Structural, electrical, and optical properties of La1ÀzYzHx switchable mirrors

Thin La12zYzHx 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 nearinfrared spectral range. For x50 the structural phase diagram is similar to that of the bulk system. Upon hydrogen absorption and desorption, the La12zYzHx 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 La12zYzHx 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 1.8760.03 eV for LaH3 to 2.6360.03 eV for YH3. The optical properties suggest that the fundamental band gap is 1 –1.8 eV lower.