  The optical and electronic properties of Mg-Ti hydrides are studied using
first-principles density functional theory. Dielectric functions are calculated
for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is
that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of
the lattice. In order to assess the effect of randomness in the Mg and Ti
occupations we consider both highly ordered structures, modeled with simple
unit cells of minimal size, and models of random alloys. These are simulated by
super cells containing up to 64 formula units (Z = 64). All compositions and
structural models turn out metallic, hence the dielectric functions contain
interband and intraband free electron contributions. The former are calculated
in the independent particle random phase approximation. The latter are modeled
based upon the intraband plasma frequencies, which are also calculated from
first-principles. Only for the models of the random alloys we obtain a black
state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.
