal PdH0.74 sample between 20 and 150 K. The μ+ depolarization rate σ has been measured as a function of crystal orientation on a magnetic field. With increasing temperature a strong reduction in the σ anisotropy occurs, whereas only a moderate decrease in average depolarization rate is observed. It is assumed that protons and muon occupy octahedral interstitial sites in the palladium lattice. The data can be fitted by a decreasing proton occupancy of the octahedral sites nearest the μ+ with increase in temperature. The proton occupancy of the remaining octahedral sites at all temperatures, as well as the occupancy of the proton sites nearest the μ+ at 20 K, is compatible with the bulk nominal hydrogen concentration x = 0.74. It appears that at 20 K the μ+ would stay at a location with “regular” nearest-neighbour proton occupancy, but that at higher temperatures diffusion or reordering would allow the μ+ to find an environment with less nearest-neighbour protons.