Electronic structure of metal hydrides. V. $x$-dependent properties of $\mathrm{La}{\mathrm{H}}_x (1.9\mathrm{}<~x<~2.9)$ and $\mathrm{Nd}{\mathrm{H}}_x (2.01\mathrm{}<~x<~2.27)$

The composition-dependent electronic structure of fcc $\mathrm{La}{\mathrm{H}}_x$, $1.9<~x<~2.9$, has been studied using photoelectron spectroscopy with synchrotron radiation ($10<~\mathrm{hv}<~50 \mathrm{eV}$). Complementary optical reflectance measurements ($0.16<~\mathrm{hv}<~4 \mathrm{eV}$) have been performed for fcc $\mathrm{La}{\mathrm{H}}_x$, $1.9<~x<~2.9$, and $\mathrm{Nd}{\mathrm{H}}_x$, $2.01<~x<~2.27$. For $\mathrm{La}{\mathrm{H}}_x$, $x\simeq 2$, the occupied $d$ bands are ∼1.5 eV wide and the hydrogen-induced bonding band is centered ∼5 eV below the Fermi level, $E_F$ (full width ∼6 eV). Hydrogen occupation of both octahedral and tetrahedral sites is revealed for $x<~2$, 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, $x\sim 2.9$, 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 ∼0.8 eV for La → La${\mathrm{H}}_2$ and 0.9 eV for La${\mathrm{H}}_2$ → 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.