D. Shaltiel

The deuterium site occupation in ZrV2Dx as a function of the deuterium concentration

Abstract The distribution of the deuterium atoms in the deuterated Laves-phase ZrV 2 D x was measured by neutron powder diffraction for deuterium concentrations x of 1.5, 2.8 and 4.9. At low deuterium concentrations only the tetrahedral interstices formed by two zirconium and two vanadium atoms are occupied whereas at medium deuterium concentrations the interstices formed by one zirconium and three vanadium atoms are also populated. At high deuterium concentrations the occupancy of the latter interstices exceeds that of the former. The interstices formed by four vanadium atoms always remain empty. The results are analysed in the light of a cellular model of hydride stability. Good agreement is found between the calculated and the observed deuterium site occupancies.

ESR in new high temperature superconductors

Abstract We report on ESR experiments on the 90K superconductor YBa 2 Cu 3 O 9−y . We observed the spectrum at X band as function of temperature down to 10K. At room temperature a weak line was observed which, on cooling, disappears around 60K. The striking result is the appearance of a broad Dysonian line below about 55K whose intensity grows exponentially with decreasing temperature. In samples doped with 1% of Gd the room temperature Gd line is split by the crystal field and narrowed by the Koringa mechanism. It again disappears at about 60K. Below 55K the spectrum is similar to that observed in the undoped sample. Some tentative interpretation regarding the magnetic behaviour in the superconducting state is offered.

Phase relations and hydrogen absorption of neodymium-iron-(boron) alloys

AbstractBased on an investigation of the phase correlations in the system Nd-Fe-B, a complete study of the hydrogen absorption of Nd-Fe-B alloys is presented, mainly concerning the permanent magnet material Nd2Fe14 B. Absorption isotherms of the compound Nd2Fe14 B and of the master alloy for permanent magnet production, Nd15Fe77B8, have been recorded and their hydrogenation behaviour is discussed. Thermal desorption spectra support the conclusion that neodymium hydride is the second hydride phase in the hydrogenated master alloy. In the Nd-Fe binary system, Nd2Fe17 was confirmed as the only equilibrium phase at 870 and 1170 K. The properties of a new ferromagnetic ternary hydride Nd2Fe17Hx,x = 0 to 5, with the Th2Zn17 type structure, space group

Hydrogen absorption in (ZrxTi1−x)B2 (B Cr, Mn) and the phenomenological model for the absorption capacity in pseudo-binary laves-phase compounds

R\bar 3m

On the occupation of interstitial sites by hydrogen atoms in intermetallic hydrides: A quantitative model

, are reported.

Effect of hydrogen absorption on the magnetic properties of some Zr-3d laves phase binary and pseudobinary intermetallics

Thermal desorption spectroscopy (TDS) of hydrogen from metal hydrides is a technique where the rate of hydrogen desorption is measured as a function of temperature while the temperature is increased linearly with time. The authors present the TDS of a powdered PdHx (particle size approximately 2 mu m) for various initial concentrations and for various heating rates. The spectra develop from a single peak (A) and a small shoulder curve at low initial H concentrations to two peaks (A,B) and a shoulder at high initial concentration (x equivalent to 0.9). The peaks are correlated to the phase diagram-the peak A to the alpha + beta region, the peak B to the beta region and the shoulder to the alpha region. Activation energies for desorption and preexponential factors are derived from the Arrhenius-like parts that the spectra exhibit in their onset regions and in the region alpha + beta . The results are discussed in terms of a model in which the desorption rate is determined by the rate of the recombination process H+H to 2 at the surface.

Unusual behavior of the Gd ESR in single crystals of GdyY1−yBa2Cu3O6+x with x=0.1−0.8 and y=0.03−0.06: Evidence for magnetic interaction in the superconductors

Abstract The hydrogen absorption of the systems (Zr x Ti 1− x )B 2 ( B  Cr , Mn; 0 ⩽ x ⩽ 1) was investigated as a function of x to check a previously proposed phenomenological model for the hydrogen capacities of Lavesphase AB 2 pseudo-binary compounds. The measurements were performed at pressures between 40 and 300 atm at room temperature and at liquid nitrogen temperatures. At 40 atm and 80 K the absorption capacity of the pseudo-binary compounds (Zr x Ti 1− x )Cr 2 increases linearly from 3.3 H atoms (formula unit (f.u.)) −1 in TiCr 2 to 4.5 H atoms (f.u.) −1 in ZrCr 2 . We show that the linear dependence of the hydrogen capacity in this system is in agreement with the phenomenological model, in which we consider a cluster composed of an A atom and its 12 B-type nearest neighbours. The maximum absorption capacity N H of the system (Zr x Ti 1− x )Mn 2 steeply increases from N H = 0.3 H atoms (f.u.) −1 for x = 0 to N H = 3.65 H atoms (f.u.) −1 for x = 0.4; it stays practically constant at approximately 4 H atoms (f.u.) −1 in the range 0.6 ⩽ x ⩽ 1. The non-linear dependence of N H on x in this system was accounted for in our model by taking into consideration the secondnearest neighbours (4A atoms) to an A atom in the Laves AB 2 compounds. The agreement between our experimental results and our model further emphasizes the importance of near-neighbour effects on the hydrogen absorption capacity in intermetallic compounds.

Thermal desorption spectra of hydrogen in HfV2Hx and ZrV2Hx

An overview of the kinetic properties in magnesium-rich systems is proposed in this report and our results on “macroscopic” and “microscopic” diffusion in MgH2 and in a mixed system of 12MgH2 + LaH2.8 are presented. We have obtained the diffusion coefficient D of the hydrogen in the MgH2 phase using nuclear magnetic resonance and note that this coefficient is nearly independent of temperature in the range 300 K < T < 600 K. In contrast, the macroscopic hydrogen absorption shows different behaviour at different temperatures and the absorption is much slower in pure magnesium than in the mixed hydride. A consistent approach which accounts for all the above observations is presented.