L Schlapbach

New structure results for hydrides and deuterides of the hydrogen storage material Mg2Ni

X-ray and neutron diffraction experiments performed on Mg2Ni(H,D)x (0 ⩽ x ⩽ 3.9) confirmed the existence of a structural phase transformation at about 235 °C. The high temperature phase (a = 6.49 A, space group Fm3m) has an antifluorite-type metal structure in which the deuterium atoms surround the nickel atoms octahedrally in a disordered manner (D−Ni =1.47 A, D−Mg = 2.30 A). Refined atomic parameters of Mg2Ni as well as absorption and desorption isotherms for the deutende and hydride phases are reported.

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 .

X-ray photoelectron spectroscopy studies of the electronic structure of the dihydrides and trihydrides of lanthanum and cerium

Abstract We analysed the valence band and the 3d and 4d core levels of LaH x ( x = 0, 2.2, 2.9) and CeH x ( x = 0, 2.1, 2.9) using X-ray photoelectron spectroscopy. A hydrogen-induced band around 5 eV is observed in all the hydrides. The density of states at E F is considerably reduced in the dihydrides and vanishes completely in the trihydrides in agreement with the transition from metallic to non-metallic behaviour. The cerium 4f level appears at 2 eV and is separated from the hydrogen-induced band for both cerium hydrides. The character and occupancy of the 4f levels of cerium is almost conserved in the hydrides as is confirmed by measurements of the magnetic susceptibility. The 3d core levels are shifted by 1.3 eV (LaH 2.2 ), 2.9 eV (LaH 2.9 ), 1.9 eV (CeH 2.1 ) and 2.1 eV (CeH 2.9 ) and exhibit intense satellites on the low binding energy side.

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.

Surface properties of hydride-forming AB2 compounds

Abstract The surface properties of ZrMn 2 and TiMn 2−x ( x = 0, 0.5) were investigated using X-ray photoelectron spectroscopy because of the readiness of hydrogen sorption of many AB 2 compounds. The concentration of the components and their chemical state were analysed in samples which had been fractured in an ultrahigh vacuum, exposed to increasing doses of oxygen and air and subjected to hydrogen absorption and desorption. It is concluded that surface segregation accounts for the ease with which hydrogen is absorbed. Manganese, but not zirconium or titanium, segregates and is preferentially oxidized. Zirconium and titanium are assumed to form metallic precipitates on the subsurface below the oxidized manganese. The precipitates and the subsurface can catalyse the H 2  2H reaction. Furthermore the precipitates themselves may form hydrides and spill over atomic hydrogen to the intermetallic.

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.

Magnetic properties of cerium hydrides at low temperatures

Abstract The magnetic properties of CeHx were investigated in the concentration range 1.8

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.

Low temperature electronic properties of cerium hydrides

Abstract We have analysed the low-temperature electronic properties of CeH x of compositions close to the metal-semiconductor transition by measuring the specific heat (1.5 to 15 K) and photoelectron spectra (25 to 300 K) at different energies ( h ω = 25 to 55 eV and 115 to 122 eV ) using synchrotron radiation. Two important findings result from our investigations. Firstly, above 5 K the coefficient γ of the electronic specific heat of CeH 2.65 is fairly large (110 mJ mol −1 K −2 ) as compared with that of the corresponding lanthanum hydride (not greater than 0.04 mJ mol −1 K −2 ) suggesting heavy-electron type behaviour. An unambiguous demonstration of heavy-electron behaviour is impeded by the occurrence of magnetic order below 4 K. Secondly, a strong peak of about 1 eV FWHM appears at E F below 70 K in the photoelectron spectra. From the energy dependence and from the presence of a similar peak in LaH 2.7 we conclude that the peak is not caused by f electrons. Evidence for a surface semiconductor-metal transition due to the formation of cerium dihydride at low temperature at the surface is given.