Structural, electrical, and optical properties ofLa1−zYzHxswitchable mirrors

Abstract

Thin ${\mathrm{La}}_{1\ensuremath{-}z}{\mathrm{Y}}_{z}{\mathrm{H}}_{x}$ films, in the composition range $0<z<1$ and $0<x<3,$ are studied using x-ray diffraction, dc resistivity measurements, reflectance-transmittance measurements, and ellipsometry in the visible and near-infrared spectral range. For $x=0$ the structural phase diagram is similar to that of the bulk system. Upon hydrogen absorption and desorption, the ${\mathrm{La}}_{1\ensuremath{-}z}{\mathrm{Y}}_{z}{\mathrm{H}}_{x}$ films do not disproportionate. All dihydrides have a fcc structure with a continuous shift of the lattice parameter, whereas the trihydrides undergo a transition from a fcc lattice structure for $0<z<0.67$ to a hexagonal lattice structure for $0.81<z<1.$ No significant thin-film effects occur in the structural, electrical, and optical properties, whereas disorder effects are observed in the x-ray coherence length, the electron relaxation time at both zero and optical frequencies, and in the optical properties of the trihydrides. In ${\mathrm{LaH}}_{2}$ a similar dihydride transmission window is observed as in ${\mathrm{YH}}_{2}.$ The suppression of this window upon alloying is a disorder effect. As in the case of their parent materials, all ${\mathrm{La}}_{1\ensuremath{-}z}{\mathrm{Y}}_{z}{\mathrm{H}}_{x}$ alloys (both cubic and hexagonal) exhibit a metal-insulator transition for $2<x<3,$ which is a clear demonstration of the robustness of the metal-insulator transition in switchable mirrors. The optical band-gap shifts from $1.87\ifmmode\pm\else\textpm\fi{}0.03$ eV for ${\mathrm{LaH}}_{3}$ to $2.63\ifmmode\pm\else\textpm\fi{}0.03$ eV for ${\mathrm{YH}}_{3}.$ The optical properties suggest that the fundamental band gap is $1--1.8$ eV lower.