  The Mg-Ni-H switchable mirror system shows a spectacular composition
dependence in the hydrogenated state. The optical transmission of a
hydrogenated Mg-Ni-H gradient film displays a stripe-like pattern that consists
of alternating regions of high and low transmission. This behavior is peculiar
as the composite Mg-Ni-H phase diagram only predicts a gradual change between
MgH2 and Mg2NiH4 which can not account for the observed composition dependence.
In order to understand the observed transmission pattern, the Mg-Ni-H system
has been investigated optically, electrically, structurally and morphologically
in the metallic and hydrogenated state. As-deposited, metallic, Mg-Ni
composition gradient thin films contain the Mg2Ni and MgNi2 phases for
well-defined composition regions. In the hydrogenated state, vibrational
spectroscopy shows the composition dependence of semiconducting Mg2NiH4 which
is formed from metallic Mg2Ni. On exposure to hydrogen, physical properties
change as well near the equiatomic composition and the composition of MgNi2,
pointing to possible hydride formation. The strong composition dependence of
the Mg-Ni-H system, expressed in the observed transmission, can eventually be
understood from the distribution of Mg2NiH4 and the occurrence of MgH2 for
higher Mg contents.

Materials Science