[en] We have performed a mechanical-spectroscopy study on hcp YH_x single crystals (x ≤ 0.1, vibrating-reed technique, 280 to 1750 Hz). In agreement with previous data from polycrystalline YH_x, we find an anelastic relaxation peak at ∼ 280 K. Our single crystals show a large anisotropy in the relaxation strength which demonstrates that this peak results from a Zener relaxation of hydrogen (H) pairs. Two types of H jumps determine the relaxation kinetics. Comparison with previous QNS data identifies the jumps that cause the relaxation, showing further that their jump rates follow, over ten orders of magnitude, an Arrhenius law with an activation energy of (0.60 ± 0.03) eV. (orig.)

[en] Solution calorimetry has been used to determine the standard enthalpy of formation, ΔH⁰sub(f,298), of two hydrides of yttrium, the dihydride and a composition close to the trihydride. Yttrium and yttrium hydride were dissolved in 1.5M and 0.3M hydrochloric acid in a semi-microcalorimeter with an isothermal jacket. The composition of the dihydride was determined by the increase in the weight of the yttrium as a result of hydriding and the composition of the trihydride from the amount of hydrogen evolved during the dissolution

[en] Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination offers an attractive possibility to tune yttrium hydride through the T=0 metal-insulator transition. Conductivity and Hall measurements are used to determine critical exponents. The unusually large value for the product of the static and dynamical critical exponents appears to signify the important role played by electron-electron interactions

No abstract available

[en] Electron spin resonance (ESR) of dilute paramagnetic ions in nonmagnetic metallic hydrides provides microscopic information about the hydrogen ions in the immediate vicinity of the impurity. By comparing ESR spectra for different host metals and several hydrogen/metal ratios, one can determine material properties including host lattice symmetry, phase boundaries and occupation of available sites by hydrogen. Examples are presented of ESR of dilute Er in group IIIB and IVB metal hydrides, demonstrating the sensitivity and versatility of ESR as a spectroscopic technique

[en] The angular correlations of positron-annihilation radiation were measured for Y, YHsub(0.99), YHsub(2.01), YHsub(2.5), and YH₃. The angular correlation for free electrons (annihilation with bound electrons of atomic cores was taken into account) was found to be broader than that for yttrium metal and to increase in width with increasing hydrogen concentration to YHsub(2.01) in a manner consistent with the ''protonic'' model of electronic structure. The numbers of free electrons per atom for Y, YHsub(0.99), and YHsub(2.01) calculated from the distribution of momentum density are respectively: 2.86, 3.89 and 5. Thus, the free-electron model is a useful approximation of the electronic structure of yttrium hydrides, especially for concentrations near H/Y=2, but this model fails for higher concentrations. (author)

No abstract available

[en] Electron spin resonance (ESR) of dilute paramagnetic ions establishes the site symmetry of these ions. In the case of metal hydrides the site symmetry is determined by the number and location of neighboring protons. Typical ESR spectra for trivalent erbium in scandium and yttrium hydrides are presented and analyzed, and this technique is shown to be a versatile microscopic probe of the location, net charge and occupation probability of nearby protons

[en] A simple explanation for the logarithmic ageing of the photoconductivity in YH_3-δ is proposed. We show that the scaling ('simple' ageing) of the relaxation response with the illumination time t_w is consistent with the superposition of independently relaxing excitations with time offsets distributed over a window of width t_w. (comment)

[en] Self-consistent band-structure calculations have been performed for ScH₂ and YH₂ using the Korringa-Kohn-Rostoker method. The results indicate a net charge transfer from the metal to the hydrogen site and a concomitant raising of the hydrogen bonding bands relative to those obtained through non-self-consistent calculations. Comparisons are made between the results of our calculations and the results of optical studies by Weaver, Rosei, and Peterson. Additional calculations were performed in which the Fermi level or band gaps were rigidly shifted by a small energy increment. These calculations were used to simulate the derivative structure obtained in thermomodulation spectra and helped to identify the k-space origin of several experimental interband features found in the thermoreflectance of ScH₂ and YH₂. The experimentally observed, low energy, stoichiometry-dependent optical features of YH₂, which had partially inspired our studies, were not interpretable within the framework of our calculations based on the CaF₂ structure in which the hydrogen occupies all of the available tetrahedral sites. Instead, indirect evidence suggests that these low-energy features are associated with partial occupation of octahedral sites