The dielectric function of MgyNiHx thin films (2 ≤ y ≤ 10)

Abstract Mg y Ni ( 2 ≤ y ≤ 10 ) thin films covered with a Pd cap layer are hydrogenated in 1 0 5  Pa H 2 between room temperature and 80  ° C and their dielectric function ɛ ˜ is determined from reflection and transmission measurements. The hydrogenated Mg y NiH x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y . Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg 2 NiH 4 host takes place at least for y ≤ 3.5 while no signature of MgH 2 is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg 2 NiH 4 (energy gap E g = 1.6  eV) and MgH 2 ( E g = 5.6  eV) is expected.