  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.
