The highly tunable Mg-Ni-H switchable mirror system

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.