Electrochromism of Mg-Ni hydride Switchable Mirrors

Switchable mirrors have so far been made of rare-earth and rare-earth-magnesium based metal-hydrides. In this investigation we study Mg-Ni-hydrides, which have been shown by Richardson et al.(1) to exhibit switchable proper-ties similar to those of the rare-earth hydrides. Cyclic voltammetry on MgzNiHx samples with 0.8 < z < 3.7 shows that addition of one Mg atom per Mg2Ni gives the best ab/desorption kinetics for hydrogen. X-ray diffraction reveals a structural change as hydrogen is absorbed. The metal-insulator transition is confirmed with simultaneous resistivity measurements. A pressure-composition isotherm of Mg2NiHx is also determined electrocemically. Optical spectrometry during gas loading gives an optical band gap of 1.6 eV for Mg2NiH4. This gap increases with increasing Mg content in a way similar to that of the Mg-doped rare-earth hydrides.