D. J. Peterman

Self-consistent band-structure calculations have been performed for Sc${\mathrm{H}}_{2}$ and Y${\mathrm{H}}_{2}$ 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-conssitent 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 Sc${\mathrm{H}}_{2}$ and Y${\mathrm{H}}_{2}$. The experimentally observed, low-energy, stoichiometry-dependent optical features of Y${\mathrm{H}}_{2}$, which had partially inspired our studies, were not interpretable within the framework of our calculations based on the Ca${\mathrm{F}}_{2}$ 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.

Photoelectron-spectroscopy studies of Ce${\mathrm{Al}}_{2}$, La${\mathrm{Al}}_{2}$, and the "chemically compressed" alloys ${\mathrm{Ce}}_{0.6}$${\mathrm{Y}}_{0.4}$${\mathrm{Al}}_{2}$ and ${\mathrm{Ce}}_{0.6}$${\mathrm{Sc}}_{0.4}$${\mathrm{Al}}_{2}$ were performed with use of synchrotron radiation. For all compounds the $4f$ binding energy is 2.55 \ifmmode\pm\else\textpm\fi{} 0.1 eV. These results, combined with previous bulk-property results, motivate a reevaluation of Anderson-model parameters and mechanisms as applied to Ce${\mathrm{Al}}_{2}$.

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.

Photoemission studies ($12\ensuremath{\le}\mathrm{hv}\ensuremath{\le}135$ eV) of room-temperature formation of the Si-Cr interface show reactive behavior with atomic intermixing and dramatic modifications of the metal-derived $d$ density of states. Self-consistent augmented-spherical-wave calculations of the total and $l$-projected densities of states for the silicides ${\mathrm{Cr}}_{3}$Si, CrSi, and Cr${\mathrm{Si}}_{3}$ in simplified cubic lattice structures allow an identification of general trends in the electronic structure upon Si-Cr heteropolar bond formation. These experimental and theoretical results suggest an interface morphology where a Si-rich intermixed phase is present for a depth of $\ensuremath{\simeq}10$ monolayers between the Si crystal and the unreacted Cr film. Evidence of Si segregation in the top layers of the Cr film is provided.

We present a photoemission study of the $\mathrm{Ce}4f$ electron in materials which range from the localized, trivalent ($4{f}^{1}$) compounds Ce${\mathrm{H}}_{2}$ and Ce${\mathrm{Al}}_{2}$ to the reportedly tetravalent ($4{f}^{0}$) compounds Ce${\mathrm{Rh}}_{3}$ and Ce${\mathrm{Ru}}_{2}$. Using synchrotron radiation in the photon energy range of $10\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}140$ eV, we compare the valence-band emission of the Ce compounds with that of the analogous La compounds, paying particular attention to resonant behavior near the $4d\ensuremath{\rightarrow}4f$ transition threshold. In addition, we use the partial photoelectron yield as an indicator of the degree of $4f$ localization as well as occupation. We find no evidence of a change in $4f$ occupation across the series Ce${\mathrm{H}}_{2.1}$\ensuremath{\rightarrow}Ce${\mathrm{Ru}}_{2.}$ Instead, we find an increase in the width of the $4f$ emission and an increase in the effect of the $4f$ electron on the remaining band states. We conclude that the apparent valence changes in these particular compounds are due primarily to hybridization effects, in agreement with recent band-structure calculations.