Structural, electrical, and optical properties of La1-zYzHx switchable mirrors - art. no. 195105

Thin La1-zYzHx films. in the composition range 0<z<1 and 0<x<3, are studied using x-ray diffraction. de resistivity measurements, reflectance-transmit tance measurements. and ellipsometry in the visible and near-infrared spect ral range. For x=0 the structural phase diagram is similar to that of the b ulk system. Upon hydrogen absorption and desorption, the La1-zYzHx films do not disproportionate. All dihydrides have a fee structure with a continuou s shift of the lattice parameter, whereas the trihydrides undergo a transit ion from a fee lattice structure for 0<z<0.67 to a hexagonal lattice struct ure 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 an d optical frequencies, and in the optical properties of the trihydrides. In LaH2 a similar dihydride transmission window is observed as in YH2. The su ppression of this window upon alloying is a disorder effect. As in the case of their parent materials, all La1-zYzHx alloys (both cubic and hexagonal) exhibit a metal-insulator transition for 2<x<3, which is a clear demonstra tion of the robustness of the metal-insulator transition in switchable mirr ors. The optical band-gap shifts from 1.87+/-0.03 eV for LaH3 to 2.63+/-0.0 3 eV for YH3. The optical properties suggest that the fundamental band gap is 1-1.8 eV lower.