The dielectric functions and optical band gaps of thin films of   amorphous and cubic crystalline Mg~2NiH~4
  Magnesium nickel hydride films have earlier been suggested for several optoelectronic applications, but the optical properties and band gap have not been firmly established. In this work, the dielectric functions and the optical band gaps of thin films of Mg2NiH4 have been determined experimentally from optical modeling using spectroscopic ellipsometry and spectrophotometry in the photon energy range between 0.7 and 4.2 eV. Samples were prepared by reactive sputtering, resulting in a single-layer geometry that could easily be studied by ellipsometry. Crystalline samples were prepared by annealing the amorphous films ex-situ. The resulting films remained in the high temperature cubic Mg2NiH4 structure even after cooling to room temperature. Tauc analysis of the dielectric function shows that Mg2NiH4 exhibits a band gap of 1.6 eV for amorphous structure films and 2.1 eV for its cubic crystalline structure. 
J. H. Selj, T. Mongstad, B. Hauback, S. Karazhanov
cond-mat.mtrl-sci