Electronic structure of metal hydrides. V.x-dependent properties ofLaHx (1.9<~x<~2.9)andNdHx (2.01<~x<~2.27)

Abstract

The composition-dependent electronic structure of fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, has been studied using photoelectron spectroscopy with synchrotron radiation ($10l~\mathrm{hv}l~50 \mathrm{eV}$). Complementary optical reflectance measurements ($0.16l~\mathrm{hv}l~4 \mathrm{eV}$) have been performed for fcc $\mathrm{La}{\mathrm{H}}_{x}$, $1.9l~xl~2.9$, and $\mathrm{Nd}{\mathrm{H}}_{x}$, $2.01l~xl~2.27$. For $\mathrm{La}{\mathrm{H}}_{x}$, $x\ensuremath{\simeq}2$, the occupied $d$ bands are \ensuremath{\sim}1.5 eV wide and the hydrogen-induced bonding band is centered \ensuremath{\sim}5 eV below the Fermi level, ${E}_{F}$ (full width \ensuremath{\sim}6 eV). Hydrogen occupation of both octahedral and tetrahedral sites is revealed for $xl~2$, analogous to what has been observed for other metal dihydrides. With increasing hydrogen concentration, emission from the $d$ bands near ${E}_{F}$ decreases and the bonding band shifts to higher binding energy; the optical spectra show a red shift of interband absorption features, increased octahedral site occupation, and increased screening of a low-energy plasmon. For $\mathrm{La}{\mathrm{H}}_{x}$ samples at the upper end of the composition range, $x\ensuremath{\sim}2.9$, the photoemission spectra show very weak valence-band emission, and the optical spectra suggest semiconducting behavior. The binding energies of the ${5p}_{\frac{1}{2},\frac{3}{2}}$ cores, ${E}_{B}$, measured relative to ${E}_{F}$ are shown to increase with $x$ (the total shift is \ensuremath{\sim}0.8 eV for La \ensuremath{\rightarrow} La${\mathrm{H}}_{2}$ and 0.9 eV for La${\mathrm{H}}_{2}$ \ensuremath{\rightarrow} La${\mathrm{H}}_{3}$). Our results are compared to band calculations by Gupta and Burger and by Misemer and Harmon and to results of NMR, specific heat, and resistivity studies.