Structural, electrical, and optical properties of La1-zYzHx switchable mirrors

Thin La_1-zY_zH_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 La_1-zY_zH_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 LaH₂ a similar dihydride transmission window is observed as in YH₂. The suppression of this window upon alloying is a disorder effect. As in the case of their parent materials, all La_1-zY_zH_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+/-0.03 eV for LaH₃ to 2.63+/-0.03 eV for YH₃. The optical properties suggest that the fundamental band gap is 1-1.8 eV lower.