The dielectric function of Mgy NiHx thin films ()

Mgy Ni (2 < =y < =10) thin films covered with a Pd cap layer are hydrogenated in 105 Pa H2 between room temperature and 80 deg C and their dielectric function is determined from reflection and transmission measurements. The hydrogenated Mgy NiHx 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 Mg2 NiH4 host takes place at least for y < =3.5 while no signature of MgH2 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 Mg2 NiH4 (energy gap Eg=1.6 eV) and MgH2 (Eg=5.6 eV) is expected.