Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE: rare earth) which have a weak red transparency window in their metallic dih ydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resis tivity measurements on a series of Y1-yMgyHx, films we show that this is du e to a disproportionation of the alloy. While the yttrium dihydride phase i s formed, Mg separates out, remaining in its metallic state. Upon further l oading, insulating MgH2 is formed together with cubic YH3-delta. In this wa y Mg acts essentially as a microscopic optical shutter, enhancing the refle ctivity of these switchable mirrors in the metallic state and increasing th e optical gap in the transparent state. (C) 1999 American Institute of Phys ics. [S0003-6951(99)04540-4].