D. T. Peterson

Measurements are reported of the temperature dependence of the proton spin-lattice and spin-spin relaxation times ${T}_{1}$ and ${T}_{2}$ in yttrium and lanthanum dihydrides containing controlled levels of gadolinium as low as 50 ppm. The results demonstrate unambiguously that paramagnetic ions in concentrations so low as to have heretofore been regarded as insignificant have marked effects on the magnitude, frequency dependence, and temperature dependence of ${T}_{1}$ and to a lesser extent on ${T}_{2}$, and on the electronic structure and hydrogen diffusion parameters derived therefrom. The ${\mathrm{Gd}}^{3+}$ ion contributes an additional spin-lattice relaxation rate ${T}_{1p}$, which in these hydrides arises entirely from the dipolar coupling between impurity and proton moments. Proton magnetization is transported to the relaxation centers by spin diffusion at low temperatures and by hydrogen-atom diffusion at intermediate and high temperatures. The rate ${R}_{1p}$ is directly proportional to Gd-ion concentration at both low and high temperatures, but in the atom diffusion regime ${R}_{1p}$ is 20-25 times greater than for spin diffusion. The impurity-induced relaxation is shown to have profound effects on the apparent nuclear-nuclear dipolar relaxation rate ${R}_{1d}$ associated with hydrogen diffusion. At impurity levels as low as 10 ppm Gd, a secondary minimum appears in the temperature dependence of ${T}_{1}$ which may be readily misinterpreted in terms of a second motional process with lower activation energy. Even lower impurity levels yield a characteristic "slope-change" effect, which may be construed as indicating a change in the activation energy for hydrogen diffusion. At low temperatures ${R}_{1p}$ interferes with the determination of the conduction-electron contribution ${R}_{1e}$ and the Korringa product ${T}_{1e}T$. Separation of ${R}_{1e}$ and ${R}_{1p}$ is complicated by the fact the ${R}_{1p}$ is not temperature independent as has typically been assumed. Methods of achieving this separation are discussed, and it is shown experimentally that this difficulty can be circumvented by replacing the major part of the hydrogen with deuterium, thereby inhibiting spin diffusion. Measurement of ${T}_{1}$ as a function of resonance frequency and of ${T}_{2}$ can also be of value in separating the various sources of relaxation.

We have examined the electronic structure of Sc${\mathrm{H}}_{2}$, Y${\mathrm{H}}_{2}$, and Y${\mathrm{H}}_{2}$ using optical absorptivity and thermoreflectance techniques in the photon energy range from 0.2 to 5 eV between 4.2 and 340 K. The measured quantities were used to determine the frequency-dependent dielectric functions and the dependence of the dielectric functions on temperature modulation. The results show that the low-energy properties ($h\ensuremath{\nu}\ensuremath{\lesssim}1.5$ eV) are dominated by intraband absorption and a plasmon falling near 1.5-1.8 eV. Interband absorption is observed to be strong and structured above the interband onsets of \ensuremath{\sim} 1.25, \ensuremath{\sim} 1.6, and \ensuremath{\sim} 1.9 eV for Sc${\mathrm{H}}_{2}$, Y${\mathrm{H}}_{2}$, and Lu${\mathrm{H}}_{2}$, respectively. The observed interband features can be interpreted in terms of the self-consistent band calculations of Sc${\mathrm{H}}_{2}$ and Y${\mathrm{H}}_{2}$ presented in the companion paper, and experimental features can be related to specific bands in particular parts of the Brillouin zone. The systematics observed in these three trivalent metal dihydrides can be correlated well to theory. Extensive studies with samples of varying hydrogen to metal ratio ($x$) within the dihydride phase were carried out to examine the influence of hydrogen sublattice disorder on the optical properties and electronic structure. It was observed that, for $x$ approaching 2, interband features which could be related to $d$-band absorption were broadened by increasing lattice disorder, and new features in the interband absorption spectra were observed which could not be interpreted without postulating the hydrogen occupancy of significant numbers of octahedral sites. The strong $x$ dependence of the optical features emphasizes the importance of studying well-characterized samples.

The electronic structures of $\mathrm{Ti}{\mathrm{H}}_{x}$, $\mathrm{Zr}{\mathrm{H}}_{x}$, and $\mathrm{Hf}{\mathrm{H}}_{x}$ have been studied using photoelectron spectroscopy and synchrotron radiation. Structures in the metal $d$-derived band within \ensuremath{\sim}3 eV of the Fermi level ${E}_{F}$ and in the bonding band (\ensuremath{\sim}3-10 eV below ${E}_{F}$) are compared with theory. In each dihydride, the bonding band center falls at -5.5 eV, at approximately the same energy as previously observed for the dihydrides of Sc and Y. Changes in the emission features near ${E}_{F}$ and at -7 eV have been observed in samples bridging the $\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{fct}$ distortion in $\mathrm{Zr}{\mathrm{H}}_{x}$, $1.63\ensuremath{\le}x\ensuremath{\le}1.94$. The changes at ${E}_{F}$ demonstrate the Jahn-Teller effect for the electronic states of $\mathrm{Zr}{\mathrm{H}}_{x}$. The binding energies of the Ti $3p$, Zr $4p$, Hf $5p$, and Hf $4f$ cores are observed to be greater than in the elemental metals, consistent with charge transfer to the hydrogen site.