Highly reflecting Y/Mg-Hx multilayered switchable mirrors

Optical, structural and thermodynamic properties of Y/Mg-H-x multilayered switchable mirrors are investigated and compared with YMgHx-alloys and pure YHx. Multilayers clearly have superior reflectance in the low-hydrogen state over the whole range of photon energies, 0.72 < hw < 3.5 eV, investigated in this work. In the transparent high-hydrogen state the absorption edge of the multilayers with the same overall composition is shifted in energy on increasing the individual layer thicknesses. Thus, the absorption edge is non-trivially depending on the distribution of Mg in the film. Moreover, the optical contrast of multilayers is much higher than for pure YHx and their optical switching does not suffer from hysteretic effects. Finally, Mg and YH2 transform almost at the same time to MgH2 and YH3, respectively, with a heat of formation of -30.9 kJ/mol H in absorption and -36.6 kJ/mol H in desorption. (C) 2002 Elsevier Science B.V. All rights reserved.