Electronic structure of metal hydrides. I. Optical studies of Sc${\mathrm{H}}_2$, Y${\mathrm{H}}_2$, and Lu${\mathrm{H}}_2$

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 ($h\nu \lesssim 1.5$ 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 ∼ 1.25, ∼ 1.6, and ∼ 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.